[RFC PATCH 0/2] Ability to allocate DMAable pages using unpopulated-alloc

[RFC PATCH 0/2] Ability to allocate DMAable pages using unpopulated-alloc

[Oleksandr Tyshchenko]

From: Oleksandr Tyshchenko <oleksandr_tyshchenko@epam.com>

Hello all.

The purpose of this RFC patch series is to get feedback about supporting the allocation
of DMAable pages by Linux's unpopulated-alloc.

The unpopulated-alloc feature has been enabled on Arm since the extended-regions support
reached upstream. With that (if, of course, we run new Xen version and Xen was able to
allocate extended regions), we don't allocate the real RAM pages from host memory and balloon
them out (in order to obtain physical memory space to map the guest pages into) anymore, we use
the unpopulated pages instead. And it seems that all users I have played with on Arm (I mean,
Xen PV and virtio backends) are happy with the pages provided by xen_alloc_unpopulated_pages().
It is worth mentioning that these pages are not contiguous, but this wasn't an issue so far.

There is one place, where we still steal RAM pages if user-space Xen PV backend tries
to establish grant mapping with a need to be backed by a DMA buffer for the sake of zero-copy
(see dma_alloc*() usage in gnttab_dma_alloc_pages()).

And, if I am not mistaken (there might be pitfalls which I am not aware of), we could avoid
wasting real RAM pages in that particular case also by adding an ability to allocate
unpopulated DMAable pages (which are guaranteed to be contiguous in IPA).
The benefit is quite clear here:
1. Avoid wasting real RAM pages (reducing the amount of CMA memory usable) for allocating
   physical memory space to map the granted buffer into (which can be big enough if
   we deal with Xen PV Display driver using multiple Full HD buffers) 
2. Avoid superpage shattering in Xen P2M when establishing stage-2 mapping for that
   granted buffer
3. Avoid extra operations needed for the granted buffer to be properly mapped and
   unmapped such as ballooning in/out real RAM pages

Please note, there are several TODOs (described in corresponding commit subjects),
which I will try to eliminate in next versions if we find a common ground regarding
the approach.

Any feedback/help would be highly appreciated.

Oleksandr Tyshchenko (2):
  xen/unpopulated-alloc: Introduce helpers for DMA allocations
  xen/grant-table: Use unpopulated DMAable pages instead of real RAM
    ones

 drivers/xen/grant-table.c       |  27 +++++++
 drivers/xen/unpopulated-alloc.c | 167 ++++++++++++++++++++++++++++++++++++++++
 include/xen/xen.h               |  15 ++++
 3 files changed, 209 insertions(+)

-- 
2.7.4

[RFC PATCH 1/2] xen/unpopulated-alloc: Introduce helpers for DMA allocations

[Oleksandr Tyshchenko]

From: Oleksandr Tyshchenko <oleksandr_tyshchenko@epam.com>

Add ability to allocate unpopulated DMAable (contiguous) pages
suitable for grant mapping into. This is going to be used by gnttab
code (see gnttab_dma_alloc_pages()).

TODO: There is a code duplication in fill_dma_pool(). Also pool
oparations likely need to be protected by the lock.

Signed-off-by: Oleksandr Tyshchenko <oleksandr_tyshchenko@epam.com>
---
 drivers/xen/unpopulated-alloc.c | 167 ++++++++++++++++++++++++++++++++++++++++
 include/xen/xen.h               |  15 ++++
 2 files changed, 182 insertions(+)

diff --git a/drivers/xen/unpopulated-alloc.c b/drivers/xen/unpopulated-alloc.c
index a39f2d3..bca0198 100644
--- a/drivers/xen/unpopulated-alloc.c
+++ b/drivers/xen/unpopulated-alloc.c
@@ -1,5 +1,6 @@
 // SPDX-License-Identifier: GPL-2.0
 #include <linux/errno.h>
+#include <linux/genalloc.h>
 #include <linux/gfp.h>
 #include <linux/kernel.h>
 #include <linux/mm.h>
@@ -16,6 +17,8 @@ static DEFINE_MUTEX(list_lock);
 static struct page *page_list;
 static unsigned int list_count;
 
+static struct gen_pool *dma_pool;
+
 static struct resource *target_resource;
 
 /*
@@ -230,6 +233,161 @@ void xen_free_unpopulated_pages(unsigned int nr_pages, struct page **pages)
 }
 EXPORT_SYMBOL(xen_free_unpopulated_pages);
 
+static int fill_dma_pool(unsigned int nr_pages)
+{
+	struct dev_pagemap *pgmap;
+	struct resource *res, *tmp_res = NULL;
+	void *vaddr;
+	unsigned int alloc_pages = round_up(nr_pages, PAGES_PER_SECTION);
+	struct range mhp_range;
+	int ret;
+
+	res = kzalloc(sizeof(*res), GFP_KERNEL);
+	if (!res)
+		return -ENOMEM;
+
+	res->name = "Xen DMA pool";
+	res->flags = IORESOURCE_MEM | IORESOURCE_BUSY;
+
+	mhp_range = mhp_get_pluggable_range(true);
+
+	ret = allocate_resource(target_resource, res,
+				alloc_pages * PAGE_SIZE, mhp_range.start, mhp_range.end,
+				PAGES_PER_SECTION * PAGE_SIZE, NULL, NULL);
+	if (ret < 0) {
+		pr_err("Cannot allocate new IOMEM resource\n");
+		goto err_resource;
+	}
+
+	/*
+	 * Reserve the region previously allocated from Xen resource to avoid
+	 * re-using it by someone else.
+	 */
+	if (target_resource != &iomem_resource) {
+		tmp_res = kzalloc(sizeof(*tmp_res), GFP_KERNEL);
+		if (!res) {
+			ret = -ENOMEM;
+			goto err_insert;
+		}
+
+		tmp_res->name = res->name;
+		tmp_res->start = res->start;
+		tmp_res->end = res->end;
+		tmp_res->flags = res->flags;
+
+		ret = request_resource(&iomem_resource, tmp_res);
+		if (ret < 0) {
+			pr_err("Cannot request resource %pR (%d)\n", tmp_res, ret);
+			kfree(tmp_res);
+			goto err_insert;
+		}
+	}
+
+	pgmap = kzalloc(sizeof(*pgmap), GFP_KERNEL);
+	if (!pgmap) {
+		ret = -ENOMEM;
+		goto err_pgmap;
+	}
+
+	pgmap->type = MEMORY_DEVICE_GENERIC;
+	pgmap->range = (struct range) {
+		.start = res->start,
+		.end = res->end,
+	};
+	pgmap->nr_range = 1;
+	pgmap->owner = res;
+
+	vaddr = memremap_pages(pgmap, NUMA_NO_NODE);
+	if (IS_ERR(vaddr)) {
+		pr_err("Cannot remap memory range\n");
+		ret = PTR_ERR(vaddr);
+		goto err_memremap;
+	}
+
+	ret = gen_pool_add_virt(dma_pool, (unsigned long)vaddr, res->start,
+			alloc_pages * PAGE_SIZE, NUMA_NO_NODE);
+	if (ret)
+		goto err_pool;
+
+	return 0;
+
+err_pool:
+	memunmap_pages(pgmap);
+err_memremap:
+	kfree(pgmap);
+err_pgmap:
+	if (tmp_res) {
+		release_resource(tmp_res);
+		kfree(tmp_res);
+	}
+err_insert:
+	release_resource(res);
+err_resource:
+	kfree(res);
+	return ret;
+}
+
+/**
+ * xen_alloc_unpopulated_dma_pages - alloc unpopulated DMAable pages
+ * @dev: valid struct device pointer
+ * @nr_pages: Number of pages
+ * @pages: pages returned
+ * @return 0 on success, error otherwise
+ */
+int xen_alloc_unpopulated_dma_pages(struct device *dev, unsigned int nr_pages,
+		struct page **pages)
+{
+	void *vaddr;
+	bool filled = false;
+	unsigned int i;
+	int ret;
+
+	if (!dma_pool)
+		return -ENODEV;
+
+	/* XXX Handle devices which support 64-bit DMA address only for now */
+	if (dma_get_mask(dev) != DMA_BIT_MASK(64))
+		return -EINVAL;
+
+	while (!(vaddr = (void *)gen_pool_alloc(dma_pool, nr_pages * PAGE_SIZE))) {
+		if (filled)
+			return -ENOMEM;
+		else {
+			ret = fill_dma_pool(nr_pages);
+			if (ret)
+				return ret;
+
+			filled = true;
+		}
+	}
+
+	for (i = 0; i < nr_pages; i++)
+		pages[i] = virt_to_page(vaddr + PAGE_SIZE * i);
+
+	return 0;
+}
+EXPORT_SYMBOL(xen_alloc_unpopulated_dma_pages);
+
+/**
+ * xen_free_unpopulated_dma_pages - return unpopulated DMAable pages
+ * @dev: valid struct device pointer
+ * @nr_pages: Number of pages
+ * @pages: pages to return
+ */
+void xen_free_unpopulated_dma_pages(struct device *dev, unsigned int nr_pages,
+		struct page **pages)
+{
+	void *vaddr;
+
+	if (!dma_pool)
+		return;
+
+	vaddr = page_to_virt(pages[0]);
+
+	gen_pool_free(dma_pool, (unsigned long)vaddr, nr_pages * PAGE_SIZE);
+}
+EXPORT_SYMBOL(xen_free_unpopulated_dma_pages);
+
 static int __init unpopulated_init(void)
 {
 	int ret;
@@ -241,8 +399,17 @@ static int __init unpopulated_init(void)
 	if (ret) {
 		pr_err("xen:unpopulated: Cannot initialize target resource\n");
 		target_resource = NULL;
+		return ret;
 	}
 
+	dma_pool = gen_pool_create(PAGE_SHIFT, NUMA_NO_NODE);
+	if (!dma_pool) {
+		pr_err("xen:unpopulated: Cannot create DMA pool\n");
+		return -ENOMEM;
+	}
+
+	gen_pool_set_algo(dma_pool, gen_pool_best_fit, NULL);
+
 	return ret;
 }
 early_initcall(unpopulated_init);
diff --git a/include/xen/xen.h b/include/xen/xen.h
index a99bab8..a6a7a59 100644
--- a/include/xen/xen.h
+++ b/include/xen/xen.h
@@ -52,9 +52,15 @@ bool xen_biovec_phys_mergeable(const struct bio_vec *vec1,
 extern u64 xen_saved_max_mem_size;
 #endif
 
+struct device;
+
 #ifdef CONFIG_XEN_UNPOPULATED_ALLOC
 int xen_alloc_unpopulated_pages(unsigned int nr_pages, struct page **pages);
 void xen_free_unpopulated_pages(unsigned int nr_pages, struct page **pages);
+int xen_alloc_unpopulated_dma_pages(struct device *dev, unsigned int nr_pages,
+		struct page **pages);
+void xen_free_unpopulated_dma_pages(struct device *dev, unsigned int nr_pages,
+		struct page **pages);
 #include <linux/ioport.h>
 int arch_xen_unpopulated_init(struct resource **res);
 #else
@@ -69,6 +75,15 @@ static inline void xen_free_unpopulated_pages(unsigned int nr_pages,
 {
 	xen_free_ballooned_pages(nr_pages, pages);
 }
+static inline int xen_alloc_unpopulated_dma_pages(struct device *dev,
+		unsigned int nr_pages, struct page **pages)
+{
+	return -1;
+}
+static inline void xen_free_unpopulated_dma_pages(struct device *dev,
+		unsigned int nr_pages, struct page **pages)
+{
+}
 #endif
 
 #endif	/* _XEN_XEN_H */
-- 
2.7.4

[RFC PATCH 2/2] xen/grant-table: Use unpopulated DMAable pages instead of real RAM ones

[Oleksandr Tyshchenko]

From: Oleksandr Tyshchenko <oleksandr_tyshchenko@epam.com>

Depends on CONFIG_XEN_UNPOPULATED_ALLOC. If enabled then unpopulated
DMAable (contiguous) pages will be allocated for grant mapping into
instead of ballooning out real RAM pages.

TODO: Fallback to real RAM pages if xen_alloc_unpopulated_dma_pages()
fails.

Signed-off-by: Oleksandr Tyshchenko <oleksandr_tyshchenko@epam.com>
---
 drivers/xen/grant-table.c | 27 +++++++++++++++++++++++++++
 1 file changed, 27 insertions(+)

diff --git a/drivers/xen/grant-table.c b/drivers/xen/grant-table.c
index 8ccccac..2bb4392 100644
--- a/drivers/xen/grant-table.c
+++ b/drivers/xen/grant-table.c
@@ -864,6 +864,25 @@ EXPORT_SYMBOL_GPL(gnttab_free_pages);
  */
 int gnttab_dma_alloc_pages(struct gnttab_dma_alloc_args *args)
 {
+#ifdef CONFIG_XEN_UNPOPULATED_ALLOC
+	int ret;
+
+	ret = xen_alloc_unpopulated_dma_pages(args->dev, args->nr_pages,
+			args->pages);
+	if (ret < 0)
+		return ret;
+
+	ret = gnttab_pages_set_private(args->nr_pages, args->pages);
+	if (ret < 0) {
+		gnttab_dma_free_pages(args);
+		return ret;
+	}
+
+	args->vaddr = page_to_virt(args->pages[0]);
+	args->dev_bus_addr = page_to_phys(args->pages[0]);
+
+	return ret;
+#else
 	unsigned long pfn, start_pfn;
 	size_t size;
 	int i, ret;
@@ -910,6 +929,7 @@ int gnttab_dma_alloc_pages(struct gnttab_dma_alloc_args *args)
 fail:
 	gnttab_dma_free_pages(args);
 	return ret;
+#endif
 }
 EXPORT_SYMBOL_GPL(gnttab_dma_alloc_pages);
 
@@ -919,6 +939,12 @@ EXPORT_SYMBOL_GPL(gnttab_dma_alloc_pages);
  */
 int gnttab_dma_free_pages(struct gnttab_dma_alloc_args *args)
 {
+#ifdef CONFIG_XEN_UNPOPULATED_ALLOC
+	gnttab_pages_clear_private(args->nr_pages, args->pages);
+	xen_free_unpopulated_dma_pages(args->dev, args->nr_pages, args->pages);
+
+	return 0;
+#else
 	size_t size;
 	int i, ret;
 
@@ -946,6 +972,7 @@ int gnttab_dma_free_pages(struct gnttab_dma_alloc_args *args)
 		dma_free_wc(args->dev, size,
 			    args->vaddr, args->dev_bus_addr);
 	return ret;
+#endif
 }
 EXPORT_SYMBOL_GPL(gnttab_dma_free_pages);
 #endif
-- 
2.7.4

Re: [RFC PATCH 2/2] xen/grant-table: Use unpopulated DMAable pages instead of real RAM ones

[Stefano Stabellini]

On Tue, 17 May 2022, Oleksandr Tyshchenko wrote:
> From: Oleksandr Tyshchenko <oleksandr_tyshchenko@epam.com>
> 
> Depends on CONFIG_XEN_UNPOPULATED_ALLOC. If enabled then unpopulated
> DMAable (contiguous) pages will be allocated for grant mapping into
> instead of ballooning out real RAM pages.
> 
> TODO: Fallback to real RAM pages if xen_alloc_unpopulated_dma_pages()
> fails.
> 
> Signed-off-by: Oleksandr Tyshchenko <oleksandr_tyshchenko@epam.com>
> ---
>  drivers/xen/grant-table.c | 27 +++++++++++++++++++++++++++
>  1 file changed, 27 insertions(+)
> 
> diff --git a/drivers/xen/grant-table.c b/drivers/xen/grant-table.c
> index 8ccccac..2bb4392 100644
> --- a/drivers/xen/grant-table.c
> +++ b/drivers/xen/grant-table.c
> @@ -864,6 +864,25 @@ EXPORT_SYMBOL_GPL(gnttab_free_pages);
>   */
>  int gnttab_dma_alloc_pages(struct gnttab_dma_alloc_args *args)
>  {
> +#ifdef CONFIG_XEN_UNPOPULATED_ALLOC
> +	int ret;

This is an alternative implementation of the same function. If we are
going to use #ifdef, then I would #ifdef the entire function, rather
than just the body. Otherwise within the function body we can use
IS_ENABLED.


> +	ret = xen_alloc_unpopulated_dma_pages(args->dev, args->nr_pages,
> +			args->pages);
> +	if (ret < 0)
> +		return ret;
> +
> +	ret = gnttab_pages_set_private(args->nr_pages, args->pages);
> +	if (ret < 0) {
> +		gnttab_dma_free_pages(args);

it should xen_free_unpopulated_dma_pages ?


> +		return ret;
> +	}
> +
> +	args->vaddr = page_to_virt(args->pages[0]);
> +	args->dev_bus_addr = page_to_phys(args->pages[0]);

There are two things to note here. 

The first thing to note is that normally we would call pfn_to_bfn to
retrieve the dev_bus_addr of a page because pfn_to_bfn takes into
account foreign mappings. However, these are freshly allocated pages
without foreign mappings, so page_to_phys/dma should be sufficient.


The second has to do with physical addresses and DMA addresses. The
functions are called gnttab_dma_alloc_pages and
xen_alloc_unpopulated_dma_pages which make you think we are retrieving a
DMA address here. However, to get a DMA address we need to call
page_to_dma rather than page_to_phys.

page_to_dma takes into account special offsets that some devices have
when accessing memory. There are real cases on ARM where the physical
address != DMA address, e.g. RPi4.

However, to call page_to_dma you need to specify as first argument the
DMA-capable device that is expected to use those pages for DMA (e.g. an
ethernet device or a MMC controller.) While the args->dev we have in
gnttab_dma_alloc_pages is the gntdev_miscdev.

So this interface cannot actually be used to allocate memory that is
supposed to be DMA-able by a DMA-capable device, such as an ethernet
device.

But I think that should be fine because the memory is meant to be used
by a userspace PV backend for grant mappings. If any of those mappings
end up being used for actual DMA in the kernel they should go through the
drivers/xen/swiotlb-xen.c and xen_phys_to_dma should be called, which
ends up calling page_to_dma as appropriate.

It would be good to double-check that the above is correct and, if so,
maybe add a short in-code comment about it:

/*
 * These are not actually DMA addresses but regular physical addresses.
 * If these pages end up being used in a DMA operation then the
 * swiotlb-xen functions are called and xen_phys_to_dma takes care of
 * the address translations:
 *
 * - from gfn to bfn in case of foreign mappings
 * - from physical to DMA addresses in case the two are different for a
 *   given DMA-mastering device
 */



> +	return ret;
> +#else
>  	unsigned long pfn, start_pfn;
>  	size_t size;
>  	int i, ret;
> @@ -910,6 +929,7 @@ int gnttab_dma_alloc_pages(struct gnttab_dma_alloc_args *args)
>  fail:
>  	gnttab_dma_free_pages(args);
>  	return ret;
> +#endif
>  }
>  EXPORT_SYMBOL_GPL(gnttab_dma_alloc_pages);
>  
> @@ -919,6 +939,12 @@ EXPORT_SYMBOL_GPL(gnttab_dma_alloc_pages);
>   */
>  int gnttab_dma_free_pages(struct gnttab_dma_alloc_args *args)
>  {
> +#ifdef CONFIG_XEN_UNPOPULATED_ALLOC
> +	gnttab_pages_clear_private(args->nr_pages, args->pages);
> +	xen_free_unpopulated_dma_pages(args->dev, args->nr_pages, args->pages);
> +
> +	return 0;
> +#else
>  	size_t size;
>  	int i, ret;
>  
> @@ -946,6 +972,7 @@ int gnttab_dma_free_pages(struct gnttab_dma_alloc_args *args)
>  		dma_free_wc(args->dev, size,
>  			    args->vaddr, args->dev_bus_addr);
>  	return ret;
> +#endif
>  }
>  EXPORT_SYMBOL_GPL(gnttab_dma_free_pages);
>  #endif
> -- 
> 2.7.4
> 

Re: [RFC PATCH 1/2] xen/unpopulated-alloc: Introduce helpers for DMA allocations

[Stefano Stabellini]

On Tue, 17 May 2022, Oleksandr Tyshchenko wrote:
> From: Oleksandr Tyshchenko <oleksandr_tyshchenko@epam.com>
> 
> Add ability to allocate unpopulated DMAable (contiguous) pages
> suitable for grant mapping into. This is going to be used by gnttab
> code (see gnttab_dma_alloc_pages()).
> 
> TODO: There is a code duplication in fill_dma_pool(). Also pool
> oparations likely need to be protected by the lock.
> 
> Signed-off-by: Oleksandr Tyshchenko <oleksandr_tyshchenko@epam.com>
> ---
>  drivers/xen/unpopulated-alloc.c | 167 ++++++++++++++++++++++++++++++++++++++++
>  include/xen/xen.h               |  15 ++++
>  2 files changed, 182 insertions(+)
> 
> diff --git a/drivers/xen/unpopulated-alloc.c b/drivers/xen/unpopulated-alloc.c
> index a39f2d3..bca0198 100644
> --- a/drivers/xen/unpopulated-alloc.c
> +++ b/drivers/xen/unpopulated-alloc.c
> @@ -1,5 +1,6 @@
>  // SPDX-License-Identifier: GPL-2.0
>  #include <linux/errno.h>
> +#include <linux/genalloc.h>
>  #include <linux/gfp.h>
>  #include <linux/kernel.h>
>  #include <linux/mm.h>
> @@ -16,6 +17,8 @@ static DEFINE_MUTEX(list_lock);
>  static struct page *page_list;
>  static unsigned int list_count;
>  
> +static struct gen_pool *dma_pool;
> +
>  static struct resource *target_resource;
>  
>  /*
> @@ -230,6 +233,161 @@ void xen_free_unpopulated_pages(unsigned int nr_pages, struct page **pages)
>  }
>  EXPORT_SYMBOL(xen_free_unpopulated_pages);
>  
> +static int fill_dma_pool(unsigned int nr_pages)
> +{

I think we shouldn't need to add this function at all as we should be
able to reuse fill_list even for contiguous pages. fill_list could
always call gen_pool_add_virt before returning.


> +	struct dev_pagemap *pgmap;
> +	struct resource *res, *tmp_res = NULL;
> +	void *vaddr;
> +	unsigned int alloc_pages = round_up(nr_pages, PAGES_PER_SECTION);
> +	struct range mhp_range;
> +	int ret;
> +
> +	res = kzalloc(sizeof(*res), GFP_KERNEL);
> +	if (!res)
> +		return -ENOMEM;
> +
> +	res->name = "Xen DMA pool";
> +	res->flags = IORESOURCE_MEM | IORESOURCE_BUSY;
> +
> +	mhp_range = mhp_get_pluggable_range(true);
> +
> +	ret = allocate_resource(target_resource, res,
> +				alloc_pages * PAGE_SIZE, mhp_range.start, mhp_range.end,
> +				PAGES_PER_SECTION * PAGE_SIZE, NULL, NULL);
> +	if (ret < 0) {
> +		pr_err("Cannot allocate new IOMEM resource\n");
> +		goto err_resource;
> +	}
> +
> +	/*
> +	 * Reserve the region previously allocated from Xen resource to avoid
> +	 * re-using it by someone else.
> +	 */
> +	if (target_resource != &iomem_resource) {
> +		tmp_res = kzalloc(sizeof(*tmp_res), GFP_KERNEL);
> +		if (!res) {
> +			ret = -ENOMEM;
> +			goto err_insert;
> +		}
> +
> +		tmp_res->name = res->name;
> +		tmp_res->start = res->start;
> +		tmp_res->end = res->end;
> +		tmp_res->flags = res->flags;
> +
> +		ret = request_resource(&iomem_resource, tmp_res);
> +		if (ret < 0) {
> +			pr_err("Cannot request resource %pR (%d)\n", tmp_res, ret);
> +			kfree(tmp_res);
> +			goto err_insert;
> +		}
> +	}
> +
> +	pgmap = kzalloc(sizeof(*pgmap), GFP_KERNEL);
> +	if (!pgmap) {
> +		ret = -ENOMEM;
> +		goto err_pgmap;
> +	}
> +
> +	pgmap->type = MEMORY_DEVICE_GENERIC;
> +	pgmap->range = (struct range) {
> +		.start = res->start,
> +		.end = res->end,
> +	};
> +	pgmap->nr_range = 1;
> +	pgmap->owner = res;
> +
> +	vaddr = memremap_pages(pgmap, NUMA_NO_NODE);
> +	if (IS_ERR(vaddr)) {
> +		pr_err("Cannot remap memory range\n");
> +		ret = PTR_ERR(vaddr);
> +		goto err_memremap;
> +	}
> +
> +	ret = gen_pool_add_virt(dma_pool, (unsigned long)vaddr, res->start,
> +			alloc_pages * PAGE_SIZE, NUMA_NO_NODE);
> +	if (ret)
> +		goto err_pool;
> +
> +	return 0;
> +
> +err_pool:
> +	memunmap_pages(pgmap);
> +err_memremap:
> +	kfree(pgmap);
> +err_pgmap:
> +	if (tmp_res) {
> +		release_resource(tmp_res);
> +		kfree(tmp_res);
> +	}
> +err_insert:
> +	release_resource(res);
> +err_resource:
> +	kfree(res);
> +	return ret;
> +}
> +
> +/**
> + * xen_alloc_unpopulated_dma_pages - alloc unpopulated DMAable pages
> + * @dev: valid struct device pointer
> + * @nr_pages: Number of pages
> + * @pages: pages returned
> + * @return 0 on success, error otherwise
> + */
> +int xen_alloc_unpopulated_dma_pages(struct device *dev, unsigned int nr_pages,
> +		struct page **pages)
> +{
> +	void *vaddr;
> +	bool filled = false;
> +	unsigned int i;
> +	int ret;


Also probably it might be better if xen_alloc_unpopulated_pages and
xen_alloc_unpopulated_dma_pages shared the implementation. Something
along these lines:

int xen_alloc_unpopulated_pages(unsigned int nr_pages, struct page **pages)
{
    return _xen_alloc_unpopulated_pages(nr_pages, pages, false);
}

int xen_alloc_unpopulated_dma_pages(struct device *dev, unsigned int nr_pages,
		struct page **pages)
{
    return _xen_alloc_unpopulated_pages(nr_pages, pages, true);
}

static int _xen_alloc_unpopulated_pages(unsigned int nr_pages,
        struct page **pages, bool contiguous)
{
	unsigned int i;
	int ret = 0;

    if (contiguous && !xen_feature(XENFEAT_auto_translated_physmap))
        return -EINVAL;

	/*
	 * Fallback to default behavior if we do not have any suitable resource
	 * to allocate required region from and as the result we won't be able to
	 * construct pages.
	 */
	if (!target_resource) {
        if (contiguous)
            return -EINVAL;
		return xen_alloc_ballooned_pages(nr_pages, pages);
    }

	mutex_lock(&list_lock);
	if (list_count < nr_pages) {
		ret = fill_list(nr_pages - list_count);
		if (ret)
			goto out;
	}

    if (contiguous) {
        vaddr = (void *)gen_pool_alloc(dma_pool, nr_pages * PAGE_SIZE);

        for (i = 0; i < nr_pages; i++)
            pages[i] = virt_to_page(vaddr + PAGE_SIZE * i);
    } else {
        for (i = 0; i < nr_pages; i++) {
            struct page *pg = page_list;

            BUG_ON(!pg);
            page_list = pg->zone_device_data;
            list_count--;
            pages[i] = pg;

    #ifdef CONFIG_XEN_HAVE_PVMMU
            if (!xen_feature(XENFEAT_auto_translated_physmap)) {
                ret = xen_alloc_p2m_entry(page_to_pfn(pg));
                if (ret < 0) {
                    unsigned int j;

                    for (j = 0; j <= i; j++) {
                        pages[j]->zone_device_data = page_list;
                        page_list = pages[j];
                        list_count++;
                    }
                    goto out;
                }
            }
    #endif
        }
    }

out:
	mutex_unlock(&list_lock);
	return ret;
}

	

> +	if (!dma_pool)
> +		return -ENODEV;
> +
> +	/* XXX Handle devices which support 64-bit DMA address only for now */
> +	if (dma_get_mask(dev) != DMA_BIT_MASK(64))
> +		return -EINVAL;
> +
> +	while (!(vaddr = (void *)gen_pool_alloc(dma_pool, nr_pages * PAGE_SIZE))) {
> +		if (filled)
> +			return -ENOMEM;
> +		else {
> +			ret = fill_dma_pool(nr_pages);
> +			if (ret)
> +				return ret;
> +
> +			filled = true;
> +		}
> +	}
> +
> +	for (i = 0; i < nr_pages; i++)
> +		pages[i] = virt_to_page(vaddr + PAGE_SIZE * i);
> +
> +	return 0;
> +}
> +EXPORT_SYMBOL(xen_alloc_unpopulated_dma_pages);
> +
> +/**
> + * xen_free_unpopulated_dma_pages - return unpopulated DMAable pages
> + * @dev: valid struct device pointer
> + * @nr_pages: Number of pages
> + * @pages: pages to return
> + */
> +void xen_free_unpopulated_dma_pages(struct device *dev, unsigned int nr_pages,
> +		struct page **pages)
> +{
> +	void *vaddr;
> +
> +	if (!dma_pool)
> +		return;
> +
> +	vaddr = page_to_virt(pages[0]);
> +
> +	gen_pool_free(dma_pool, (unsigned long)vaddr, nr_pages * PAGE_SIZE);
> +}
> +EXPORT_SYMBOL(xen_free_unpopulated_dma_pages);
> +
>  static int __init unpopulated_init(void)
>  {
>  	int ret;
> @@ -241,8 +399,17 @@ static int __init unpopulated_init(void)
>  	if (ret) {
>  		pr_err("xen:unpopulated: Cannot initialize target resource\n");
>  		target_resource = NULL;
> +		return ret;
>  	}
>  
> +	dma_pool = gen_pool_create(PAGE_SHIFT, NUMA_NO_NODE);
> +	if (!dma_pool) {
> +		pr_err("xen:unpopulated: Cannot create DMA pool\n");
> +		return -ENOMEM;
> +	}
> +
> +	gen_pool_set_algo(dma_pool, gen_pool_best_fit, NULL);
> +
>  	return ret;
>  }
>  early_initcall(unpopulated_init);
> diff --git a/include/xen/xen.h b/include/xen/xen.h
> index a99bab8..a6a7a59 100644
> --- a/include/xen/xen.h
> +++ b/include/xen/xen.h
> @@ -52,9 +52,15 @@ bool xen_biovec_phys_mergeable(const struct bio_vec *vec1,
>  extern u64 xen_saved_max_mem_size;
>  #endif
>  
> +struct device;
> +
>  #ifdef CONFIG_XEN_UNPOPULATED_ALLOC
>  int xen_alloc_unpopulated_pages(unsigned int nr_pages, struct page **pages);
>  void xen_free_unpopulated_pages(unsigned int nr_pages, struct page **pages);
> +int xen_alloc_unpopulated_dma_pages(struct device *dev, unsigned int nr_pages,
> +		struct page **pages);
> +void xen_free_unpopulated_dma_pages(struct device *dev, unsigned int nr_pages,
> +		struct page **pages);
>  #include <linux/ioport.h>
>  int arch_xen_unpopulated_init(struct resource **res);
>  #else
> @@ -69,6 +75,15 @@ static inline void xen_free_unpopulated_pages(unsigned int nr_pages,
>  {
>  	xen_free_ballooned_pages(nr_pages, pages);
>  }
> +static inline int xen_alloc_unpopulated_dma_pages(struct device *dev,
> +		unsigned int nr_pages, struct page **pages)
> +{
> +	return -1;
> +}
> +static inline void xen_free_unpopulated_dma_pages(struct device *dev,
> +		unsigned int nr_pages, struct page **pages)
> +{
> +}
>  #endif

Given that we have these stubs, maybe we don't need to #ifdef the so
much code in the next patch

Re: [RFC PATCH 1/2] xen/unpopulated-alloc: Introduce helpers for DMA allocations

[Oleksandr]

On 04.06.22 00:52, Stefano Stabellini wrote:


Hello Stefano

Thank you for having a look and sorry for the late response.

> On Tue, 17 May 2022, Oleksandr Tyshchenko wrote:
>> From: Oleksandr Tyshchenko <oleksandr_tyshchenko@epam.com>
>>
>> Add ability to allocate unpopulated DMAable (contiguous) pages
>> suitable for grant mapping into. This is going to be used by gnttab
>> code (see gnttab_dma_alloc_pages()).
>>
>> TODO: There is a code duplication in fill_dma_pool(). Also pool
>> oparations likely need to be protected by the lock.
>>
>> Signed-off-by: Oleksandr Tyshchenko <oleksandr_tyshchenko@epam.com>
>> ---
>>   drivers/xen/unpopulated-alloc.c | 167 ++++++++++++++++++++++++++++++++++++++++
>>   include/xen/xen.h               |  15 ++++
>>   2 files changed, 182 insertions(+)
>>
>> diff --git a/drivers/xen/unpopulated-alloc.c b/drivers/xen/unpopulated-alloc.c
>> index a39f2d3..bca0198 100644
>> --- a/drivers/xen/unpopulated-alloc.c
>> +++ b/drivers/xen/unpopulated-alloc.c
>> @@ -1,5 +1,6 @@
>>   // SPDX-License-Identifier: GPL-2.0
>>   #include <linux/errno.h>
>> +#include <linux/genalloc.h>
>>   #include <linux/gfp.h>
>>   #include <linux/kernel.h>
>>   #include <linux/mm.h>
>> @@ -16,6 +17,8 @@ static DEFINE_MUTEX(list_lock);
>>   static struct page *page_list;
>>   static unsigned int list_count;
>>   
>> +static struct gen_pool *dma_pool;
>> +
>>   static struct resource *target_resource;
>>   
>>   /*
>> @@ -230,6 +233,161 @@ void xen_free_unpopulated_pages(unsigned int nr_pages, struct page **pages)
>>   }
>>   EXPORT_SYMBOL(xen_free_unpopulated_pages);
>>   
>> +static int fill_dma_pool(unsigned int nr_pages)
>> +{
> I think we shouldn't need to add this function at all as we should be
> able to reuse fill_list even for contiguous pages. fill_list could
> always call gen_pool_add_virt before returning.


First of all, I agree that fill_dma_pool() has a lot in common with 
fill_list(), so we indeed can avoid code duplication (this was mentioned 
in TODO).
I am not quite sure regarding "to always call gen_pool_add_virt before 
returning" (does this mean that the same pages will be in the 
"page_list" and "dma_pool" simultaneously?),
but I completely agree that we can reuse fill_list() for contiguous 
pages as well slightly updating it.

Please see below.


>
>
>> +	struct dev_pagemap *pgmap;
>> +	struct resource *res, *tmp_res = NULL;
>> +	void *vaddr;
>> +	unsigned int alloc_pages = round_up(nr_pages, PAGES_PER_SECTION);
>> +	struct range mhp_range;
>> +	int ret;
>> +
>> +	res = kzalloc(sizeof(*res), GFP_KERNEL);
>> +	if (!res)
>> +		return -ENOMEM;
>> +
>> +	res->name = "Xen DMA pool";
>> +	res->flags = IORESOURCE_MEM | IORESOURCE_BUSY;
>> +
>> +	mhp_range = mhp_get_pluggable_range(true);
>> +
>> +	ret = allocate_resource(target_resource, res,
>> +				alloc_pages * PAGE_SIZE, mhp_range.start, mhp_range.end,
>> +				PAGES_PER_SECTION * PAGE_SIZE, NULL, NULL);
>> +	if (ret < 0) {
>> +		pr_err("Cannot allocate new IOMEM resource\n");
>> +		goto err_resource;
>> +	}
>> +
>> +	/*
>> +	 * Reserve the region previously allocated from Xen resource to avoid
>> +	 * re-using it by someone else.
>> +	 */
>> +	if (target_resource != &iomem_resource) {
>> +		tmp_res = kzalloc(sizeof(*tmp_res), GFP_KERNEL);
>> +		if (!res) {
>> +			ret = -ENOMEM;
>> +			goto err_insert;
>> +		}
>> +
>> +		tmp_res->name = res->name;
>> +		tmp_res->start = res->start;
>> +		tmp_res->end = res->end;
>> +		tmp_res->flags = res->flags;
>> +
>> +		ret = request_resource(&iomem_resource, tmp_res);
>> +		if (ret < 0) {
>> +			pr_err("Cannot request resource %pR (%d)\n", tmp_res, ret);
>> +			kfree(tmp_res);
>> +			goto err_insert;
>> +		}
>> +	}
>> +
>> +	pgmap = kzalloc(sizeof(*pgmap), GFP_KERNEL);
>> +	if (!pgmap) {
>> +		ret = -ENOMEM;
>> +		goto err_pgmap;
>> +	}
>> +
>> +	pgmap->type = MEMORY_DEVICE_GENERIC;
>> +	pgmap->range = (struct range) {
>> +		.start = res->start,
>> +		.end = res->end,
>> +	};
>> +	pgmap->nr_range = 1;
>> +	pgmap->owner = res;
>> +
>> +	vaddr = memremap_pages(pgmap, NUMA_NO_NODE);
>> +	if (IS_ERR(vaddr)) {
>> +		pr_err("Cannot remap memory range\n");
>> +		ret = PTR_ERR(vaddr);
>> +		goto err_memremap;
>> +	}
>> +
>> +	ret = gen_pool_add_virt(dma_pool, (unsigned long)vaddr, res->start,
>> +			alloc_pages * PAGE_SIZE, NUMA_NO_NODE);
>> +	if (ret)
>> +		goto err_pool;
>> +
>> +	return 0;
>> +
>> +err_pool:
>> +	memunmap_pages(pgmap);
>> +err_memremap:
>> +	kfree(pgmap);
>> +err_pgmap:
>> +	if (tmp_res) {
>> +		release_resource(tmp_res);
>> +		kfree(tmp_res);
>> +	}
>> +err_insert:
>> +	release_resource(res);
>> +err_resource:
>> +	kfree(res);
>> +	return ret;
>> +}
>> +
>> +/**
>> + * xen_alloc_unpopulated_dma_pages - alloc unpopulated DMAable pages
>> + * @dev: valid struct device pointer
>> + * @nr_pages: Number of pages
>> + * @pages: pages returned
>> + * @return 0 on success, error otherwise
>> + */
>> +int xen_alloc_unpopulated_dma_pages(struct device *dev, unsigned int nr_pages,
>> +		struct page **pages)
>> +{
>> +	void *vaddr;
>> +	bool filled = false;
>> +	unsigned int i;
>> +	int ret;
>
> Also probably it might be better if xen_alloc_unpopulated_pages and
> xen_alloc_unpopulated_dma_pages shared the implementation. Something
> along these lines:
>
> int xen_alloc_unpopulated_pages(unsigned int nr_pages, struct page **pages)
> {
>      return _xen_alloc_unpopulated_pages(nr_pages, pages, false);
> }
>
> int xen_alloc_unpopulated_dma_pages(struct device *dev, unsigned int nr_pages,
> 		struct page **pages)
> {
>      return _xen_alloc_unpopulated_pages(nr_pages, pages, true);
> }

I think, this makes sense, although it depends on how the resulting 
_xen_alloc_unpopulated_pages() will look like. I'd 
s/_xen_alloc_unpopulated_pages/alloc_unpopulated_pages

Please see below.


>
> static int _xen_alloc_unpopulated_pages(unsigned int nr_pages,
>          struct page **pages, bool contiguous)
> {
> 	unsigned int i;
> 	int ret = 0;
>
>      if (contiguous && !xen_feature(XENFEAT_auto_translated_physmap))
>          return -EINVAL;
>
> 	/*
> 	 * Fallback to default behavior if we do not have any suitable resource
> 	 * to allocate required region from and as the result we won't be able to
> 	 * construct pages.
> 	 */
> 	if (!target_resource) {
>          if (contiguous)
>              return -EINVAL;
> 		return xen_alloc_ballooned_pages(nr_pages, pages);
>      }
>
> 	mutex_lock(&list_lock);
> 	if (list_count < nr_pages) {
> 		ret = fill_list(nr_pages - list_count);

As I understand, this might not work if we need contiguous pages. The 
check might not be precise as "list_count" is a number of available 
pages in the list, which are not guaranteed to be contiguous, also the 
amount of pages to be added to the pool here "nr_pages - list_count" 
might not be enough to allocate contiguous region with "nr_pages" size.


> 		if (ret)
> 			goto out;
> 	}
>
>      if (contiguous) {
>          vaddr = (void *)gen_pool_alloc(dma_pool, nr_pages * PAGE_SIZE);
>
>          for (i = 0; i < nr_pages; i++)
>              pages[i] = virt_to_page(vaddr + PAGE_SIZE * i);
>      } else {
>          for (i = 0; i < nr_pages; i++) {
>              struct page *pg = page_list;
>
>              BUG_ON(!pg);
>              page_list = pg->zone_device_data;
>              list_count--;
>              pages[i] = pg;

I think, if we keep the same pages in the "page_list" and "dma_pool" 
simultaneously we will need to reflect that here, otherwise we might end 
up reusing already allocated pages.
What I mean is if we allocate pages from "dma_pool" we will need to 
remove them from the "page_list" as well and vice versa, so this might 
add a complexity to the code. I or missed something?



According to the suggestions I see two options, both follow suggestion 
where xen_alloc(free)_unpopulated_pages() and 
xen_alloc(free)_unpopulated_dma_pages() share the implementation.

1. Keep "page_list" and "dma_pool" separately. So they don't share 
pages. Like how it was implemented in current patch but with eliminating 
both TODOs. This doesn't change behavior for all users of 
xen_alloc_unpopulated_pages().

Below the diff for unpopulated-alloc.c. The patch is also available at:

https://github.com/otyshchenko1/linux/commit/1c629abc37478c108a5f4c37ae8076b766c4d5cc


diff --git a/drivers/xen/unpopulated-alloc.c 
b/drivers/xen/unpopulated-alloc.c
index a39f2d3..c3a86c09 100644
--- a/drivers/xen/unpopulated-alloc.c
+++ b/drivers/xen/unpopulated-alloc.c
@@ -1,5 +1,7 @@
  // SPDX-License-Identifier: GPL-2.0
+#include <linux/dma-mapping.h>
  #include <linux/errno.h>
+#include <linux/genalloc.h>
  #include <linux/gfp.h>
  #include <linux/kernel.h>
  #include <linux/mm.h>
@@ -16,6 +18,8 @@ static DEFINE_MUTEX(list_lock);
  static struct page *page_list;
  static unsigned int list_count;

+static struct gen_pool *dma_pool;
+
  static struct resource *target_resource;

  /*
@@ -31,7 +35,7 @@ int __weak __init arch_xen_unpopulated_init(struct 
resource **res)
         return 0;
  }

-static int fill_list(unsigned int nr_pages)
+static int fill_list(unsigned int nr_pages, bool use_pool)
  {
         struct dev_pagemap *pgmap;
         struct resource *res, *tmp_res = NULL;
@@ -125,12 +129,21 @@ static int fill_list(unsigned int nr_pages)
                 goto err_memremap;
         }

-       for (i = 0; i < alloc_pages; i++) {
-               struct page *pg = virt_to_page(vaddr + PAGE_SIZE * i);
+       if (use_pool) {
+               ret = gen_pool_add_virt(dma_pool, (unsigned long)vaddr, 
res->start,
+                               alloc_pages * PAGE_SIZE, NUMA_NO_NODE);
+               if (ret) {
+                       memunmap_pages(pgmap);
+                       goto err_memremap;
+               }
+       } else {
+               for (i = 0; i < alloc_pages; i++) {
+                       struct page *pg = virt_to_page(vaddr + PAGE_SIZE 
* i);

-               pg->zone_device_data = page_list;
-               page_list = pg;
-               list_count++;
+                       pg->zone_device_data = page_list;
+                       page_list = pg;
+                       list_count++;
+               }
         }

         return 0;
@@ -149,13 +162,8 @@ static int fill_list(unsigned int nr_pages)
         return ret;
  }

-/**
- * xen_alloc_unpopulated_pages - alloc unpopulated pages
- * @nr_pages: Number of pages
- * @pages: pages returned
- * @return 0 on success, error otherwise
- */
-int xen_alloc_unpopulated_pages(unsigned int nr_pages, struct page **pages)
+static int alloc_unpopulated_pages(unsigned int nr_pages, struct page 
**pages,
+               bool contiguous)
  {
         unsigned int i;
         int ret = 0;
@@ -165,71 +173,167 @@ int xen_alloc_unpopulated_pages(unsigned int 
nr_pages, struct page **pages)
          * to allocate required region from and as the result we won't 
be able to
          * construct pages.
          */
-       if (!target_resource)
+       if (!target_resource) {
+               if (contiguous)
+                       return -ENODEV;
+
                 return xen_alloc_ballooned_pages(nr_pages, pages);
+       }

         mutex_lock(&list_lock);
-       if (list_count < nr_pages) {
-               ret = fill_list(nr_pages - list_count);
-               if (ret)
-                       goto out;
-       }

-       for (i = 0; i < nr_pages; i++) {
-               struct page *pg = page_list;
+       if (contiguous) {
+               void *vaddr;
+               bool filled = false;
+
+               while (!(vaddr = (void *)gen_pool_alloc(dma_pool, 
nr_pages * PAGE_SIZE))) {
+                       if (filled)
+                               ret = -ENOMEM;
+                       else {
+                               ret = fill_list(nr_pages, true);
+                               filled = true;
+                       }
+                       if (ret)
+                               goto out;
+               }

-               BUG_ON(!pg);
-               page_list = pg->zone_device_data;
-               list_count--;
-               pages[i] = pg;
+               for (i = 0; i < nr_pages; i++) {
+                       pages[i] = virt_to_page(vaddr + PAGE_SIZE * i);

  #ifdef CONFIG_XEN_HAVE_PVMMU
-               if (!xen_feature(XENFEAT_auto_translated_physmap)) {
-                       ret = xen_alloc_p2m_entry(page_to_pfn(pg));
-                       if (ret < 0) {
-                               unsigned int j;
-
-                               for (j = 0; j <= i; j++) {
- pages[j]->zone_device_data = page_list;
-                                       page_list = pages[j];
-                                       list_count++;
+                       if (!xen_feature(XENFEAT_auto_translated_physmap)) {
+                               ret = 
xen_alloc_p2m_entry(page_to_pfn(pages[i]));
+                               if (ret < 0) {
+                                       /* XXX Do we need to zeroed 
pages[i]? */
+                                       gen_pool_free(dma_pool, 
(unsigned long)vaddr,
+                                                       nr_pages * 
PAGE_SIZE);
+                                       goto out;
                                 }
-                               goto out;
                         }
+#endif
+               }
+       } else {
+               if (list_count < nr_pages) {
+                       ret = fill_list(nr_pages - list_count, false);
+                       if (ret)
+                               goto out;
                 }
+
+               for (i = 0; i < nr_pages; i++) {
+                       struct page *pg = page_list;
+
+                       BUG_ON(!pg);
+                       page_list = pg->zone_device_data;
+                       list_count--;
+                       pages[i] = pg;
+
+#ifdef CONFIG_XEN_HAVE_PVMMU
+                       if (!xen_feature(XENFEAT_auto_translated_physmap)) {
+                               ret = xen_alloc_p2m_entry(page_to_pfn(pg));
+                               if (ret < 0) {
+                                       unsigned int j;
+
+                                       for (j = 0; j <= i; j++) {
+ pages[j]->zone_device_data = page_list;
+                                               page_list = pages[j];
+                                               list_count++;
+                                       }
+                                       goto out;
+                               }
+                       }
  #endif
+               }
         }

  out:
         mutex_unlock(&list_lock);
         return ret;
  }
-EXPORT_SYMBOL(xen_alloc_unpopulated_pages);

-/**
- * xen_free_unpopulated_pages - return unpopulated pages
- * @nr_pages: Number of pages
- * @pages: pages to return
- */
-void xen_free_unpopulated_pages(unsigned int nr_pages, struct page **pages)
+static void free_unpopulated_pages(unsigned int nr_pages, struct page 
**pages,
+               bool contiguous)
  {
-       unsigned int i;
-
         if (!target_resource) {
+               if (contiguous)
+                       return;
+
                 xen_free_ballooned_pages(nr_pages, pages);
                 return;
         }

         mutex_lock(&list_lock);
-       for (i = 0; i < nr_pages; i++) {
-               pages[i]->zone_device_data = page_list;
-               page_list = pages[i];
-               list_count++;
+
+       /* XXX Do we need to check the range (gen_pool_has_addr)? */
+       if (contiguous)
+               gen_pool_free(dma_pool, (unsigned 
long)page_to_virt(pages[0]),
+                               nr_pages * PAGE_SIZE);
+       else {
+               unsigned int i;
+
+               for (i = 0; i < nr_pages; i++) {
+                       pages[i]->zone_device_data = page_list;
+                       page_list = pages[i];
+                       list_count++;
+               }
         }
+
         mutex_unlock(&list_lock);
  }
+
+/**
+ * xen_alloc_unpopulated_pages - alloc unpopulated pages
+ * @nr_pages: Number of pages
+ * @pages: pages returned
+ * @return 0 on success, error otherwise
+ */
+int xen_alloc_unpopulated_pages(unsigned int nr_pages, struct page **pages)
+{
+       return alloc_unpopulated_pages(nr_pages, pages, false);
+}
+EXPORT_SYMBOL(xen_alloc_unpopulated_pages);
+
+/**
+ * xen_free_unpopulated_pages - return unpopulated pages
+ * @nr_pages: Number of pages
+ * @pages: pages to return
+ */
+void xen_free_unpopulated_pages(unsigned int nr_pages, struct page **pages)
+{
+       free_unpopulated_pages(nr_pages, pages, false);
+}
  EXPORT_SYMBOL(xen_free_unpopulated_pages);

+/**
+ * xen_alloc_unpopulated_dma_pages - alloc unpopulated DMAable pages
+ * @dev: valid struct device pointer
+ * @nr_pages: Number of pages
+ * @pages: pages returned
+ * @return 0 on success, error otherwise
+ */
+int xen_alloc_unpopulated_dma_pages(struct device *dev, unsigned int 
nr_pages,
+               struct page **pages)
+{
+       /* XXX Handle devices which support 64-bit DMA address only for 
now */
+       if (dma_get_mask(dev) != DMA_BIT_MASK(64))
+               return -EINVAL;
+
+       return alloc_unpopulated_pages(nr_pages, pages, true);
+}
+EXPORT_SYMBOL(xen_alloc_unpopulated_dma_pages);
+
+/**
+ * xen_free_unpopulated_dma_pages - return unpopulated DMAable pages
+ * @dev: valid struct device pointer
+ * @nr_pages: Number of pages
+ * @pages: pages to return
+ */
+void xen_free_unpopulated_dma_pages(struct device *dev, unsigned int 
nr_pages,
+               struct page **pages)
+{
+       free_unpopulated_pages(nr_pages, pages, true);
+}
+EXPORT_SYMBOL(xen_free_unpopulated_dma_pages);
+
  static int __init unpopulated_init(void)
  {
         int ret;
@@ -237,9 +341,19 @@ static int __init unpopulated_init(void)
         if (!xen_domain())
                 return -ENODEV;

+       dma_pool = gen_pool_create(PAGE_SHIFT, NUMA_NO_NODE);
+       if (!dma_pool) {
+               pr_err("xen:unpopulated: Cannot create DMA pool\n");
+               return -ENOMEM;
+       }
+
+       gen_pool_set_algo(dma_pool, gen_pool_best_fit, NULL);
+
         ret = arch_xen_unpopulated_init(&target_resource);
         if (ret) {
                 pr_err("xen:unpopulated: Cannot initialize target 
resource\n");
+               gen_pool_destroy(dma_pool);
+               dma_pool = NULL;
                 target_resource = NULL;
         }

[snip]


2. Drop the "page_list" entirely and use "dma_pool" for all (contiguous 
and non-contiguous) allocations. After all, all pages are initially 
contiguous in fill_list() as they are built from the resource. This 
changes behavior for all users of xen_alloc_unpopulated_pages()

Below the diff for unpopulated-alloc.c. The patch is also available at:

https://github.com/otyshchenko1/linux/commit/7be569f113a4acbdc4bcb9b20cb3995b3151387a


diff --git a/drivers/xen/unpopulated-alloc.c 
b/drivers/xen/unpopulated-alloc.c
index a39f2d3..ab5c7bd 100644
--- a/drivers/xen/unpopulated-alloc.c
+++ b/drivers/xen/unpopulated-alloc.c
@@ -1,5 +1,7 @@
  // SPDX-License-Identifier: GPL-2.0
+#include <linux/dma-mapping.h>
  #include <linux/errno.h>
+#include <linux/genalloc.h>
  #include <linux/gfp.h>
  #include <linux/kernel.h>
  #include <linux/mm.h>
@@ -13,8 +15,8 @@
  #include <xen/xen.h>

  static DEFINE_MUTEX(list_lock);
-static struct page *page_list;
-static unsigned int list_count;
+
+static struct gen_pool *dma_pool;

  static struct resource *target_resource;

@@ -36,7 +38,7 @@ static int fill_list(unsigned int nr_pages)
         struct dev_pagemap *pgmap;
         struct resource *res, *tmp_res = NULL;
         void *vaddr;
-       unsigned int i, alloc_pages = round_up(nr_pages, PAGES_PER_SECTION);
+       unsigned int alloc_pages = round_up(nr_pages, PAGES_PER_SECTION);
         struct range mhp_range;
         int ret;

@@ -106,6 +108,7 @@ static int fill_list(unsigned int nr_pages)
           * conflict with any devices.
           */
         if (!xen_feature(XENFEAT_auto_translated_physmap)) {
+               unsigned int i;
                 xen_pfn_t pfn = PFN_DOWN(res->start);

                 for (i = 0; i < alloc_pages; i++) {
@@ -125,16 +128,17 @@ static int fill_list(unsigned int nr_pages)
                 goto err_memremap;
         }

-       for (i = 0; i < alloc_pages; i++) {
-               struct page *pg = virt_to_page(vaddr + PAGE_SIZE * i);
-
-               pg->zone_device_data = page_list;
-               page_list = pg;
-               list_count++;
+       ret = gen_pool_add_virt(dma_pool, (unsigned long)vaddr, res->start,
+                       alloc_pages * PAGE_SIZE, NUMA_NO_NODE);
+       if (ret) {
+               pr_err("Cannot add memory range to the pool\n");
+               goto err_pool;
         }

         return 0;

+err_pool:
+       memunmap_pages(pgmap);
  err_memremap:
         kfree(pgmap);
  err_pgmap:
@@ -149,51 +153,49 @@ static int fill_list(unsigned int nr_pages)
         return ret;
  }

-/**
- * xen_alloc_unpopulated_pages - alloc unpopulated pages
- * @nr_pages: Number of pages
- * @pages: pages returned
- * @return 0 on success, error otherwise
- */
-int xen_alloc_unpopulated_pages(unsigned int nr_pages, struct page **pages)
+static int alloc_unpopulated_pages(unsigned int nr_pages, struct page 
**pages,
+               bool contiguous)
  {
         unsigned int i;
         int ret = 0;
+       void *vaddr;
+       bool filled = false;

         /*
          * Fallback to default behavior if we do not have any suitable 
resource
          * to allocate required region from and as the result we won't 
be able to
          * construct pages.
          */
-       if (!target_resource)
+       if (!target_resource) {
+               if (contiguous)
+                       return -ENODEV;
+
                 return xen_alloc_ballooned_pages(nr_pages, pages);
+       }

         mutex_lock(&list_lock);
-       if (list_count < nr_pages) {
-               ret = fill_list(nr_pages - list_count);
+
+       while (!(vaddr = (void *)gen_pool_alloc(dma_pool, nr_pages * 
PAGE_SIZE))) {
+               if (filled)
+                       ret = -ENOMEM;
+               else {
+                       ret = fill_list(nr_pages);
+                       filled = true;
+               }
                 if (ret)
                         goto out;
         }

         for (i = 0; i < nr_pages; i++) {
-               struct page *pg = page_list;
-
-               BUG_ON(!pg);
-               page_list = pg->zone_device_data;
-               list_count--;
-               pages[i] = pg;
+               pages[i] = virt_to_page(vaddr + PAGE_SIZE * i);

  #ifdef CONFIG_XEN_HAVE_PVMMU
                 if (!xen_feature(XENFEAT_auto_translated_physmap)) {
-                       ret = xen_alloc_p2m_entry(page_to_pfn(pg));
+                       ret = xen_alloc_p2m_entry(page_to_pfn(pages[i]));
                         if (ret < 0) {
-                               unsigned int j;
-
-                               for (j = 0; j <= i; j++) {
- pages[j]->zone_device_data = page_list;
-                                       page_list = pages[j];
-                                       list_count++;
-                               }
+                               /* XXX Do we need to zeroed pages[i]? */
+                               gen_pool_free(dma_pool, (unsigned 
long)vaddr,
+                                               nr_pages * PAGE_SIZE);
                                 goto out;
                         }
                 }
@@ -204,32 +206,89 @@ int xen_alloc_unpopulated_pages(unsigned int 
nr_pages, struct page **pages)
         mutex_unlock(&list_lock);
         return ret;
  }
-EXPORT_SYMBOL(xen_alloc_unpopulated_pages);

-/**
- * xen_free_unpopulated_pages - return unpopulated pages
- * @nr_pages: Number of pages
- * @pages: pages to return
- */
-void xen_free_unpopulated_pages(unsigned int nr_pages, struct page **pages)
+static void free_unpopulated_pages(unsigned int nr_pages, struct page 
**pages,
+               bool contiguous)
  {
-       unsigned int i;
-
         if (!target_resource) {
+               if (contiguous)
+                       return;
+
                 xen_free_ballooned_pages(nr_pages, pages);
                 return;
         }

         mutex_lock(&list_lock);
-       for (i = 0; i < nr_pages; i++) {
-               pages[i]->zone_device_data = page_list;
-               page_list = pages[i];
-               list_count++;
+
+       /* XXX Do we need to check the range (gen_pool_has_addr)? */
+       if (contiguous)
+               gen_pool_free(dma_pool, (unsigned 
long)page_to_virt(pages[0]),
+                               nr_pages * PAGE_SIZE);
+       else {
+               unsigned int i;
+
+               for (i = 0; i < nr_pages; i++)
+                       gen_pool_free(dma_pool, (unsigned 
long)page_to_virt(pages[i]),
+                                       PAGE_SIZE);
         }
+
         mutex_unlock(&list_lock);
  }
+
+/**
+ * xen_alloc_unpopulated_pages - alloc unpopulated pages
+ * @nr_pages: Number of pages
+ * @pages: pages returned
+ * @return 0 on success, error otherwise
+ */
+int xen_alloc_unpopulated_pages(unsigned int nr_pages, struct page **pages)
+{
+       return alloc_unpopulated_pages(nr_pages, pages, false);
+}
+EXPORT_SYMBOL(xen_alloc_unpopulated_pages);
+
+/**
+ * xen_free_unpopulated_pages - return unpopulated pages
+ * @nr_pages: Number of pages
+ * @pages: pages to return
+ */
+void xen_free_unpopulated_pages(unsigned int nr_pages, struct page **pages)
+{
+       free_unpopulated_pages(nr_pages, pages, false);
+}
  EXPORT_SYMBOL(xen_free_unpopulated_pages);

+/**
+ * xen_alloc_unpopulated_dma_pages - alloc unpopulated DMAable pages
+ * @dev: valid struct device pointer
+ * @nr_pages: Number of pages
+ * @pages: pages returned
+ * @return 0 on success, error otherwise
+ */
+int xen_alloc_unpopulated_dma_pages(struct device *dev, unsigned int 
nr_pages,
+               struct page **pages)
+{
+       /* XXX Handle devices which support 64-bit DMA address only for 
now */
+       if (dma_get_mask(dev) != DMA_BIT_MASK(64))
+               return -EINVAL;
+
+       return alloc_unpopulated_pages(nr_pages, pages, true);
+}
+EXPORT_SYMBOL(xen_alloc_unpopulated_dma_pages);
+
+/**
+ * xen_free_unpopulated_dma_pages - return unpopulated DMAable pages
+ * @dev: valid struct device pointer
+ * @nr_pages: Number of pages
+ * @pages: pages to return
+ */
+void xen_free_unpopulated_dma_pages(struct device *dev, unsigned int 
nr_pages,
+               struct page **pages)
+{
+       free_unpopulated_pages(nr_pages, pages, true);
+}
+EXPORT_SYMBOL(xen_free_unpopulated_dma_pages);
+
  static int __init unpopulated_init(void)
  {
         int ret;
@@ -237,9 +296,19 @@ static int __init unpopulated_init(void)
         if (!xen_domain())
                 return -ENODEV;

+       dma_pool = gen_pool_create(PAGE_SHIFT, NUMA_NO_NODE);
+       if (!dma_pool) {
+               pr_err("xen:unpopulated: Cannot create DMA pool\n");
+               return -ENOMEM;
+       }
+
+       gen_pool_set_algo(dma_pool, gen_pool_best_fit, NULL);
+
         ret = arch_xen_unpopulated_init(&target_resource);
         if (ret) {
                 pr_err("xen:unpopulated: Cannot initialize target 
resource\n");
+               gen_pool_destroy(dma_pool);
+               dma_pool = NULL;
                 target_resource = NULL;
         }

[snip]


I think, regarding on the approach we would likely need to do some 
renaming for fill_list, page_list, list_lock, etc.


Both options work in my Arm64 based environment, not sure regarding x86.
Or do we have another option here?
I would be happy to go any route. What do you think?


>
>      #ifdef CONFIG_XEN_HAVE_PVMMU
>              if (!xen_feature(XENFEAT_auto_translated_physmap)) {
>                  ret = xen_alloc_p2m_entry(page_to_pfn(pg));
>                  if (ret < 0) {
>                      unsigned int j;
>
>                      for (j = 0; j <= i; j++) {
>                          pages[j]->zone_device_data = page_list;
>                          page_list = pages[j];
>                          list_count++;
>                      }
>                      goto out;
>                  }
>              }
>      #endif
>          }
>      }
>
> out:
> 	mutex_unlock(&list_lock);
> 	return ret;
> }
>
> 	
>
>> +	if (!dma_pool)
>> +		return -ENODEV;
>> +
>> +	/* XXX Handle devices which support 64-bit DMA address only for now */
>> +	if (dma_get_mask(dev) != DMA_BIT_MASK(64))
>> +		return -EINVAL;
>> +
>> +	while (!(vaddr = (void *)gen_pool_alloc(dma_pool, nr_pages * PAGE_SIZE))) {
>> +		if (filled)
>> +			return -ENOMEM;
>> +		else {
>> +			ret = fill_dma_pool(nr_pages);
>> +			if (ret)
>> +				return ret;
>> +
>> +			filled = true;
>> +		}
>> +	}
>> +
>> +	for (i = 0; i < nr_pages; i++)
>> +		pages[i] = virt_to_page(vaddr + PAGE_SIZE * i);
>> +
>> +	return 0;
>> +}
>> +EXPORT_SYMBOL(xen_alloc_unpopulated_dma_pages);
>> +
>> +/**
>> + * xen_free_unpopulated_dma_pages - return unpopulated DMAable pages
>> + * @dev: valid struct device pointer
>> + * @nr_pages: Number of pages
>> + * @pages: pages to return
>> + */
>> +void xen_free_unpopulated_dma_pages(struct device *dev, unsigned int nr_pages,
>> +		struct page **pages)
>> +{
>> +	void *vaddr;
>> +
>> +	if (!dma_pool)
>> +		return;
>> +
>> +	vaddr = page_to_virt(pages[0]);
>> +
>> +	gen_pool_free(dma_pool, (unsigned long)vaddr, nr_pages * PAGE_SIZE);
>> +}
>> +EXPORT_SYMBOL(xen_free_unpopulated_dma_pages);
>> +
>>   static int __init unpopulated_init(void)
>>   {
>>   	int ret;
>> @@ -241,8 +399,17 @@ static int __init unpopulated_init(void)
>>   	if (ret) {
>>   		pr_err("xen:unpopulated: Cannot initialize target resource\n");
>>   		target_resource = NULL;
>> +		return ret;
>>   	}
>>   
>> +	dma_pool = gen_pool_create(PAGE_SHIFT, NUMA_NO_NODE);
>> +	if (!dma_pool) {
>> +		pr_err("xen:unpopulated: Cannot create DMA pool\n");
>> +		return -ENOMEM;
>> +	}
>> +
>> +	gen_pool_set_algo(dma_pool, gen_pool_best_fit, NULL);
>> +
>>   	return ret;
>>   }
>>   early_initcall(unpopulated_init);
>> diff --git a/include/xen/xen.h b/include/xen/xen.h
>> index a99bab8..a6a7a59 100644
>> --- a/include/xen/xen.h
>> +++ b/include/xen/xen.h
>> @@ -52,9 +52,15 @@ bool xen_biovec_phys_mergeable(const struct bio_vec *vec1,
>>   extern u64 xen_saved_max_mem_size;
>>   #endif
>>   
>> +struct device;
>> +
>>   #ifdef CONFIG_XEN_UNPOPULATED_ALLOC
>>   int xen_alloc_unpopulated_pages(unsigned int nr_pages, struct page **pages);
>>   void xen_free_unpopulated_pages(unsigned int nr_pages, struct page **pages);
>> +int xen_alloc_unpopulated_dma_pages(struct device *dev, unsigned int nr_pages,
>> +		struct page **pages);
>> +void xen_free_unpopulated_dma_pages(struct device *dev, unsigned int nr_pages,
>> +		struct page **pages);
>>   #include <linux/ioport.h>
>>   int arch_xen_unpopulated_init(struct resource **res);
>>   #else
>> @@ -69,6 +75,15 @@ static inline void xen_free_unpopulated_pages(unsigned int nr_pages,
>>   {
>>   	xen_free_ballooned_pages(nr_pages, pages);
>>   }
>> +static inline int xen_alloc_unpopulated_dma_pages(struct device *dev,
>> +		unsigned int nr_pages, struct page **pages)
>> +{
>> +	return -1;
>> +}
>> +static inline void xen_free_unpopulated_dma_pages(struct device *dev,
>> +		unsigned int nr_pages, struct page **pages)
>> +{
>> +}
>>   #endif
> Given that we have these stubs, maybe we don't need to #ifdef the so
> much code in the next patch

ok, I will analyze


-- 
Regards,

Oleksandr Tyshchenko

Re: [RFC PATCH 2/2] xen/grant-table: Use unpopulated DMAable pages instead of real RAM ones

[Oleksandr]

On 04.06.22 00:19, Stefano Stabellini wrote:


Hello Stefano

Thank you for having a look and sorry for the late response.

> On Tue, 17 May 2022, Oleksandr Tyshchenko wrote:
>> From: Oleksandr Tyshchenko <oleksandr_tyshchenko@epam.com>
>>
>> Depends on CONFIG_XEN_UNPOPULATED_ALLOC. If enabled then unpopulated
>> DMAable (contiguous) pages will be allocated for grant mapping into
>> instead of ballooning out real RAM pages.
>>
>> TODO: Fallback to real RAM pages if xen_alloc_unpopulated_dma_pages()
>> fails.
>>
>> Signed-off-by: Oleksandr Tyshchenko <oleksandr_tyshchenko@epam.com>
>> ---
>>   drivers/xen/grant-table.c | 27 +++++++++++++++++++++++++++
>>   1 file changed, 27 insertions(+)
>>
>> diff --git a/drivers/xen/grant-table.c b/drivers/xen/grant-table.c
>> index 8ccccac..2bb4392 100644
>> --- a/drivers/xen/grant-table.c
>> +++ b/drivers/xen/grant-table.c
>> @@ -864,6 +864,25 @@ EXPORT_SYMBOL_GPL(gnttab_free_pages);
>>    */
>>   int gnttab_dma_alloc_pages(struct gnttab_dma_alloc_args *args)
>>   {
>> +#ifdef CONFIG_XEN_UNPOPULATED_ALLOC
>> +	int ret;
> This is an alternative implementation of the same function.

Currently, yes.


>   If we are
> going to use #ifdef, then I would #ifdef the entire function, rather
> than just the body. Otherwise within the function body we can use
> IS_ENABLED.


Good point. Note, there is one missing thing in current patch which is 
described in TODO.

"Fallback to real RAM pages if xen_alloc_unpopulated_dma_pages() 
fails."  So I will likely use IS_ENABLED within the function body.

If CONFIG_XEN_UNPOPULATED_ALLOC is enabled then gnttab_dma_alloc_pages() 
will try to call xen_alloc_unpopulated_dma_pages() the first and if 
fails then fallback to allocate RAM pages and balloon them out.

One moment is not entirely clear to me. If we use fallback in 
gnttab_dma_alloc_pages() then we must use fallback in 
gnttab_dma_free_pages() as well, we cannot use 
xen_free_unpopulated_dma_pages() for real RAM pages. The question is how 
to pass this information to the gnttab_dma_free_pages()? The first idea 
which comes to mind is to add a flag to struct gnttab_dma_alloc_args...


>
>> +	ret = xen_alloc_unpopulated_dma_pages(args->dev, args->nr_pages,
>> +			args->pages);
>> +	if (ret < 0)
>> +		return ret;
>> +
>> +	ret = gnttab_pages_set_private(args->nr_pages, args->pages);
>> +	if (ret < 0) {
>> +		gnttab_dma_free_pages(args);
> it should xen_free_unpopulated_dma_pages ?

Besides calling the xen_free_unpopulated_dma_pages(), we also need to 
call gnttab_pages_clear_private() here, this is what 
gnttab_dma_free_pages() is doing.

I can change to call both function instead:

     gnttab_pages_clear_private(args->nr_pages, args->pages);
     xen_free_unpopulated_dma_pages(args->dev, args->nr_pages, args->pages);

Shall I?


>
>
>> +		return ret;
>> +	}
>> +
>> +	args->vaddr = page_to_virt(args->pages[0]);
>> +	args->dev_bus_addr = page_to_phys(args->pages[0]);
> There are two things to note here.
>
> The first thing to note is that normally we would call pfn_to_bfn to
> retrieve the dev_bus_addr of a page because pfn_to_bfn takes into
> account foreign mappings. However, these are freshly allocated pages
> without foreign mappings, so page_to_phys/dma should be sufficient.

agree


>
>
> The second has to do with physical addresses and DMA addresses. The
> functions are called gnttab_dma_alloc_pages and
> xen_alloc_unpopulated_dma_pages which make you think we are retrieving a
> DMA address here. However, to get a DMA address we need to call
> page_to_dma rather than page_to_phys.
>
> page_to_dma takes into account special offsets that some devices have
> when accessing memory. There are real cases on ARM where the physical
> address != DMA address, e.g. RPi4.

I got it. Now I am in doubt whether it would be better to name the API:

xen_alloc_unpopulated_cma_pages()

or

xen_alloc_unpopulated_contiguous_pages()

What do you think?


>
> However, to call page_to_dma you need to specify as first argument the
> DMA-capable device that is expected to use those pages for DMA (e.g. an
> ethernet device or a MMC controller.) While the args->dev we have in
> gnttab_dma_alloc_pages is the gntdev_miscdev.

agree

As I understand, at this time it is unknown for what exactly device 
these pages are supposed to be used at the end.

For now, it is only known that these pages to be used by userspace PV 
backend for grant mappings.


>
> So this interface cannot actually be used to allocate memory that is
> supposed to be DMA-able by a DMA-capable device, such as an ethernet
> device.

agree


>
> But I think that should be fine because the memory is meant to be used
> by a userspace PV backend for grant mappings. If any of those mappings
> end up being used for actual DMA in the kernel they should go through the
> drivers/xen/swiotlb-xen.c and xen_phys_to_dma should be called, which
> ends up calling page_to_dma as appropriate.
>
> It would be good to double-check that the above is correct and, if so,
> maybe add a short in-code comment about it:
>
> /*
>   * These are not actually DMA addresses but regular physical addresses.
>   * If these pages end up being used in a DMA operation then the
>   * swiotlb-xen functions are called and xen_phys_to_dma takes care of
>   * the address translations:
>   *
>   * - from gfn to bfn in case of foreign mappings
>   * - from physical to DMA addresses in case the two are different for a
>   *   given DMA-mastering device
>   */

I agree this needs to be re-checked. But, there is one moment here, if 
userspace PV backend runs in other than Dom0 domain (non 1:1 mapped 
domain), the xen-swiotlb seems not to be in use then? How to be in this 
case?


>
>
>
>> +	return ret;
>> +#else
>>   	unsigned long pfn, start_pfn;
>>   	size_t size;
>>   	int i, ret;
>> @@ -910,6 +929,7 @@ int gnttab_dma_alloc_pages(struct gnttab_dma_alloc_args *args)
>>   fail:
>>   	gnttab_dma_free_pages(args);
>>   	return ret;
>> +#endif
>>   }
>>   EXPORT_SYMBOL_GPL(gnttab_dma_alloc_pages);
>>   
>> @@ -919,6 +939,12 @@ EXPORT_SYMBOL_GPL(gnttab_dma_alloc_pages);
>>    */
>>   int gnttab_dma_free_pages(struct gnttab_dma_alloc_args *args)
>>   {
>> +#ifdef CONFIG_XEN_UNPOPULATED_ALLOC
>> +	gnttab_pages_clear_private(args->nr_pages, args->pages);
>> +	xen_free_unpopulated_dma_pages(args->dev, args->nr_pages, args->pages);
>> +
>> +	return 0;
>> +#else
>>   	size_t size;
>>   	int i, ret;
>>   
>> @@ -946,6 +972,7 @@ int gnttab_dma_free_pages(struct gnttab_dma_alloc_args *args)
>>   		dma_free_wc(args->dev, size,
>>   			    args->vaddr, args->dev_bus_addr);
>>   	return ret;
>> +#endif
>>   }
>>   EXPORT_SYMBOL_GPL(gnttab_dma_free_pages);
>>   #endif
>> -- 
>> 2.7.4
>>
-- 
Regards,

Oleksandr Tyshchenko

Re: [RFC PATCH 2/2] xen/grant-table: Use unpopulated DMAable pages instead of real RAM ones

[Stefano Stabellini]

[-- Attachment #1: Type: text/plain, Size: 8324 bytes --]

On Thu, 9 Jun 2022, Oleksandr wrote:
> On 04.06.22 00:19, Stefano Stabellini wrote:
> Hello Stefano
> 
> Thank you for having a look and sorry for the late response.
> 
> > On Tue, 17 May 2022, Oleksandr Tyshchenko wrote:
> > > From: Oleksandr Tyshchenko <oleksandr_tyshchenko@epam.com>
> > > 
> > > Depends on CONFIG_XEN_UNPOPULATED_ALLOC. If enabled then unpopulated
> > > DMAable (contiguous) pages will be allocated for grant mapping into
> > > instead of ballooning out real RAM pages.
> > > 
> > > TODO: Fallback to real RAM pages if xen_alloc_unpopulated_dma_pages()
> > > fails.
> > > 
> > > Signed-off-by: Oleksandr Tyshchenko <oleksandr_tyshchenko@epam.com>
> > > ---
> > >   drivers/xen/grant-table.c | 27 +++++++++++++++++++++++++++
> > >   1 file changed, 27 insertions(+)
> > > 
> > > diff --git a/drivers/xen/grant-table.c b/drivers/xen/grant-table.c
> > > index 8ccccac..2bb4392 100644
> > > --- a/drivers/xen/grant-table.c
> > > +++ b/drivers/xen/grant-table.c
> > > @@ -864,6 +864,25 @@ EXPORT_SYMBOL_GPL(gnttab_free_pages);
> > >    */
> > >   int gnttab_dma_alloc_pages(struct gnttab_dma_alloc_args *args)
> > >   {
> > > +#ifdef CONFIG_XEN_UNPOPULATED_ALLOC
> > > +	int ret;
> > This is an alternative implementation of the same function.
> 
> Currently, yes.
> 
> 
> >   If we are
> > going to use #ifdef, then I would #ifdef the entire function, rather
> > than just the body. Otherwise within the function body we can use
> > IS_ENABLED.
> 
> 
> Good point. Note, there is one missing thing in current patch which is
> described in TODO.
> 
> "Fallback to real RAM pages if xen_alloc_unpopulated_dma_pages() fails."  So I
> will likely use IS_ENABLED within the function body.
> 
> If CONFIG_XEN_UNPOPULATED_ALLOC is enabled then gnttab_dma_alloc_pages() will
> try to call xen_alloc_unpopulated_dma_pages() the first and if fails then
> fallback to allocate RAM pages and balloon them out.
> 
> One moment is not entirely clear to me. If we use fallback in
> gnttab_dma_alloc_pages() then we must use fallback in gnttab_dma_free_pages()
> as well, we cannot use xen_free_unpopulated_dma_pages() for real RAM pages.
> The question is how to pass this information to the gnttab_dma_free_pages()?
> The first idea which comes to mind is to add a flag to struct
> gnttab_dma_alloc_args...
 
You can check if the page is within the mhp_range range or part of
iomem_resource? If not, you can free it as a normal page.

If we do this, then the fallback is better implemented in
unpopulated-alloc.c because that is the one that is aware about
page addresses.

 
 
> > > +	ret = xen_alloc_unpopulated_dma_pages(args->dev, args->nr_pages,
> > > +			args->pages);
> > > +	if (ret < 0)
> > > +		return ret;
> > > +
> > > +	ret = gnttab_pages_set_private(args->nr_pages, args->pages);
> > > +	if (ret < 0) {
> > > +		gnttab_dma_free_pages(args);
> > it should xen_free_unpopulated_dma_pages ?
> 
> Besides calling the xen_free_unpopulated_dma_pages(), we also need to call
> gnttab_pages_clear_private() here, this is what gnttab_dma_free_pages() is
> doing.
> 
> I can change to call both function instead:
> 
>     gnttab_pages_clear_private(args->nr_pages, args->pages);
>     xen_free_unpopulated_dma_pages(args->dev, args->nr_pages, args->pages);
> 
> Shall I?

No, leave it as is. I didn't realize that gnttab_pages_set_private can
fail half-way through.

 
> > 
> > 
> > > +		return ret;
> > > +	}
> > > +
> > > +	args->vaddr = page_to_virt(args->pages[0]);
> > > +	args->dev_bus_addr = page_to_phys(args->pages[0]);
> > There are two things to note here.
> > 
> > The first thing to note is that normally we would call pfn_to_bfn to
> > retrieve the dev_bus_addr of a page because pfn_to_bfn takes into
> > account foreign mappings. However, these are freshly allocated pages
> > without foreign mappings, so page_to_phys/dma should be sufficient.
> 
> agree
> 
> 
> > 
> > 
> > The second has to do with physical addresses and DMA addresses. The
> > functions are called gnttab_dma_alloc_pages and
> > xen_alloc_unpopulated_dma_pages which make you think we are retrieving a
> > DMA address here. However, to get a DMA address we need to call
> > page_to_dma rather than page_to_phys.
> > 
> > page_to_dma takes into account special offsets that some devices have
> > when accessing memory. There are real cases on ARM where the physical
> > address != DMA address, e.g. RPi4.
> 
> I got it. Now I am in doubt whether it would be better to name the API:
> 
> xen_alloc_unpopulated_cma_pages()
> 
> or
> 
> xen_alloc_unpopulated_contiguous_pages()
> 
> What do you think?

Yeah actually I think it is better to stay away from "dma" in the name.
I like xen_alloc_unpopulated_contiguous_pages().
 
 
> > However, to call page_to_dma you need to specify as first argument the
> > DMA-capable device that is expected to use those pages for DMA (e.g. an
> > ethernet device or a MMC controller.) While the args->dev we have in
> > gnttab_dma_alloc_pages is the gntdev_miscdev.
> 
> agree
> 
> As I understand, at this time it is unknown for what exactly device these
> pages are supposed to be used at the end.
> 
> For now, it is only known that these pages to be used by userspace PV backend
> for grant mappings.

Yeah
 

> > So this interface cannot actually be used to allocate memory that is
> > supposed to be DMA-able by a DMA-capable device, such as an ethernet
> > device.
> 
> agree
> 
> 
> > 
> > But I think that should be fine because the memory is meant to be used
> > by a userspace PV backend for grant mappings. If any of those mappings
> > end up being used for actual DMA in the kernel they should go through the
> > drivers/xen/swiotlb-xen.c and xen_phys_to_dma should be called, which
> > ends up calling page_to_dma as appropriate.
> > 
> > It would be good to double-check that the above is correct and, if so,
> > maybe add a short in-code comment about it:
> > 
> > /*
> >   * These are not actually DMA addresses but regular physical addresses.
> >   * If these pages end up being used in a DMA operation then the
> >   * swiotlb-xen functions are called and xen_phys_to_dma takes care of
> >   * the address translations:
> >   *
> >   * - from gfn to bfn in case of foreign mappings
> >   * - from physical to DMA addresses in case the two are different for a
> >   *   given DMA-mastering device
> >   */
> 
> I agree this needs to be re-checked. But, there is one moment here, if
> userspace PV backend runs in other than Dom0 domain (non 1:1 mapped domain),
> the xen-swiotlb seems not to be in use then? How to be in this case?
 
In that case, an IOMMU is required. If an IOMMU is setup correct, then
the gfn->bfn translation is not necessary because it is done
automatically by the IOMMU. That is because when the foreign page is
mapped in the domain, the mapping also applies to the IOMMU pagetable.

So the device is going to do DMA to "gfn" and the IOMMU will translate
it to the right "mfn", the one corresponding to "bfn".

The physical to DMA address should be done automatically by the default
(non-swiotlb_xen) dma_ops in Linux. E.g.
kernel/dma/direct.c:dma_direct_map_sg correctly calls
dma_direct_map_page, which calls phys_to_dma.
 
 
 
> > > +	return ret;
> > > +#else
> > >   	unsigned long pfn, start_pfn;
> > >   	size_t size;
> > >   	int i, ret;
> > > @@ -910,6 +929,7 @@ int gnttab_dma_alloc_pages(struct
> > > gnttab_dma_alloc_args *args)
> > >   fail:
> > >   	gnttab_dma_free_pages(args);
> > >   	return ret;
> > > +#endif
> > >   }
> > >   EXPORT_SYMBOL_GPL(gnttab_dma_alloc_pages);
> > >   @@ -919,6 +939,12 @@ EXPORT_SYMBOL_GPL(gnttab_dma_alloc_pages);
> > >    */
> > >   int gnttab_dma_free_pages(struct gnttab_dma_alloc_args *args)
> > >   {
> > > +#ifdef CONFIG_XEN_UNPOPULATED_ALLOC
> > > +	gnttab_pages_clear_private(args->nr_pages, args->pages);
> > > +	xen_free_unpopulated_dma_pages(args->dev, args->nr_pages,
> > > args->pages);
> > > +
> > > +	return 0;
> > > +#else
> > >   	size_t size;
> > >   	int i, ret;
> > >   @@ -946,6 +972,7 @@ int gnttab_dma_free_pages(struct
> > > gnttab_dma_alloc_args *args)
> > >   		dma_free_wc(args->dev, size,
> > >   			    args->vaddr, args->dev_bus_addr);
> > >   	return ret;
> > > +#endif
> > >   }
> > >   EXPORT_SYMBOL_GPL(gnttab_dma_free_pages);
> > >   #endif
> > > -- 
> > > 2.7.4
> > > 

Re: [RFC PATCH 1/2] xen/unpopulated-alloc: Introduce helpers for DMA allocations

[Stefano Stabellini]

[-- Attachment #1: Type: text/plain, Size: 11699 bytes --]

On Wed, 8 Jun 2022, Oleksandr wrote:
> 2. Drop the "page_list" entirely and use "dma_pool" for all (contiguous and
> non-contiguous) allocations. After all, all pages are initially contiguous in
> fill_list() as they are built from the resource. This changes behavior for all
> users of xen_alloc_unpopulated_pages()
> 
> Below the diff for unpopulated-alloc.c. The patch is also available at:
> 
> https://github.com/otyshchenko1/linux/commit/7be569f113a4acbdc4bcb9b20cb3995b3151387a
> 
> 
> diff --git a/drivers/xen/unpopulated-alloc.c b/drivers/xen/unpopulated-alloc.c
> index a39f2d3..ab5c7bd 100644
> --- a/drivers/xen/unpopulated-alloc.c
> +++ b/drivers/xen/unpopulated-alloc.c
> @@ -1,5 +1,7 @@
>  // SPDX-License-Identifier: GPL-2.0
> +#include <linux/dma-mapping.h>
>  #include <linux/errno.h>
> +#include <linux/genalloc.h>
>  #include <linux/gfp.h>
>  #include <linux/kernel.h>
>  #include <linux/mm.h>
> @@ -13,8 +15,8 @@
>  #include <xen/xen.h>
> 
>  static DEFINE_MUTEX(list_lock);
> -static struct page *page_list;
> -static unsigned int list_count;
> +
> +static struct gen_pool *dma_pool;
> 
>  static struct resource *target_resource;
> 
> @@ -36,7 +38,7 @@ static int fill_list(unsigned int nr_pages)
>         struct dev_pagemap *pgmap;
>         struct resource *res, *tmp_res = NULL;
>         void *vaddr;
> -       unsigned int i, alloc_pages = round_up(nr_pages, PAGES_PER_SECTION);
> +       unsigned int alloc_pages = round_up(nr_pages, PAGES_PER_SECTION);
>         struct range mhp_range;
>         int ret;
> 
> @@ -106,6 +108,7 @@ static int fill_list(unsigned int nr_pages)
>           * conflict with any devices.
>           */
>         if (!xen_feature(XENFEAT_auto_translated_physmap)) {
> +               unsigned int i;
>                 xen_pfn_t pfn = PFN_DOWN(res->start);
> 
>                 for (i = 0; i < alloc_pages; i++) {
> @@ -125,16 +128,17 @@ static int fill_list(unsigned int nr_pages)
>                 goto err_memremap;
>         }
> 
> -       for (i = 0; i < alloc_pages; i++) {
> -               struct page *pg = virt_to_page(vaddr + PAGE_SIZE * i);
> -
> -               pg->zone_device_data = page_list;
> -               page_list = pg;
> -               list_count++;
> +       ret = gen_pool_add_virt(dma_pool, (unsigned long)vaddr, res->start,
> +                       alloc_pages * PAGE_SIZE, NUMA_NO_NODE);
> +       if (ret) {
> +               pr_err("Cannot add memory range to the pool\n");
> +               goto err_pool;
>         }
> 
>         return 0;
> 
> +err_pool:
> +       memunmap_pages(pgmap);
>  err_memremap:
>         kfree(pgmap);
>  err_pgmap:
> @@ -149,51 +153,49 @@ static int fill_list(unsigned int nr_pages)
>         return ret;
>  }
> 
> -/**
> - * xen_alloc_unpopulated_pages - alloc unpopulated pages
> - * @nr_pages: Number of pages
> - * @pages: pages returned
> - * @return 0 on success, error otherwise
> - */
> -int xen_alloc_unpopulated_pages(unsigned int nr_pages, struct page **pages)
> +static int alloc_unpopulated_pages(unsigned int nr_pages, struct page
> **pages,
> +               bool contiguous)
>  {
>         unsigned int i;
>         int ret = 0;
> +       void *vaddr;
> +       bool filled = false;
> 
>         /*
>          * Fallback to default behavior if we do not have any suitable
> resource
>          * to allocate required region from and as the result we won't be able
> to
>          * construct pages.
>          */
> -       if (!target_resource)
> +       if (!target_resource) {
> +               if (contiguous)
> +                       return -ENODEV;
> +
>                 return xen_alloc_ballooned_pages(nr_pages, pages);
> +       }
> 
>         mutex_lock(&list_lock);
> -       if (list_count < nr_pages) {
> -               ret = fill_list(nr_pages - list_count);
> +
> +       while (!(vaddr = (void *)gen_pool_alloc(dma_pool, nr_pages *
> PAGE_SIZE))) {
> +               if (filled)
> +                       ret = -ENOMEM;
> +               else {
> +                       ret = fill_list(nr_pages);
> +                       filled = true;
> +               }
>                 if (ret)
>                         goto out;
>         }
> 
>         for (i = 0; i < nr_pages; i++) {
> -               struct page *pg = page_list;
> -
> -               BUG_ON(!pg);
> -               page_list = pg->zone_device_data;
> -               list_count--;
> -               pages[i] = pg;
> +               pages[i] = virt_to_page(vaddr + PAGE_SIZE * i);
> 
>  #ifdef CONFIG_XEN_HAVE_PVMMU
>                 if (!xen_feature(XENFEAT_auto_translated_physmap)) {
> -                       ret = xen_alloc_p2m_entry(page_to_pfn(pg));
> +                       ret = xen_alloc_p2m_entry(page_to_pfn(pages[i]));
>                         if (ret < 0) {
> -                               unsigned int j;
> -
> -                               for (j = 0; j <= i; j++) {
> - pages[j]->zone_device_data = page_list;
> -                                       page_list = pages[j];
> -                                       list_count++;
> -                               }
> +                               /* XXX Do we need to zeroed pages[i]? */
> +                               gen_pool_free(dma_pool, (unsigned long)vaddr,
> +                                               nr_pages * PAGE_SIZE);
>                                 goto out;
>                         }
>                 }
> @@ -204,32 +206,89 @@ int xen_alloc_unpopulated_pages(unsigned int nr_pages,
> struct page **pages)
>         mutex_unlock(&list_lock);
>         return ret;
>  }
> -EXPORT_SYMBOL(xen_alloc_unpopulated_pages);
> 
> -/**
> - * xen_free_unpopulated_pages - return unpopulated pages
> - * @nr_pages: Number of pages
> - * @pages: pages to return
> - */
> -void xen_free_unpopulated_pages(unsigned int nr_pages, struct page **pages)
> +static void free_unpopulated_pages(unsigned int nr_pages, struct page
> **pages,
> +               bool contiguous)
>  {
> -       unsigned int i;
> -
>         if (!target_resource) {
> +               if (contiguous)
> +                       return;
> +
>                 xen_free_ballooned_pages(nr_pages, pages);
>                 return;
>         }
> 
>         mutex_lock(&list_lock);
> -       for (i = 0; i < nr_pages; i++) {
> -               pages[i]->zone_device_data = page_list;
> -               page_list = pages[i];
> -               list_count++;
> +
> +       /* XXX Do we need to check the range (gen_pool_has_addr)? */
> +       if (contiguous)
> +               gen_pool_free(dma_pool, (unsigned long)page_to_virt(pages[0]),
> +                               nr_pages * PAGE_SIZE);
> +       else {
> +               unsigned int i;
> +
> +               for (i = 0; i < nr_pages; i++)
> +                       gen_pool_free(dma_pool, (unsigned
> long)page_to_virt(pages[i]),
> +                                       PAGE_SIZE);
>         }
> +
>         mutex_unlock(&list_lock);
>  }
> +
> +/**
> + * xen_alloc_unpopulated_pages - alloc unpopulated pages
> + * @nr_pages: Number of pages
> + * @pages: pages returned
> + * @return 0 on success, error otherwise
> + */
> +int xen_alloc_unpopulated_pages(unsigned int nr_pages, struct page **pages)
> +{
> +       return alloc_unpopulated_pages(nr_pages, pages, false);
> +}
> +EXPORT_SYMBOL(xen_alloc_unpopulated_pages);
> +
> +/**
> + * xen_free_unpopulated_pages - return unpopulated pages
> + * @nr_pages: Number of pages
> + * @pages: pages to return
> + */
> +void xen_free_unpopulated_pages(unsigned int nr_pages, struct page **pages)
> +{
> +       free_unpopulated_pages(nr_pages, pages, false);
> +}
>  EXPORT_SYMBOL(xen_free_unpopulated_pages);
> 
> +/**
> + * xen_alloc_unpopulated_dma_pages - alloc unpopulated DMAable pages
> + * @dev: valid struct device pointer
> + * @nr_pages: Number of pages
> + * @pages: pages returned
> + * @return 0 on success, error otherwise
> + */
> +int xen_alloc_unpopulated_dma_pages(struct device *dev, unsigned int
> nr_pages,
> +               struct page **pages)
> +{
> +       /* XXX Handle devices which support 64-bit DMA address only for now */
> +       if (dma_get_mask(dev) != DMA_BIT_MASK(64))
> +               return -EINVAL;
> +
> +       return alloc_unpopulated_pages(nr_pages, pages, true);
> +}
> +EXPORT_SYMBOL(xen_alloc_unpopulated_dma_pages);
> +
> +/**
> + * xen_free_unpopulated_dma_pages - return unpopulated DMAable pages
> + * @dev: valid struct device pointer
> + * @nr_pages: Number of pages
> + * @pages: pages to return
> + */
> +void xen_free_unpopulated_dma_pages(struct device *dev, unsigned int
> nr_pages,
> +               struct page **pages)
> +{
> +       free_unpopulated_pages(nr_pages, pages, true);
> +}
> +EXPORT_SYMBOL(xen_free_unpopulated_dma_pages);
> +
>  static int __init unpopulated_init(void)
>  {
>         int ret;
> @@ -237,9 +296,19 @@ static int __init unpopulated_init(void)
>         if (!xen_domain())
>                 return -ENODEV;
> 
> +       dma_pool = gen_pool_create(PAGE_SHIFT, NUMA_NO_NODE);
> +       if (!dma_pool) {
> +               pr_err("xen:unpopulated: Cannot create DMA pool\n");
> +               return -ENOMEM;
> +       }
> +
> +       gen_pool_set_algo(dma_pool, gen_pool_best_fit, NULL);
> +
>         ret = arch_xen_unpopulated_init(&target_resource);
>         if (ret) {
>                 pr_err("xen:unpopulated: Cannot initialize target
> resource\n");
> +               gen_pool_destroy(dma_pool);
> +               dma_pool = NULL;
>                 target_resource = NULL;
>         }
> 
> [snip]
> 
> 
> I think, regarding on the approach we would likely need to do some renaming
> for fill_list, page_list, list_lock, etc.
> 
> 
> Both options work in my Arm64 based environment, not sure regarding x86.
> Or do we have another option here?
> I would be happy to go any route. What do you think?

The second option (use "dma_pool" for all) looks great, thank you for
looking into it!

Re: [RFC PATCH 1/2] xen/unpopulated-alloc: Introduce helpers for DMA allocations

[Oleksandr]

On 11.06.22 03:12, Stefano Stabellini wrote:


Hello Stefano


> On Wed, 8 Jun 2022, Oleksandr wrote:
>> 2. Drop the "page_list" entirely and use "dma_pool" for all (contiguous and
>> non-contiguous) allocations. After all, all pages are initially contiguous in
>> fill_list() as they are built from the resource. This changes behavior for all
>> users of xen_alloc_unpopulated_pages()
>>
>> Below the diff for unpopulated-alloc.c. The patch is also available at:
>>
>> https://github.com/otyshchenko1/linux/commit/7be569f113a4acbdc4bcb9b20cb3995b3151387a
>>
>>
>> diff --git a/drivers/xen/unpopulated-alloc.c b/drivers/xen/unpopulated-alloc.c
>> index a39f2d3..ab5c7bd 100644
>> --- a/drivers/xen/unpopulated-alloc.c
>> +++ b/drivers/xen/unpopulated-alloc.c
>> @@ -1,5 +1,7 @@
>>   // SPDX-License-Identifier: GPL-2.0
>> +#include <linux/dma-mapping.h>
>>   #include <linux/errno.h>
>> +#include <linux/genalloc.h>
>>   #include <linux/gfp.h>
>>   #include <linux/kernel.h>
>>   #include <linux/mm.h>
>> @@ -13,8 +15,8 @@
>>   #include <xen/xen.h>
>>
>>   static DEFINE_MUTEX(list_lock);
>> -static struct page *page_list;
>> -static unsigned int list_count;
>> +
>> +static struct gen_pool *dma_pool;
>>
>>   static struct resource *target_resource;
>>
>> @@ -36,7 +38,7 @@ static int fill_list(unsigned int nr_pages)
>>          struct dev_pagemap *pgmap;
>>          struct resource *res, *tmp_res = NULL;
>>          void *vaddr;
>> -       unsigned int i, alloc_pages = round_up(nr_pages, PAGES_PER_SECTION);
>> +       unsigned int alloc_pages = round_up(nr_pages, PAGES_PER_SECTION);
>>          struct range mhp_range;
>>          int ret;
>>
>> @@ -106,6 +108,7 @@ static int fill_list(unsigned int nr_pages)
>>            * conflict with any devices.
>>            */
>>          if (!xen_feature(XENFEAT_auto_translated_physmap)) {
>> +               unsigned int i;
>>                  xen_pfn_t pfn = PFN_DOWN(res->start);
>>
>>                  for (i = 0; i < alloc_pages; i++) {
>> @@ -125,16 +128,17 @@ static int fill_list(unsigned int nr_pages)
>>                  goto err_memremap;
>>          }
>>
>> -       for (i = 0; i < alloc_pages; i++) {
>> -               struct page *pg = virt_to_page(vaddr + PAGE_SIZE * i);
>> -
>> -               pg->zone_device_data = page_list;
>> -               page_list = pg;
>> -               list_count++;
>> +       ret = gen_pool_add_virt(dma_pool, (unsigned long)vaddr, res->start,
>> +                       alloc_pages * PAGE_SIZE, NUMA_NO_NODE);
>> +       if (ret) {
>> +               pr_err("Cannot add memory range to the pool\n");
>> +               goto err_pool;
>>          }
>>
>>          return 0;
>>
>> +err_pool:
>> +       memunmap_pages(pgmap);
>>   err_memremap:
>>          kfree(pgmap);
>>   err_pgmap:
>> @@ -149,51 +153,49 @@ static int fill_list(unsigned int nr_pages)
>>          return ret;
>>   }
>>
>> -/**
>> - * xen_alloc_unpopulated_pages - alloc unpopulated pages
>> - * @nr_pages: Number of pages
>> - * @pages: pages returned
>> - * @return 0 on success, error otherwise
>> - */
>> -int xen_alloc_unpopulated_pages(unsigned int nr_pages, struct page **pages)
>> +static int alloc_unpopulated_pages(unsigned int nr_pages, struct page
>> **pages,
>> +               bool contiguous)
>>   {
>>          unsigned int i;
>>          int ret = 0;
>> +       void *vaddr;
>> +       bool filled = false;
>>
>>          /*
>>           * Fallback to default behavior if we do not have any suitable
>> resource
>>           * to allocate required region from and as the result we won't be able
>> to
>>           * construct pages.
>>           */
>> -       if (!target_resource)
>> +       if (!target_resource) {
>> +               if (contiguous)
>> +                       return -ENODEV;
>> +
>>                  return xen_alloc_ballooned_pages(nr_pages, pages);
>> +       }
>>
>>          mutex_lock(&list_lock);
>> -       if (list_count < nr_pages) {
>> -               ret = fill_list(nr_pages - list_count);
>> +
>> +       while (!(vaddr = (void *)gen_pool_alloc(dma_pool, nr_pages *
>> PAGE_SIZE))) {
>> +               if (filled)
>> +                       ret = -ENOMEM;
>> +               else {
>> +                       ret = fill_list(nr_pages);
>> +                       filled = true;
>> +               }
>>                  if (ret)
>>                          goto out;
>>          }
>>
>>          for (i = 0; i < nr_pages; i++) {
>> -               struct page *pg = page_list;
>> -
>> -               BUG_ON(!pg);
>> -               page_list = pg->zone_device_data;
>> -               list_count--;
>> -               pages[i] = pg;
>> +               pages[i] = virt_to_page(vaddr + PAGE_SIZE * i);
>>
>>   #ifdef CONFIG_XEN_HAVE_PVMMU
>>                  if (!xen_feature(XENFEAT_auto_translated_physmap)) {
>> -                       ret = xen_alloc_p2m_entry(page_to_pfn(pg));
>> +                       ret = xen_alloc_p2m_entry(page_to_pfn(pages[i]));
>>                          if (ret < 0) {
>> -                               unsigned int j;
>> -
>> -                               for (j = 0; j <= i; j++) {
>> - pages[j]->zone_device_data = page_list;
>> -                                       page_list = pages[j];
>> -                                       list_count++;
>> -                               }
>> +                               /* XXX Do we need to zeroed pages[i]? */
>> +                               gen_pool_free(dma_pool, (unsigned long)vaddr,
>> +                                               nr_pages * PAGE_SIZE);
>>                                  goto out;
>>                          }
>>                  }
>> @@ -204,32 +206,89 @@ int xen_alloc_unpopulated_pages(unsigned int nr_pages,
>> struct page **pages)
>>          mutex_unlock(&list_lock);
>>          return ret;
>>   }
>> -EXPORT_SYMBOL(xen_alloc_unpopulated_pages);
>>
>> -/**
>> - * xen_free_unpopulated_pages - return unpopulated pages
>> - * @nr_pages: Number of pages
>> - * @pages: pages to return
>> - */
>> -void xen_free_unpopulated_pages(unsigned int nr_pages, struct page **pages)
>> +static void free_unpopulated_pages(unsigned int nr_pages, struct page
>> **pages,
>> +               bool contiguous)
>>   {
>> -       unsigned int i;
>> -
>>          if (!target_resource) {
>> +               if (contiguous)
>> +                       return;
>> +
>>                  xen_free_ballooned_pages(nr_pages, pages);
>>                  return;
>>          }
>>
>>          mutex_lock(&list_lock);
>> -       for (i = 0; i < nr_pages; i++) {
>> -               pages[i]->zone_device_data = page_list;
>> -               page_list = pages[i];
>> -               list_count++;
>> +
>> +       /* XXX Do we need to check the range (gen_pool_has_addr)? */
>> +       if (contiguous)
>> +               gen_pool_free(dma_pool, (unsigned long)page_to_virt(pages[0]),
>> +                               nr_pages * PAGE_SIZE);
>> +       else {
>> +               unsigned int i;
>> +
>> +               for (i = 0; i < nr_pages; i++)
>> +                       gen_pool_free(dma_pool, (unsigned
>> long)page_to_virt(pages[i]),
>> +                                       PAGE_SIZE);
>>          }
>> +
>>          mutex_unlock(&list_lock);
>>   }
>> +
>> +/**
>> + * xen_alloc_unpopulated_pages - alloc unpopulated pages
>> + * @nr_pages: Number of pages
>> + * @pages: pages returned
>> + * @return 0 on success, error otherwise
>> + */
>> +int xen_alloc_unpopulated_pages(unsigned int nr_pages, struct page **pages)
>> +{
>> +       return alloc_unpopulated_pages(nr_pages, pages, false);
>> +}
>> +EXPORT_SYMBOL(xen_alloc_unpopulated_pages);
>> +
>> +/**
>> + * xen_free_unpopulated_pages - return unpopulated pages
>> + * @nr_pages: Number of pages
>> + * @pages: pages to return
>> + */
>> +void xen_free_unpopulated_pages(unsigned int nr_pages, struct page **pages)
>> +{
>> +       free_unpopulated_pages(nr_pages, pages, false);
>> +}
>>   EXPORT_SYMBOL(xen_free_unpopulated_pages);
>>
>> +/**
>> + * xen_alloc_unpopulated_dma_pages - alloc unpopulated DMAable pages
>> + * @dev: valid struct device pointer
>> + * @nr_pages: Number of pages
>> + * @pages: pages returned
>> + * @return 0 on success, error otherwise
>> + */
>> +int xen_alloc_unpopulated_dma_pages(struct device *dev, unsigned int
>> nr_pages,
>> +               struct page **pages)
>> +{
>> +       /* XXX Handle devices which support 64-bit DMA address only for now */
>> +       if (dma_get_mask(dev) != DMA_BIT_MASK(64))
>> +               return -EINVAL;
>> +
>> +       return alloc_unpopulated_pages(nr_pages, pages, true);
>> +}
>> +EXPORT_SYMBOL(xen_alloc_unpopulated_dma_pages);
>> +
>> +/**
>> + * xen_free_unpopulated_dma_pages - return unpopulated DMAable pages
>> + * @dev: valid struct device pointer
>> + * @nr_pages: Number of pages
>> + * @pages: pages to return
>> + */
>> +void xen_free_unpopulated_dma_pages(struct device *dev, unsigned int
>> nr_pages,
>> +               struct page **pages)
>> +{
>> +       free_unpopulated_pages(nr_pages, pages, true);
>> +}
>> +EXPORT_SYMBOL(xen_free_unpopulated_dma_pages);
>> +
>>   static int __init unpopulated_init(void)
>>   {
>>          int ret;
>> @@ -237,9 +296,19 @@ static int __init unpopulated_init(void)
>>          if (!xen_domain())
>>                  return -ENODEV;
>>
>> +       dma_pool = gen_pool_create(PAGE_SHIFT, NUMA_NO_NODE);
>> +       if (!dma_pool) {
>> +               pr_err("xen:unpopulated: Cannot create DMA pool\n");
>> +               return -ENOMEM;
>> +       }
>> +
>> +       gen_pool_set_algo(dma_pool, gen_pool_best_fit, NULL);
>> +
>>          ret = arch_xen_unpopulated_init(&target_resource);
>>          if (ret) {
>>                  pr_err("xen:unpopulated: Cannot initialize target
>> resource\n");
>> +               gen_pool_destroy(dma_pool);
>> +               dma_pool = NULL;
>>                  target_resource = NULL;
>>          }
>>
>> [snip]
>>
>>
>> I think, regarding on the approach we would likely need to do some renaming
>> for fill_list, page_list, list_lock, etc.
>>
>>
>> Both options work in my Arm64 based environment, not sure regarding x86.
>> Or do we have another option here?
>> I would be happy to go any route. What do you think?
> The second option (use "dma_pool" for all) looks great, thank you for
> looking into it!


ok, great


May I please clarify a few moments before starting preparing non-RFC 
version:


1. According to the discussion at "[RFC PATCH 2/2] xen/grant-table: Use 
unpopulated DMAable pages instead of real RAM ones" we decided
to stay away from the "dma" in the name, also the second option (use 
"dma_pool" for all) implies dropping the "page_list" entirely, so I am 
going to do some renaming:

- 
s/xen_alloc_unpopulated_dma_pages()/xen_alloc_unpopulated_contiguous_pages()
- s/dma_pool/unpopulated_pool
- s/list_lock/pool_lock
- s/fill_list()/fill_pool()

Any objections?


2. I don't like much the fact that in free_unpopulated_pages() we have 
to free "page by page" if contiguous is false, but unfortunately we 
cannot avoid doing that.
I noticed that many users of unpopulated pages retain initially 
allocated pages[i] array, so it passed here absolutely unmodified since 
being allocated, but there is a code (for example, 
gnttab_page_cache_shrink() in grant-table.c) that can pass pages[i] 
which contains arbitrary pages.

static void free_unpopulated_pages(unsigned int nr_pages, struct page 
**pages,
         bool contiguous)
{

[snip]

     /* XXX Do we need to check the range (gen_pool_has_addr)? */
     if (contiguous)
         gen_pool_free(dma_pool, (unsigned long)page_to_virt(pages[0]),
                 nr_pages * PAGE_SIZE);
     else {
         unsigned int i;

         for (i = 0; i < nr_pages; i++)
             gen_pool_free(dma_pool, (unsigned long)page_to_virt(pages[i]),
                     PAGE_SIZE);
     }

[snip]

}

I think, it wouldn't be a big deal for the small allocations, but for 
the big allocations it might be not optimal for the speed.

What do you think if we update some places which always require big 
allocations to allocate (and free) contiguous pages instead?
The possible candidate is 
gem_create()/xen_drm_front_gem_free_object_unlocked() in 
drivers/gpu/drm/xen/xen_drm_front_gem.c.
OTOH I realize this might be inefficient use of resources. Or better not?


3. The alloc_unpopulated_pages() might be optimized for the 
non-contiguous allocations, currently we always try to allocate a single 
chunk even if contiguous is false.

static int alloc_unpopulated_pages(unsigned int nr_pages, struct page 
**pages,
         bool contiguous)
{

[snip]

     /* XXX: Optimize for non-contiguous allocations */
     while (!(vaddr = (void *)gen_pool_alloc(dma_pool, nr_pages * 
PAGE_SIZE))) {
         if (filled)
             ret = -ENOMEM;
         else {
             ret = fill_list(nr_pages);
             filled = true;
         }
         if (ret)
             goto out;
     }

[snip]

}


But we can allocate "page by page" for the non-contiguous allocations, 
it might be not optimal for the speed, but would be optimal for the 
resource usage. What do you think?

static int alloc_unpopulated_pages(unsigned int nr_pages, struct page 
**pages,
         bool contiguous)
{

[snip]

     if (contiguous) {
         while (!(vaddr = (void *)gen_pool_alloc(dma_pool, nr_pages * 
PAGE_SIZE))) {
             if (filled)
                 ret = -ENOMEM;
             else {
                 ret = fill_list(nr_pages);
                 filled = true;
             }
             if (ret)
                 goto out;
         }

         for (i = 0; i < nr_pages; i++)
             pages[i] = virt_to_page(vaddr + PAGE_SIZE * i);
     } else {
         if (gen_pool_avail(dma_pool) < nr_pages) {
             ret = fill_list(nr_pages - gen_pool_avail(dma_pool));
             if (ret)
                 goto out;
         }

         for (i = 0; i < nr_pages; i++) {
             vaddr = (void *)gen_pool_alloc(dma_pool, PAGE_SIZE);
             if (!vaddr) {
                 while (i--)
                     gen_pool_free(dma_pool, (unsigned 
long)page_to_virt(pages[i]),
                             PAGE_SIZE);

                 ret = -ENOMEM;
                 goto out;
             }

             pages[i] = virt_to_page(vaddr);
         }
     }

[snip]

}


Thank you.

-- 
Regards,

Oleksandr Tyshchenko

Re: [RFC PATCH 2/2] xen/grant-table: Use unpopulated DMAable pages instead of real RAM ones

[Oleksandr]

On 11.06.22 02:55, Stefano Stabellini wrote:

Hello Stefano

> On Thu, 9 Jun 2022, Oleksandr wrote:
>> On 04.06.22 00:19, Stefano Stabellini wrote:
>> Hello Stefano
>>
>> Thank you for having a look and sorry for the late response.
>>
>>> On Tue, 17 May 2022, Oleksandr Tyshchenko wrote:
>>>> From: Oleksandr Tyshchenko <oleksandr_tyshchenko@epam.com>
>>>>
>>>> Depends on CONFIG_XEN_UNPOPULATED_ALLOC. If enabled then unpopulated
>>>> DMAable (contiguous) pages will be allocated for grant mapping into
>>>> instead of ballooning out real RAM pages.
>>>>
>>>> TODO: Fallback to real RAM pages if xen_alloc_unpopulated_dma_pages()
>>>> fails.
>>>>
>>>> Signed-off-by: Oleksandr Tyshchenko <oleksandr_tyshchenko@epam.com>
>>>> ---
>>>>    drivers/xen/grant-table.c | 27 +++++++++++++++++++++++++++
>>>>    1 file changed, 27 insertions(+)
>>>>
>>>> diff --git a/drivers/xen/grant-table.c b/drivers/xen/grant-table.c
>>>> index 8ccccac..2bb4392 100644
>>>> --- a/drivers/xen/grant-table.c
>>>> +++ b/drivers/xen/grant-table.c
>>>> @@ -864,6 +864,25 @@ EXPORT_SYMBOL_GPL(gnttab_free_pages);
>>>>     */
>>>>    int gnttab_dma_alloc_pages(struct gnttab_dma_alloc_args *args)
>>>>    {
>>>> +#ifdef CONFIG_XEN_UNPOPULATED_ALLOC
>>>> +	int ret;
>>> This is an alternative implementation of the same function.
>> Currently, yes.
>>
>>
>>>    If we are
>>> going to use #ifdef, then I would #ifdef the entire function, rather
>>> than just the body. Otherwise within the function body we can use
>>> IS_ENABLED.
>>
>> Good point. Note, there is one missing thing in current patch which is
>> described in TODO.
>>
>> "Fallback to real RAM pages if xen_alloc_unpopulated_dma_pages() fails."  So I
>> will likely use IS_ENABLED within the function body.
>>
>> If CONFIG_XEN_UNPOPULATED_ALLOC is enabled then gnttab_dma_alloc_pages() will
>> try to call xen_alloc_unpopulated_dma_pages() the first and if fails then
>> fallback to allocate RAM pages and balloon them out.
>>
>> One moment is not entirely clear to me. If we use fallback in
>> gnttab_dma_alloc_pages() then we must use fallback in gnttab_dma_free_pages()
>> as well, we cannot use xen_free_unpopulated_dma_pages() for real RAM pages.
>> The question is how to pass this information to the gnttab_dma_free_pages()?
>> The first idea which comes to mind is to add a flag to struct
>> gnttab_dma_alloc_args...
>   
> You can check if the page is within the mhp_range range or part of
> iomem_resource? If not, you can free it as a normal page.
>
> If we do this, then the fallback is better implemented in
> unpopulated-alloc.c because that is the one that is aware about
> page addresses.


I got your idea and agree this can work technically. Or if we finally 
decide to use the second option (use "dma_pool" for all) in the first patch
"[RFC PATCH 1/2] xen/unpopulated-alloc: Introduce helpers for DMA 
allocations" then we will likely be able to check whether a page in question
is within a "dma_pool" using gen_pool_has_addr().

I am still wondering, can we avoid the fallback implementation in 
unpopulated-alloc.c.
Because for that purpose we will need to pull more code into 
unpopulated-alloc.c (to be more precise, almost everything which 
gnttab_dma_free_pages() already has except gnttab_pages_clear_private()) 
and pass more arguments to xen_free_unpopulated_dma_pages(). Also I 
might mistake, but having a fallback split between grant-table.c (to 
allocate RAM pages in gnttab_dma_alloc_pages()) and unpopulated-alloc.c 
(to free RAM pages in xen_free_unpopulated_dma_pages()) would look a bit 
weird.

I see two possible options for the fallback implementation in grant-table.c:
1. (less preferable) by introducing new flag in struct gnttab_dma_alloc_args
2. (more preferable) by guessing unpopulated (non real RAM) page using 
is_zone_device_page(), etc


For example, with the second option the resulting code will look quite 
simple (only build tested):

diff --git a/drivers/xen/grant-table.c b/drivers/xen/grant-table.c
index 738029d..3bda71f 100644
--- a/drivers/xen/grant-table.c
+++ b/drivers/xen/grant-table.c
@@ -1047,6 +1047,23 @@ int gnttab_dma_alloc_pages(struct 
gnttab_dma_alloc_args *args)
         size_t size;
         int i, ret;

+       if (IS_ENABLED(CONFIG_XEN_UNPOPULATED_ALLOC)) {
+               ret = xen_alloc_unpopulated_dma_pages(args->dev, 
args->nr_pages,
+                               args->pages);
+               if (ret < 0)
+                       goto fallback;
+
+               ret = gnttab_pages_set_private(args->nr_pages, args->pages);
+               if (ret < 0)
+                       goto fail;
+
+               args->vaddr = page_to_virt(args->pages[0]);
+               args->dev_bus_addr = page_to_phys(args->pages[0]);
+
+               return ret;
+       }
+
+fallback:
         size = args->nr_pages << PAGE_SHIFT;
         if (args->coherent)
                 args->vaddr = dma_alloc_coherent(args->dev, size,
@@ -1103,6 +1120,12 @@ int gnttab_dma_free_pages(struct 
gnttab_dma_alloc_args *args)

         gnttab_pages_clear_private(args->nr_pages, args->pages);

+       if (IS_ENABLED(CONFIG_XEN_UNPOPULATED_ALLOC) &&
+                       is_zone_device_page(args->pages[0])) {
+               xen_free_unpopulated_dma_pages(args->dev, 
args->nr_pages, args->pages);
+               return 0;
+       }
+
         for (i = 0; i < args->nr_pages; i++)
                 args->frames[i] = page_to_xen_pfn(args->pages[i]);


What do you think?


>
>   
>   
>>>> +	ret = xen_alloc_unpopulated_dma_pages(args->dev, args->nr_pages,
>>>> +			args->pages);
>>>> +	if (ret < 0)
>>>> +		return ret;
>>>> +
>>>> +	ret = gnttab_pages_set_private(args->nr_pages, args->pages);
>>>> +	if (ret < 0) {
>>>> +		gnttab_dma_free_pages(args);
>>> it should xen_free_unpopulated_dma_pages ?
>> Besides calling the xen_free_unpopulated_dma_pages(), we also need to call
>> gnttab_pages_clear_private() here, this is what gnttab_dma_free_pages() is
>> doing.
>>
>> I can change to call both function instead:
>>
>>      gnttab_pages_clear_private(args->nr_pages, args->pages);
>>      xen_free_unpopulated_dma_pages(args->dev, args->nr_pages, args->pages);
>>
>> Shall I?
> No, leave it as is. I didn't realize that gnttab_pages_set_private can
> fail half-way through.


ok, thank you for the confirmation.


>
>   
>>>
>>>> +		return ret;
>>>> +	}
>>>> +
>>>> +	args->vaddr = page_to_virt(args->pages[0]);
>>>> +	args->dev_bus_addr = page_to_phys(args->pages[0]);
>>> There are two things to note here.
>>>
>>> The first thing to note is that normally we would call pfn_to_bfn to
>>> retrieve the dev_bus_addr of a page because pfn_to_bfn takes into
>>> account foreign mappings. However, these are freshly allocated pages
>>> without foreign mappings, so page_to_phys/dma should be sufficient.
>> agree
>>
>>
>>>
>>> The second has to do with physical addresses and DMA addresses. The
>>> functions are called gnttab_dma_alloc_pages and
>>> xen_alloc_unpopulated_dma_pages which make you think we are retrieving a
>>> DMA address here. However, to get a DMA address we need to call
>>> page_to_dma rather than page_to_phys.
>>>
>>> page_to_dma takes into account special offsets that some devices have
>>> when accessing memory. There are real cases on ARM where the physical
>>> address != DMA address, e.g. RPi4.
>> I got it. Now I am in doubt whether it would be better to name the API:
>>
>> xen_alloc_unpopulated_cma_pages()
>>
>> or
>>
>> xen_alloc_unpopulated_contiguous_pages()
>>
>> What do you think?
> Yeah actually I think it is better to stay away from "dma" in the name.
> I like xen_alloc_unpopulated_contiguous_pages().


perfect, I will rename then, thank you for the confirmation.


>   
>   
>>> However, to call page_to_dma you need to specify as first argument the
>>> DMA-capable device that is expected to use those pages for DMA (e.g. an
>>> ethernet device or a MMC controller.) While the args->dev we have in
>>> gnttab_dma_alloc_pages is the gntdev_miscdev.
>> agree
>>
>> As I understand, at this time it is unknown for what exactly device these
>> pages are supposed to be used at the end.
>>
>> For now, it is only known that these pages to be used by userspace PV backend
>> for grant mappings.
> Yeah
>   
>
>>> So this interface cannot actually be used to allocate memory that is
>>> supposed to be DMA-able by a DMA-capable device, such as an ethernet
>>> device.
>> agree
>>
>>
>>> But I think that should be fine because the memory is meant to be used
>>> by a userspace PV backend for grant mappings. If any of those mappings
>>> end up being used for actual DMA in the kernel they should go through the
>>> drivers/xen/swiotlb-xen.c and xen_phys_to_dma should be called, which
>>> ends up calling page_to_dma as appropriate.
>>>
>>> It would be good to double-check that the above is correct and, if so,
>>> maybe add a short in-code comment about it:
>>>
>>> /*
>>>    * These are not actually DMA addresses but regular physical addresses.
>>>    * If these pages end up being used in a DMA operation then the
>>>    * swiotlb-xen functions are called and xen_phys_to_dma takes care of
>>>    * the address translations:
>>>    *
>>>    * - from gfn to bfn in case of foreign mappings
>>>    * - from physical to DMA addresses in case the two are different for a
>>>    *   given DMA-mastering device
>>>    */
>> I agree this needs to be re-checked. But, there is one moment here, if
>> userspace PV backend runs in other than Dom0 domain (non 1:1 mapped domain),
>> the xen-swiotlb seems not to be in use then? How to be in this case?
>   
> In that case, an IOMMU is required. If an IOMMU is setup correct, then
> the gfn->bfn translation is not necessary because it is done
> automatically by the IOMMU. That is because when the foreign page is
> mapped in the domain, the mapping also applies to the IOMMU pagetable.
>
> So the device is going to do DMA to "gfn" and the IOMMU will translate
> it to the right "mfn", the one corresponding to "bfn".
>
> The physical to DMA address should be done automatically by the default
> (non-swiotlb_xen) dma_ops in Linux. E.g.
> kernel/dma/direct.c:dma_direct_map_sg correctly calls
> dma_direct_map_page, which calls phys_to_dma.


Thank you for the explanation.


>   
>   
>   
>>>> +	return ret;
>>>> +#else
>>>>    	unsigned long pfn, start_pfn;
>>>>    	size_t size;
>>>>    	int i, ret;
>>>> @@ -910,6 +929,7 @@ int gnttab_dma_alloc_pages(struct
>>>> gnttab_dma_alloc_args *args)
>>>>    fail:
>>>>    	gnttab_dma_free_pages(args);
>>>>    	return ret;
>>>> +#endif
>>>>    }
>>>>    EXPORT_SYMBOL_GPL(gnttab_dma_alloc_pages);
>>>>    @@ -919,6 +939,12 @@ EXPORT_SYMBOL_GPL(gnttab_dma_alloc_pages);
>>>>     */
>>>>    int gnttab_dma_free_pages(struct gnttab_dma_alloc_args *args)
>>>>    {
>>>> +#ifdef CONFIG_XEN_UNPOPULATED_ALLOC
>>>> +	gnttab_pages_clear_private(args->nr_pages, args->pages);
>>>> +	xen_free_unpopulated_dma_pages(args->dev, args->nr_pages,
>>>> args->pages);
>>>> +
>>>> +	return 0;
>>>> +#else
>>>>    	size_t size;
>>>>    	int i, ret;
>>>>    @@ -946,6 +972,7 @@ int gnttab_dma_free_pages(struct
>>>> gnttab_dma_alloc_args *args)
>>>>    		dma_free_wc(args->dev, size,
>>>>    			    args->vaddr, args->dev_bus_addr);
>>>>    	return ret;
>>>> +#endif
>>>>    }
>>>>    EXPORT_SYMBOL_GPL(gnttab_dma_free_pages);
>>>>    #endif
>>>> -- 
>>>> 2.7.4

-- 
Regards,

Oleksandr Tyshchenko

Re: [RFC PATCH 1/2] xen/unpopulated-alloc: Introduce helpers for DMA allocations

[Stefano Stabellini]

[-- Attachment #1: Type: text/plain, Size: 18430 bytes --]

On Tue, 14 Jun 2022, Oleksandr wrote:
> On 11.06.22 03:12, Stefano Stabellini wrote:
> > On Wed, 8 Jun 2022, Oleksandr wrote:
> > > 2. Drop the "page_list" entirely and use "dma_pool" for all (contiguous
> > > and
> > > non-contiguous) allocations. After all, all pages are initially contiguous
> > > in
> > > fill_list() as they are built from the resource. This changes behavior for
> > > all
> > > users of xen_alloc_unpopulated_pages()
> > > 
> > > Below the diff for unpopulated-alloc.c. The patch is also available at:
> > > 
> > > https://github.com/otyshchenko1/linux/commit/7be569f113a4acbdc4bcb9b20cb3995b3151387a
> > > 
> > > 
> > > diff --git a/drivers/xen/unpopulated-alloc.c
> > > b/drivers/xen/unpopulated-alloc.c
> > > index a39f2d3..ab5c7bd 100644
> > > --- a/drivers/xen/unpopulated-alloc.c
> > > +++ b/drivers/xen/unpopulated-alloc.c
> > > @@ -1,5 +1,7 @@
> > >   // SPDX-License-Identifier: GPL-2.0
> > > +#include <linux/dma-mapping.h>
> > >   #include <linux/errno.h>
> > > +#include <linux/genalloc.h>
> > >   #include <linux/gfp.h>
> > >   #include <linux/kernel.h>
> > >   #include <linux/mm.h>
> > > @@ -13,8 +15,8 @@
> > >   #include <xen/xen.h>
> > > 
> > >   static DEFINE_MUTEX(list_lock);
> > > -static struct page *page_list;
> > > -static unsigned int list_count;
> > > +
> > > +static struct gen_pool *dma_pool;
> > > 
> > >   static struct resource *target_resource;
> > > 
> > > @@ -36,7 +38,7 @@ static int fill_list(unsigned int nr_pages)
> > >          struct dev_pagemap *pgmap;
> > >          struct resource *res, *tmp_res = NULL;
> > >          void *vaddr;
> > > -       unsigned int i, alloc_pages = round_up(nr_pages,
> > > PAGES_PER_SECTION);
> > > +       unsigned int alloc_pages = round_up(nr_pages, PAGES_PER_SECTION);
> > >          struct range mhp_range;
> > >          int ret;
> > > 
> > > @@ -106,6 +108,7 @@ static int fill_list(unsigned int nr_pages)
> > >            * conflict with any devices.
> > >            */
> > >          if (!xen_feature(XENFEAT_auto_translated_physmap)) {
> > > +               unsigned int i;
> > >                  xen_pfn_t pfn = PFN_DOWN(res->start);
> > > 
> > >                  for (i = 0; i < alloc_pages; i++) {
> > > @@ -125,16 +128,17 @@ static int fill_list(unsigned int nr_pages)
> > >                  goto err_memremap;
> > >          }
> > > 
> > > -       for (i = 0; i < alloc_pages; i++) {
> > > -               struct page *pg = virt_to_page(vaddr + PAGE_SIZE * i);
> > > -
> > > -               pg->zone_device_data = page_list;
> > > -               page_list = pg;
> > > -               list_count++;
> > > +       ret = gen_pool_add_virt(dma_pool, (unsigned long)vaddr,
> > > res->start,
> > > +                       alloc_pages * PAGE_SIZE, NUMA_NO_NODE);
> > > +       if (ret) {
> > > +               pr_err("Cannot add memory range to the pool\n");
> > > +               goto err_pool;
> > >          }
> > > 
> > >          return 0;
> > > 
> > > +err_pool:
> > > +       memunmap_pages(pgmap);
> > >   err_memremap:
> > >          kfree(pgmap);
> > >   err_pgmap:
> > > @@ -149,51 +153,49 @@ static int fill_list(unsigned int nr_pages)
> > >          return ret;
> > >   }
> > > 
> > > -/**
> > > - * xen_alloc_unpopulated_pages - alloc unpopulated pages
> > > - * @nr_pages: Number of pages
> > > - * @pages: pages returned
> > > - * @return 0 on success, error otherwise
> > > - */
> > > -int xen_alloc_unpopulated_pages(unsigned int nr_pages, struct page
> > > **pages)
> > > +static int alloc_unpopulated_pages(unsigned int nr_pages, struct page
> > > **pages,
> > > +               bool contiguous)
> > >   {
> > >          unsigned int i;
> > >          int ret = 0;
> > > +       void *vaddr;
> > > +       bool filled = false;
> > > 
> > >          /*
> > >           * Fallback to default behavior if we do not have any suitable
> > > resource
> > >           * to allocate required region from and as the result we won't be
> > > able
> > > to
> > >           * construct pages.
> > >           */
> > > -       if (!target_resource)
> > > +       if (!target_resource) {
> > > +               if (contiguous)
> > > +                       return -ENODEV;
> > > +
> > >                  return xen_alloc_ballooned_pages(nr_pages, pages);
> > > +       }
> > > 
> > >          mutex_lock(&list_lock);
> > > -       if (list_count < nr_pages) {
> > > -               ret = fill_list(nr_pages - list_count);
> > > +
> > > +       while (!(vaddr = (void *)gen_pool_alloc(dma_pool, nr_pages *
> > > PAGE_SIZE))) {
> > > +               if (filled)
> > > +                       ret = -ENOMEM;
> > > +               else {
> > > +                       ret = fill_list(nr_pages);
> > > +                       filled = true;
> > > +               }
> > >                  if (ret)
> > >                          goto out;
> > >          }
> > > 
> > >          for (i = 0; i < nr_pages; i++) {
> > > -               struct page *pg = page_list;
> > > -
> > > -               BUG_ON(!pg);
> > > -               page_list = pg->zone_device_data;
> > > -               list_count--;
> > > -               pages[i] = pg;
> > > +               pages[i] = virt_to_page(vaddr + PAGE_SIZE * i);
> > > 
> > >   #ifdef CONFIG_XEN_HAVE_PVMMU
> > >                  if (!xen_feature(XENFEAT_auto_translated_physmap)) {
> > > -                       ret = xen_alloc_p2m_entry(page_to_pfn(pg));
> > > +                       ret = xen_alloc_p2m_entry(page_to_pfn(pages[i]));
> > >                          if (ret < 0) {
> > > -                               unsigned int j;
> > > -
> > > -                               for (j = 0; j <= i; j++) {
> > > - pages[j]->zone_device_data = page_list;
> > > -                                       page_list = pages[j];
> > > -                                       list_count++;
> > > -                               }
> > > +                               /* XXX Do we need to zeroed pages[i]? */
> > > +                               gen_pool_free(dma_pool, (unsigned
> > > long)vaddr,
> > > +                                               nr_pages * PAGE_SIZE);
> > >                                  goto out;
> > >                          }
> > >                  }
> > > @@ -204,32 +206,89 @@ int xen_alloc_unpopulated_pages(unsigned int
> > > nr_pages,
> > > struct page **pages)
> > >          mutex_unlock(&list_lock);
> > >          return ret;
> > >   }
> > > -EXPORT_SYMBOL(xen_alloc_unpopulated_pages);
> > > 
> > > -/**
> > > - * xen_free_unpopulated_pages - return unpopulated pages
> > > - * @nr_pages: Number of pages
> > > - * @pages: pages to return
> > > - */
> > > -void xen_free_unpopulated_pages(unsigned int nr_pages, struct page
> > > **pages)
> > > +static void free_unpopulated_pages(unsigned int nr_pages, struct page
> > > **pages,
> > > +               bool contiguous)
> > >   {
> > > -       unsigned int i;
> > > -
> > >          if (!target_resource) {
> > > +               if (contiguous)
> > > +                       return;
> > > +
> > >                  xen_free_ballooned_pages(nr_pages, pages);
> > >                  return;
> > >          }
> > > 
> > >          mutex_lock(&list_lock);
> > > -       for (i = 0; i < nr_pages; i++) {
> > > -               pages[i]->zone_device_data = page_list;
> > > -               page_list = pages[i];
> > > -               list_count++;
> > > +
> > > +       /* XXX Do we need to check the range (gen_pool_has_addr)? */
> > > +       if (contiguous)
> > > +               gen_pool_free(dma_pool, (unsigned
> > > long)page_to_virt(pages[0]),
> > > +                               nr_pages * PAGE_SIZE);
> > > +       else {
> > > +               unsigned int i;
> > > +
> > > +               for (i = 0; i < nr_pages; i++)
> > > +                       gen_pool_free(dma_pool, (unsigned
> > > long)page_to_virt(pages[i]),
> > > +                                       PAGE_SIZE);
> > >          }
> > > +
> > >          mutex_unlock(&list_lock);
> > >   }
> > > +
> > > +/**
> > > + * xen_alloc_unpopulated_pages - alloc unpopulated pages
> > > + * @nr_pages: Number of pages
> > > + * @pages: pages returned
> > > + * @return 0 on success, error otherwise
> > > + */
> > > +int xen_alloc_unpopulated_pages(unsigned int nr_pages, struct page
> > > **pages)
> > > +{
> > > +       return alloc_unpopulated_pages(nr_pages, pages, false);
> > > +}
> > > +EXPORT_SYMBOL(xen_alloc_unpopulated_pages);
> > > +
> > > +/**
> > > + * xen_free_unpopulated_pages - return unpopulated pages
> > > + * @nr_pages: Number of pages
> > > + * @pages: pages to return
> > > + */
> > > +void xen_free_unpopulated_pages(unsigned int nr_pages, struct page
> > > **pages)
> > > +{
> > > +       free_unpopulated_pages(nr_pages, pages, false);
> > > +}
> > >   EXPORT_SYMBOL(xen_free_unpopulated_pages);
> > > 
> > > +/**
> > > + * xen_alloc_unpopulated_dma_pages - alloc unpopulated DMAable pages
> > > + * @dev: valid struct device pointer
> > > + * @nr_pages: Number of pages
> > > + * @pages: pages returned
> > > + * @return 0 on success, error otherwise
> > > + */
> > > +int xen_alloc_unpopulated_dma_pages(struct device *dev, unsigned int
> > > nr_pages,
> > > +               struct page **pages)
> > > +{
> > > +       /* XXX Handle devices which support 64-bit DMA address only for
> > > now */
> > > +       if (dma_get_mask(dev) != DMA_BIT_MASK(64))
> > > +               return -EINVAL;
> > > +
> > > +       return alloc_unpopulated_pages(nr_pages, pages, true);
> > > +}
> > > +EXPORT_SYMBOL(xen_alloc_unpopulated_dma_pages);
> > > +
> > > +/**
> > > + * xen_free_unpopulated_dma_pages - return unpopulated DMAable pages
> > > + * @dev: valid struct device pointer
> > > + * @nr_pages: Number of pages
> > > + * @pages: pages to return
> > > + */
> > > +void xen_free_unpopulated_dma_pages(struct device *dev, unsigned int
> > > nr_pages,
> > > +               struct page **pages)
> > > +{
> > > +       free_unpopulated_pages(nr_pages, pages, true);
> > > +}
> > > +EXPORT_SYMBOL(xen_free_unpopulated_dma_pages);
> > > +
> > >   static int __init unpopulated_init(void)
> > >   {
> > >          int ret;
> > > @@ -237,9 +296,19 @@ static int __init unpopulated_init(void)
> > >          if (!xen_domain())
> > >                  return -ENODEV;
> > > 
> > > +       dma_pool = gen_pool_create(PAGE_SHIFT, NUMA_NO_NODE);
> > > +       if (!dma_pool) {
> > > +               pr_err("xen:unpopulated: Cannot create DMA pool\n");
> > > +               return -ENOMEM;
> > > +       }
> > > +
> > > +       gen_pool_set_algo(dma_pool, gen_pool_best_fit, NULL);
> > > +
> > >          ret = arch_xen_unpopulated_init(&target_resource);
> > >          if (ret) {
> > >                  pr_err("xen:unpopulated: Cannot initialize target
> > > resource\n");
> > > +               gen_pool_destroy(dma_pool);
> > > +               dma_pool = NULL;
> > >                  target_resource = NULL;
> > >          }
> > > 
> > > [snip]
> > > 
> > > 
> > > I think, regarding on the approach we would likely need to do some
> > > renaming
> > > for fill_list, page_list, list_lock, etc.
> > > 
> > > 
> > > Both options work in my Arm64 based environment, not sure regarding x86.
> > > Or do we have another option here?
> > > I would be happy to go any route. What do you think?
> > The second option (use "dma_pool" for all) looks great, thank you for
> > looking into it!
> 
> 
> ok, great
> 
> 
> May I please clarify a few moments before starting preparing non-RFC version:
> 
> 
> 1. According to the discussion at "[RFC PATCH 2/2] xen/grant-table: Use
> unpopulated DMAable pages instead of real RAM ones" we decided
> to stay away from the "dma" in the name, also the second option (use
> "dma_pool" for all) implies dropping the "page_list" entirely, so I am going
> to do some renaming:
> 
> - s/xen_alloc_unpopulated_dma_pages()/xen_alloc_unpopulated_contiguous_pages()
> - s/dma_pool/unpopulated_pool
> - s/list_lock/pool_lock
> - s/fill_list()/fill_pool()
> 
> Any objections?
 
Looks good

 
> 2. I don't like much the fact that in free_unpopulated_pages() we have to free
> "page by page" if contiguous is false, but unfortunately we cannot avoid doing
> that.
> I noticed that many users of unpopulated pages retain initially allocated
> pages[i] array, so it passed here absolutely unmodified since being allocated,
> but there is a code (for example, gnttab_page_cache_shrink() in grant-table.c)
> that can pass pages[i] which contains arbitrary pages.
> 
> static void free_unpopulated_pages(unsigned int nr_pages, struct page **pages,
>         bool contiguous)
> {
> 
> [snip]
> 
>     /* XXX Do we need to check the range (gen_pool_has_addr)? */
>     if (contiguous)
>         gen_pool_free(dma_pool, (unsigned long)page_to_virt(pages[0]),
>                 nr_pages * PAGE_SIZE);
>     else {
>         unsigned int i;
> 
>         for (i = 0; i < nr_pages; i++)
>             gen_pool_free(dma_pool, (unsigned long)page_to_virt(pages[i]),
>                     PAGE_SIZE);
>     }
> 
> [snip]
> 
> }
> 
> I think, it wouldn't be a big deal for the small allocations, but for the big
> allocations it might be not optimal for the speed.
> 
> What do you think if we update some places which always require big
> allocations to allocate (and free) contiguous pages instead?
> The possible candidate is
> gem_create()/xen_drm_front_gem_free_object_unlocked() in
> drivers/gpu/drm/xen/xen_drm_front_gem.c.
> OTOH I realize this might be inefficient use of resources. Or better not?
 
Yes I think it is a good idea, more on this below.

 
> 3. The alloc_unpopulated_pages() might be optimized for the non-contiguous
> allocations, currently we always try to allocate a single chunk even if
> contiguous is false.
> 
> static int alloc_unpopulated_pages(unsigned int nr_pages, struct page **pages,
>         bool contiguous)
> {
> 
> [snip]
> 
>     /* XXX: Optimize for non-contiguous allocations */
>     while (!(vaddr = (void *)gen_pool_alloc(dma_pool, nr_pages * PAGE_SIZE)))
> {
>         if (filled)
>             ret = -ENOMEM;
>         else {
>             ret = fill_list(nr_pages);
>             filled = true;
>         }
>         if (ret)
>             goto out;
>     }
> 
> [snip]
> 
> }
> 
> 
> But we can allocate "page by page" for the non-contiguous allocations, it
> might be not optimal for the speed, but would be optimal for the resource
> usage. What do you think?
> 
> static int alloc_unpopulated_pages(unsigned int nr_pages, struct page **pages,
>         bool contiguous)
> {
> 
> [snip]
> 
>     if (contiguous) {
>         while (!(vaddr = (void *)gen_pool_alloc(dma_pool, nr_pages *
> PAGE_SIZE))) {
>             if (filled)
>                 ret = -ENOMEM;
>             else {
>                 ret = fill_list(nr_pages);
>                 filled = true;
>             }
>             if (ret)
>                 goto out;
>         }
> 
>         for (i = 0; i < nr_pages; i++)
>             pages[i] = virt_to_page(vaddr + PAGE_SIZE * i);
>     } else {
>         if (gen_pool_avail(dma_pool) < nr_pages) {
>             ret = fill_list(nr_pages - gen_pool_avail(dma_pool));
>             if (ret)
>                 goto out;
>         }
> 
>         for (i = 0; i < nr_pages; i++) {
>             vaddr = (void *)gen_pool_alloc(dma_pool, PAGE_SIZE);
>             if (!vaddr) {
>                 while (i--)
>                     gen_pool_free(dma_pool, (unsigned
> long)page_to_virt(pages[i]),
>                             PAGE_SIZE);
> 
>                 ret = -ENOMEM;
>                 goto out;
>             }
> 
>             pages[i] = virt_to_page(vaddr);
>         }
>     }
> 
> [snip]
> 
> }

Basically, if we allocate (and free) page-by-page it leads to more
efficient resource utilization but it is slower. If we allocate larger
contiguous chunks it is faster but it leads to less efficient resource
utilization.

Given that on both x86 and ARM the unpopulated memory resource is
arbitrarily large, I don't think we need to worry about resource
utilization. It is not backed by real memory. The only limitation is the
address space size which is very large.

So I would say optimize for speed and use larger contiguous chunks even
when continuity is not strictly required.

Re: [RFC PATCH 2/2] xen/grant-table: Use unpopulated DMAable pages instead of real RAM ones

[Stefano Stabellini]

[-- Attachment #1: Type: text/plain, Size: 6464 bytes --]

On Tue, 14 Jun 2022, Oleksandr wrote:
> On 11.06.22 02:55, Stefano Stabellini wrote:
> 
> Hello Stefano
> 
> > On Thu, 9 Jun 2022, Oleksandr wrote:
> > > On 04.06.22 00:19, Stefano Stabellini wrote:
> > > Hello Stefano
> > > 
> > > Thank you for having a look and sorry for the late response.
> > > 
> > > > On Tue, 17 May 2022, Oleksandr Tyshchenko wrote:
> > > > > From: Oleksandr Tyshchenko <oleksandr_tyshchenko@epam.com>
> > > > > 
> > > > > Depends on CONFIG_XEN_UNPOPULATED_ALLOC. If enabled then unpopulated
> > > > > DMAable (contiguous) pages will be allocated for grant mapping into
> > > > > instead of ballooning out real RAM pages.
> > > > > 
> > > > > TODO: Fallback to real RAM pages if xen_alloc_unpopulated_dma_pages()
> > > > > fails.
> > > > > 
> > > > > Signed-off-by: Oleksandr Tyshchenko <oleksandr_tyshchenko@epam.com>
> > > > > ---
> > > > >    drivers/xen/grant-table.c | 27 +++++++++++++++++++++++++++
> > > > >    1 file changed, 27 insertions(+)
> > > > > 
> > > > > diff --git a/drivers/xen/grant-table.c b/drivers/xen/grant-table.c
> > > > > index 8ccccac..2bb4392 100644
> > > > > --- a/drivers/xen/grant-table.c
> > > > > +++ b/drivers/xen/grant-table.c
> > > > > @@ -864,6 +864,25 @@ EXPORT_SYMBOL_GPL(gnttab_free_pages);
> > > > >     */
> > > > >    int gnttab_dma_alloc_pages(struct gnttab_dma_alloc_args *args)
> > > > >    {
> > > > > +#ifdef CONFIG_XEN_UNPOPULATED_ALLOC
> > > > > +	int ret;
> > > > This is an alternative implementation of the same function.
> > > Currently, yes.
> > > 
> > > 
> > > >    If we are
> > > > going to use #ifdef, then I would #ifdef the entire function, rather
> > > > than just the body. Otherwise within the function body we can use
> > > > IS_ENABLED.
> > > 
> > > Good point. Note, there is one missing thing in current patch which is
> > > described in TODO.
> > > 
> > > "Fallback to real RAM pages if xen_alloc_unpopulated_dma_pages() fails." 
> > > So I
> > > will likely use IS_ENABLED within the function body.
> > > 
> > > If CONFIG_XEN_UNPOPULATED_ALLOC is enabled then gnttab_dma_alloc_pages()
> > > will
> > > try to call xen_alloc_unpopulated_dma_pages() the first and if fails then
> > > fallback to allocate RAM pages and balloon them out.
> > > 
> > > One moment is not entirely clear to me. If we use fallback in
> > > gnttab_dma_alloc_pages() then we must use fallback in
> > > gnttab_dma_free_pages()
> > > as well, we cannot use xen_free_unpopulated_dma_pages() for real RAM
> > > pages.
> > > The question is how to pass this information to the
> > > gnttab_dma_free_pages()?
> > > The first idea which comes to mind is to add a flag to struct
> > > gnttab_dma_alloc_args...
> >   You can check if the page is within the mhp_range range or part of
> > iomem_resource? If not, you can free it as a normal page.
> > 
> > If we do this, then the fallback is better implemented in
> > unpopulated-alloc.c because that is the one that is aware about
> > page addresses.
> 
> 
> I got your idea and agree this can work technically. Or if we finally decide
> to use the second option (use "dma_pool" for all) in the first patch
> "[RFC PATCH 1/2] xen/unpopulated-alloc: Introduce helpers for DMA allocations"
> then we will likely be able to check whether a page in question
> is within a "dma_pool" using gen_pool_has_addr().
> 
> I am still wondering, can we avoid the fallback implementation in
> unpopulated-alloc.c.
> Because for that purpose we will need to pull more code into
> unpopulated-alloc.c (to be more precise, almost everything which
> gnttab_dma_free_pages() already has except gnttab_pages_clear_private()) and
> pass more arguments to xen_free_unpopulated_dma_pages(). Also I might mistake,
> but having a fallback split between grant-table.c (to allocate RAM pages in
> gnttab_dma_alloc_pages()) and unpopulated-alloc.c (to free RAM pages in
> xen_free_unpopulated_dma_pages()) would look a bit weird.
> 
> I see two possible options for the fallback implementation in grant-table.c:
> 1. (less preferable) by introducing new flag in struct gnttab_dma_alloc_args
> 2. (more preferable) by guessing unpopulated (non real RAM) page using
> is_zone_device_page(), etc
> 
> 
> For example, with the second option the resulting code will look quite simple
> (only build tested):
> 
> diff --git a/drivers/xen/grant-table.c b/drivers/xen/grant-table.c
> index 738029d..3bda71f 100644
> --- a/drivers/xen/grant-table.c
> +++ b/drivers/xen/grant-table.c
> @@ -1047,6 +1047,23 @@ int gnttab_dma_alloc_pages(struct gnttab_dma_alloc_args
> *args)
>         size_t size;
>         int i, ret;
> 
> +       if (IS_ENABLED(CONFIG_XEN_UNPOPULATED_ALLOC)) {
> +               ret = xen_alloc_unpopulated_dma_pages(args->dev,
> args->nr_pages,
> +                               args->pages);
> +               if (ret < 0)
> +                       goto fallback;
> +
> +               ret = gnttab_pages_set_private(args->nr_pages, args->pages);
> +               if (ret < 0)
> +                       goto fail;
> +
> +               args->vaddr = page_to_virt(args->pages[0]);
> +               args->dev_bus_addr = page_to_phys(args->pages[0]);
> +
> +               return ret;
> +       }
> +
> +fallback:
>         size = args->nr_pages << PAGE_SHIFT;
>         if (args->coherent)
>                 args->vaddr = dma_alloc_coherent(args->dev, size,
> @@ -1103,6 +1120,12 @@ int gnttab_dma_free_pages(struct gnttab_dma_alloc_args
> *args)
> 
>         gnttab_pages_clear_private(args->nr_pages, args->pages);
> 
> +       if (IS_ENABLED(CONFIG_XEN_UNPOPULATED_ALLOC) &&
> +                       is_zone_device_page(args->pages[0])) {
> +               xen_free_unpopulated_dma_pages(args->dev, args->nr_pages,
> args->pages);
> +               return 0;
> +       }
> +
>         for (i = 0; i < args->nr_pages; i++)
>                 args->frames[i] = page_to_xen_pfn(args->pages[i]);
> 
> 
> What do you think?
 
I have another idea. Why don't we introduce a function implemented in
drivers/xen/unpopulated-alloc.c called is_xen_unpopulated_page() and
call it from here? is_xen_unpopulated_page can be implemented by using
gen_pool_has_addr.

Re: [RFC PATCH 2/2] xen/grant-table: Use unpopulated DMAable pages instead of real RAM ones

[Oleksandr]

On 15.06.22 03:51, Stefano Stabellini wrote:

Hello Stefano

> On Tue, 14 Jun 2022, Oleksandr wrote:
>> On 11.06.22 02:55, Stefano Stabellini wrote:
>>
>> Hello Stefano
>>
>>> On Thu, 9 Jun 2022, Oleksandr wrote:
>>>> On 04.06.22 00:19, Stefano Stabellini wrote:
>>>> Hello Stefano
>>>>
>>>> Thank you for having a look and sorry for the late response.
>>>>
>>>>> On Tue, 17 May 2022, Oleksandr Tyshchenko wrote:
>>>>>> From: Oleksandr Tyshchenko <oleksandr_tyshchenko@epam.com>
>>>>>>
>>>>>> Depends on CONFIG_XEN_UNPOPULATED_ALLOC. If enabled then unpopulated
>>>>>> DMAable (contiguous) pages will be allocated for grant mapping into
>>>>>> instead of ballooning out real RAM pages.
>>>>>>
>>>>>> TODO: Fallback to real RAM pages if xen_alloc_unpopulated_dma_pages()
>>>>>> fails.
>>>>>>
>>>>>> Signed-off-by: Oleksandr Tyshchenko <oleksandr_tyshchenko@epam.com>
>>>>>> ---
>>>>>>     drivers/xen/grant-table.c | 27 +++++++++++++++++++++++++++
>>>>>>     1 file changed, 27 insertions(+)
>>>>>>
>>>>>> diff --git a/drivers/xen/grant-table.c b/drivers/xen/grant-table.c
>>>>>> index 8ccccac..2bb4392 100644
>>>>>> --- a/drivers/xen/grant-table.c
>>>>>> +++ b/drivers/xen/grant-table.c
>>>>>> @@ -864,6 +864,25 @@ EXPORT_SYMBOL_GPL(gnttab_free_pages);
>>>>>>      */
>>>>>>     int gnttab_dma_alloc_pages(struct gnttab_dma_alloc_args *args)
>>>>>>     {
>>>>>> +#ifdef CONFIG_XEN_UNPOPULATED_ALLOC
>>>>>> +	int ret;
>>>>> This is an alternative implementation of the same function.
>>>> Currently, yes.
>>>>
>>>>
>>>>>     If we are
>>>>> going to use #ifdef, then I would #ifdef the entire function, rather
>>>>> than just the body. Otherwise within the function body we can use
>>>>> IS_ENABLED.
>>>> Good point. Note, there is one missing thing in current patch which is
>>>> described in TODO.
>>>>
>>>> "Fallback to real RAM pages if xen_alloc_unpopulated_dma_pages() fails."
>>>> So I
>>>> will likely use IS_ENABLED within the function body.
>>>>
>>>> If CONFIG_XEN_UNPOPULATED_ALLOC is enabled then gnttab_dma_alloc_pages()
>>>> will
>>>> try to call xen_alloc_unpopulated_dma_pages() the first and if fails then
>>>> fallback to allocate RAM pages and balloon them out.
>>>>
>>>> One moment is not entirely clear to me. If we use fallback in
>>>> gnttab_dma_alloc_pages() then we must use fallback in
>>>> gnttab_dma_free_pages()
>>>> as well, we cannot use xen_free_unpopulated_dma_pages() for real RAM
>>>> pages.
>>>> The question is how to pass this information to the
>>>> gnttab_dma_free_pages()?
>>>> The first idea which comes to mind is to add a flag to struct
>>>> gnttab_dma_alloc_args...
>>>    You can check if the page is within the mhp_range range or part of
>>> iomem_resource? If not, you can free it as a normal page.
>>>
>>> If we do this, then the fallback is better implemented in
>>> unpopulated-alloc.c because that is the one that is aware about
>>> page addresses.
>>
>> I got your idea and agree this can work technically. Or if we finally decide
>> to use the second option (use "dma_pool" for all) in the first patch
>> "[RFC PATCH 1/2] xen/unpopulated-alloc: Introduce helpers for DMA allocations"
>> then we will likely be able to check whether a page in question
>> is within a "dma_pool" using gen_pool_has_addr().
>>
>> I am still wondering, can we avoid the fallback implementation in
>> unpopulated-alloc.c.
>> Because for that purpose we will need to pull more code into
>> unpopulated-alloc.c (to be more precise, almost everything which
>> gnttab_dma_free_pages() already has except gnttab_pages_clear_private()) and
>> pass more arguments to xen_free_unpopulated_dma_pages(). Also I might mistake,
>> but having a fallback split between grant-table.c (to allocate RAM pages in
>> gnttab_dma_alloc_pages()) and unpopulated-alloc.c (to free RAM pages in
>> xen_free_unpopulated_dma_pages()) would look a bit weird.
>>
>> I see two possible options for the fallback implementation in grant-table.c:
>> 1. (less preferable) by introducing new flag in struct gnttab_dma_alloc_args
>> 2. (more preferable) by guessing unpopulated (non real RAM) page using
>> is_zone_device_page(), etc
>>
>>
>> For example, with the second option the resulting code will look quite simple
>> (only build tested):
>>
>> diff --git a/drivers/xen/grant-table.c b/drivers/xen/grant-table.c
>> index 738029d..3bda71f 100644
>> --- a/drivers/xen/grant-table.c
>> +++ b/drivers/xen/grant-table.c
>> @@ -1047,6 +1047,23 @@ int gnttab_dma_alloc_pages(struct gnttab_dma_alloc_args
>> *args)
>>          size_t size;
>>          int i, ret;
>>
>> +       if (IS_ENABLED(CONFIG_XEN_UNPOPULATED_ALLOC)) {
>> +               ret = xen_alloc_unpopulated_dma_pages(args->dev,
>> args->nr_pages,
>> +                               args->pages);
>> +               if (ret < 0)
>> +                       goto fallback;
>> +
>> +               ret = gnttab_pages_set_private(args->nr_pages, args->pages);
>> +               if (ret < 0)
>> +                       goto fail;
>> +
>> +               args->vaddr = page_to_virt(args->pages[0]);
>> +               args->dev_bus_addr = page_to_phys(args->pages[0]);
>> +
>> +               return ret;
>> +       }
>> +
>> +fallback:
>>          size = args->nr_pages << PAGE_SHIFT;
>>          if (args->coherent)
>>                  args->vaddr = dma_alloc_coherent(args->dev, size,
>> @@ -1103,6 +1120,12 @@ int gnttab_dma_free_pages(struct gnttab_dma_alloc_args
>> *args)
>>
>>          gnttab_pages_clear_private(args->nr_pages, args->pages);
>>
>> +       if (IS_ENABLED(CONFIG_XEN_UNPOPULATED_ALLOC) &&
>> +                       is_zone_device_page(args->pages[0])) {
>> +               xen_free_unpopulated_dma_pages(args->dev, args->nr_pages,
>> args->pages);
>> +               return 0;
>> +       }
>> +
>>          for (i = 0; i < args->nr_pages; i++)
>>                  args->frames[i] = page_to_xen_pfn(args->pages[i]);
>>
>>
>> What do you think?
>   
> I have another idea. Why don't we introduce a function implemented in
> drivers/xen/unpopulated-alloc.c called is_xen_unpopulated_page() and
> call it from here? is_xen_unpopulated_page can be implemented by using
> gen_pool_has_addr.

I like the idea, will do


-- 
Regards,

Oleksandr Tyshchenko

Re: [RFC PATCH 1/2] xen/unpopulated-alloc: Introduce helpers for DMA allocations

[Oleksandr]

On 15.06.22 03:45, Stefano Stabellini wrote:

Hello Stefano


> On Tue, 14 Jun 2022, Oleksandr wrote:
>> On 11.06.22 03:12, Stefano Stabellini wrote:
>>> On Wed, 8 Jun 2022, Oleksandr wrote:
>>>> 2. Drop the "page_list" entirely and use "dma_pool" for all (contiguous
>>>> and
>>>> non-contiguous) allocations. After all, all pages are initially contiguous
>>>> in
>>>> fill_list() as they are built from the resource. This changes behavior for
>>>> all
>>>> users of xen_alloc_unpopulated_pages()
>>>>
>>>> Below the diff for unpopulated-alloc.c. The patch is also available at:
>>>>
>>>> https://github.com/otyshchenko1/linux/commit/7be569f113a4acbdc4bcb9b20cb3995b3151387a
>>>>
>>>>
>>>> diff --git a/drivers/xen/unpopulated-alloc.c
>>>> b/drivers/xen/unpopulated-alloc.c
>>>> index a39f2d3..ab5c7bd 100644
>>>> --- a/drivers/xen/unpopulated-alloc.c
>>>> +++ b/drivers/xen/unpopulated-alloc.c
>>>> @@ -1,5 +1,7 @@
>>>>    // SPDX-License-Identifier: GPL-2.0
>>>> +#include <linux/dma-mapping.h>
>>>>    #include <linux/errno.h>
>>>> +#include <linux/genalloc.h>
>>>>    #include <linux/gfp.h>
>>>>    #include <linux/kernel.h>
>>>>    #include <linux/mm.h>
>>>> @@ -13,8 +15,8 @@
>>>>    #include <xen/xen.h>
>>>>
>>>>    static DEFINE_MUTEX(list_lock);
>>>> -static struct page *page_list;
>>>> -static unsigned int list_count;
>>>> +
>>>> +static struct gen_pool *dma_pool;
>>>>
>>>>    static struct resource *target_resource;
>>>>
>>>> @@ -36,7 +38,7 @@ static int fill_list(unsigned int nr_pages)
>>>>           struct dev_pagemap *pgmap;
>>>>           struct resource *res, *tmp_res = NULL;
>>>>           void *vaddr;
>>>> -       unsigned int i, alloc_pages = round_up(nr_pages,
>>>> PAGES_PER_SECTION);
>>>> +       unsigned int alloc_pages = round_up(nr_pages, PAGES_PER_SECTION);
>>>>           struct range mhp_range;
>>>>           int ret;
>>>>
>>>> @@ -106,6 +108,7 @@ static int fill_list(unsigned int nr_pages)
>>>>             * conflict with any devices.
>>>>             */
>>>>           if (!xen_feature(XENFEAT_auto_translated_physmap)) {
>>>> +               unsigned int i;
>>>>                   xen_pfn_t pfn = PFN_DOWN(res->start);
>>>>
>>>>                   for (i = 0; i < alloc_pages; i++) {
>>>> @@ -125,16 +128,17 @@ static int fill_list(unsigned int nr_pages)
>>>>                   goto err_memremap;
>>>>           }
>>>>
>>>> -       for (i = 0; i < alloc_pages; i++) {
>>>> -               struct page *pg = virt_to_page(vaddr + PAGE_SIZE * i);
>>>> -
>>>> -               pg->zone_device_data = page_list;
>>>> -               page_list = pg;
>>>> -               list_count++;
>>>> +       ret = gen_pool_add_virt(dma_pool, (unsigned long)vaddr,
>>>> res->start,
>>>> +                       alloc_pages * PAGE_SIZE, NUMA_NO_NODE);
>>>> +       if (ret) {
>>>> +               pr_err("Cannot add memory range to the pool\n");
>>>> +               goto err_pool;
>>>>           }
>>>>
>>>>           return 0;
>>>>
>>>> +err_pool:
>>>> +       memunmap_pages(pgmap);
>>>>    err_memremap:
>>>>           kfree(pgmap);
>>>>    err_pgmap:
>>>> @@ -149,51 +153,49 @@ static int fill_list(unsigned int nr_pages)
>>>>           return ret;
>>>>    }
>>>>
>>>> -/**
>>>> - * xen_alloc_unpopulated_pages - alloc unpopulated pages
>>>> - * @nr_pages: Number of pages
>>>> - * @pages: pages returned
>>>> - * @return 0 on success, error otherwise
>>>> - */
>>>> -int xen_alloc_unpopulated_pages(unsigned int nr_pages, struct page
>>>> **pages)
>>>> +static int alloc_unpopulated_pages(unsigned int nr_pages, struct page
>>>> **pages,
>>>> +               bool contiguous)
>>>>    {
>>>>           unsigned int i;
>>>>           int ret = 0;
>>>> +       void *vaddr;
>>>> +       bool filled = false;
>>>>
>>>>           /*
>>>>            * Fallback to default behavior if we do not have any suitable
>>>> resource
>>>>            * to allocate required region from and as the result we won't be
>>>> able
>>>> to
>>>>            * construct pages.
>>>>            */
>>>> -       if (!target_resource)
>>>> +       if (!target_resource) {
>>>> +               if (contiguous)
>>>> +                       return -ENODEV;
>>>> +
>>>>                   return xen_alloc_ballooned_pages(nr_pages, pages);
>>>> +       }
>>>>
>>>>           mutex_lock(&list_lock);
>>>> -       if (list_count < nr_pages) {
>>>> -               ret = fill_list(nr_pages - list_count);
>>>> +
>>>> +       while (!(vaddr = (void *)gen_pool_alloc(dma_pool, nr_pages *
>>>> PAGE_SIZE))) {
>>>> +               if (filled)
>>>> +                       ret = -ENOMEM;
>>>> +               else {
>>>> +                       ret = fill_list(nr_pages);
>>>> +                       filled = true;
>>>> +               }
>>>>                   if (ret)
>>>>                           goto out;
>>>>           }
>>>>
>>>>           for (i = 0; i < nr_pages; i++) {
>>>> -               struct page *pg = page_list;
>>>> -
>>>> -               BUG_ON(!pg);
>>>> -               page_list = pg->zone_device_data;
>>>> -               list_count--;
>>>> -               pages[i] = pg;
>>>> +               pages[i] = virt_to_page(vaddr + PAGE_SIZE * i);
>>>>
>>>>    #ifdef CONFIG_XEN_HAVE_PVMMU
>>>>                   if (!xen_feature(XENFEAT_auto_translated_physmap)) {
>>>> -                       ret = xen_alloc_p2m_entry(page_to_pfn(pg));
>>>> +                       ret = xen_alloc_p2m_entry(page_to_pfn(pages[i]));
>>>>                           if (ret < 0) {
>>>> -                               unsigned int j;
>>>> -
>>>> -                               for (j = 0; j <= i; j++) {
>>>> - pages[j]->zone_device_data = page_list;
>>>> -                                       page_list = pages[j];
>>>> -                                       list_count++;
>>>> -                               }
>>>> +                               /* XXX Do we need to zeroed pages[i]? */
>>>> +                               gen_pool_free(dma_pool, (unsigned
>>>> long)vaddr,
>>>> +                                               nr_pages * PAGE_SIZE);
>>>>                                   goto out;
>>>>                           }
>>>>                   }
>>>> @@ -204,32 +206,89 @@ int xen_alloc_unpopulated_pages(unsigned int
>>>> nr_pages,
>>>> struct page **pages)
>>>>           mutex_unlock(&list_lock);
>>>>           return ret;
>>>>    }
>>>> -EXPORT_SYMBOL(xen_alloc_unpopulated_pages);
>>>>
>>>> -/**
>>>> - * xen_free_unpopulated_pages - return unpopulated pages
>>>> - * @nr_pages: Number of pages
>>>> - * @pages: pages to return
>>>> - */
>>>> -void xen_free_unpopulated_pages(unsigned int nr_pages, struct page
>>>> **pages)
>>>> +static void free_unpopulated_pages(unsigned int nr_pages, struct page
>>>> **pages,
>>>> +               bool contiguous)
>>>>    {
>>>> -       unsigned int i;
>>>> -
>>>>           if (!target_resource) {
>>>> +               if (contiguous)
>>>> +                       return;
>>>> +
>>>>                   xen_free_ballooned_pages(nr_pages, pages);
>>>>                   return;
>>>>           }
>>>>
>>>>           mutex_lock(&list_lock);
>>>> -       for (i = 0; i < nr_pages; i++) {
>>>> -               pages[i]->zone_device_data = page_list;
>>>> -               page_list = pages[i];
>>>> -               list_count++;
>>>> +
>>>> +       /* XXX Do we need to check the range (gen_pool_has_addr)? */
>>>> +       if (contiguous)
>>>> +               gen_pool_free(dma_pool, (unsigned
>>>> long)page_to_virt(pages[0]),
>>>> +                               nr_pages * PAGE_SIZE);
>>>> +       else {
>>>> +               unsigned int i;
>>>> +
>>>> +               for (i = 0; i < nr_pages; i++)
>>>> +                       gen_pool_free(dma_pool, (unsigned
>>>> long)page_to_virt(pages[i]),
>>>> +                                       PAGE_SIZE);
>>>>           }
>>>> +
>>>>           mutex_unlock(&list_lock);
>>>>    }
>>>> +
>>>> +/**
>>>> + * xen_alloc_unpopulated_pages - alloc unpopulated pages
>>>> + * @nr_pages: Number of pages
>>>> + * @pages: pages returned
>>>> + * @return 0 on success, error otherwise
>>>> + */
>>>> +int xen_alloc_unpopulated_pages(unsigned int nr_pages, struct page
>>>> **pages)
>>>> +{
>>>> +       return alloc_unpopulated_pages(nr_pages, pages, false);
>>>> +}
>>>> +EXPORT_SYMBOL(xen_alloc_unpopulated_pages);
>>>> +
>>>> +/**
>>>> + * xen_free_unpopulated_pages - return unpopulated pages
>>>> + * @nr_pages: Number of pages
>>>> + * @pages: pages to return
>>>> + */
>>>> +void xen_free_unpopulated_pages(unsigned int nr_pages, struct page
>>>> **pages)
>>>> +{
>>>> +       free_unpopulated_pages(nr_pages, pages, false);
>>>> +}
>>>>    EXPORT_SYMBOL(xen_free_unpopulated_pages);
>>>>
>>>> +/**
>>>> + * xen_alloc_unpopulated_dma_pages - alloc unpopulated DMAable pages
>>>> + * @dev: valid struct device pointer
>>>> + * @nr_pages: Number of pages
>>>> + * @pages: pages returned
>>>> + * @return 0 on success, error otherwise
>>>> + */
>>>> +int xen_alloc_unpopulated_dma_pages(struct device *dev, unsigned int
>>>> nr_pages,
>>>> +               struct page **pages)
>>>> +{
>>>> +       /* XXX Handle devices which support 64-bit DMA address only for
>>>> now */
>>>> +       if (dma_get_mask(dev) != DMA_BIT_MASK(64))
>>>> +               return -EINVAL;
>>>> +
>>>> +       return alloc_unpopulated_pages(nr_pages, pages, true);
>>>> +}
>>>> +EXPORT_SYMBOL(xen_alloc_unpopulated_dma_pages);
>>>> +
>>>> +/**
>>>> + * xen_free_unpopulated_dma_pages - return unpopulated DMAable pages
>>>> + * @dev: valid struct device pointer
>>>> + * @nr_pages: Number of pages
>>>> + * @pages: pages to return
>>>> + */
>>>> +void xen_free_unpopulated_dma_pages(struct device *dev, unsigned int
>>>> nr_pages,
>>>> +               struct page **pages)
>>>> +{
>>>> +       free_unpopulated_pages(nr_pages, pages, true);
>>>> +}
>>>> +EXPORT_SYMBOL(xen_free_unpopulated_dma_pages);
>>>> +
>>>>    static int __init unpopulated_init(void)
>>>>    {
>>>>           int ret;
>>>> @@ -237,9 +296,19 @@ static int __init unpopulated_init(void)
>>>>           if (!xen_domain())
>>>>                   return -ENODEV;
>>>>
>>>> +       dma_pool = gen_pool_create(PAGE_SHIFT, NUMA_NO_NODE);
>>>> +       if (!dma_pool) {
>>>> +               pr_err("xen:unpopulated: Cannot create DMA pool\n");
>>>> +               return -ENOMEM;
>>>> +       }
>>>> +
>>>> +       gen_pool_set_algo(dma_pool, gen_pool_best_fit, NULL);
>>>> +
>>>>           ret = arch_xen_unpopulated_init(&target_resource);
>>>>           if (ret) {
>>>>                   pr_err("xen:unpopulated: Cannot initialize target
>>>> resource\n");
>>>> +               gen_pool_destroy(dma_pool);
>>>> +               dma_pool = NULL;
>>>>                   target_resource = NULL;
>>>>           }
>>>>
>>>> [snip]
>>>>
>>>>
>>>> I think, regarding on the approach we would likely need to do some
>>>> renaming
>>>> for fill_list, page_list, list_lock, etc.
>>>>
>>>>
>>>> Both options work in my Arm64 based environment, not sure regarding x86.
>>>> Or do we have another option here?
>>>> I would be happy to go any route. What do you think?
>>> The second option (use "dma_pool" for all) looks great, thank you for
>>> looking into it!
>>
>> ok, great
>>
>>
>> May I please clarify a few moments before starting preparing non-RFC version:
>>
>>
>> 1. According to the discussion at "[RFC PATCH 2/2] xen/grant-table: Use
>> unpopulated DMAable pages instead of real RAM ones" we decided
>> to stay away from the "dma" in the name, also the second option (use
>> "dma_pool" for all) implies dropping the "page_list" entirely, so I am going
>> to do some renaming:
>>
>> - s/xen_alloc_unpopulated_dma_pages()/xen_alloc_unpopulated_contiguous_pages()
>> - s/dma_pool/unpopulated_pool
>> - s/list_lock/pool_lock
>> - s/fill_list()/fill_pool()
>>
>> Any objections?
>   
> Looks good


thank you for the clarification


>
>   
>> 2. I don't like much the fact that in free_unpopulated_pages() we have to free
>> "page by page" if contiguous is false, but unfortunately we cannot avoid doing
>> that.
>> I noticed that many users of unpopulated pages retain initially allocated
>> pages[i] array, so it passed here absolutely unmodified since being allocated,
>> but there is a code (for example, gnttab_page_cache_shrink() in grant-table.c)
>> that can pass pages[i] which contains arbitrary pages.
>>
>> static void free_unpopulated_pages(unsigned int nr_pages, struct page **pages,
>>          bool contiguous)
>> {
>>
>> [snip]
>>
>>      /* XXX Do we need to check the range (gen_pool_has_addr)? */
>>      if (contiguous)
>>          gen_pool_free(dma_pool, (unsigned long)page_to_virt(pages[0]),
>>                  nr_pages * PAGE_SIZE);
>>      else {
>>          unsigned int i;
>>
>>          for (i = 0; i < nr_pages; i++)
>>              gen_pool_free(dma_pool, (unsigned long)page_to_virt(pages[i]),
>>                      PAGE_SIZE);
>>      }
>>
>> [snip]
>>
>> }
>>
>> I think, it wouldn't be a big deal for the small allocations, but for the big
>> allocations it might be not optimal for the speed.
>>
>> What do you think if we update some places which always require big
>> allocations to allocate (and free) contiguous pages instead?
>> The possible candidate is
>> gem_create()/xen_drm_front_gem_free_object_unlocked() in
>> drivers/gpu/drm/xen/xen_drm_front_gem.c.
>> OTOH I realize this might be inefficient use of resources. Or better not?
>   
> Yes I think it is a good idea, more on this below.

thanks


>
>   
>> 3. The alloc_unpopulated_pages() might be optimized for the non-contiguous
>> allocations, currently we always try to allocate a single chunk even if
>> contiguous is false.
>>
>> static int alloc_unpopulated_pages(unsigned int nr_pages, struct page **pages,
>>          bool contiguous)
>> {
>>
>> [snip]
>>
>>      /* XXX: Optimize for non-contiguous allocations */
>>      while (!(vaddr = (void *)gen_pool_alloc(dma_pool, nr_pages * PAGE_SIZE)))
>> {
>>          if (filled)
>>              ret = -ENOMEM;
>>          else {
>>              ret = fill_list(nr_pages);
>>              filled = true;
>>          }
>>          if (ret)
>>              goto out;
>>      }
>>
>> [snip]
>>
>> }
>>
>>
>> But we can allocate "page by page" for the non-contiguous allocations, it
>> might be not optimal for the speed, but would be optimal for the resource
>> usage. What do you think?
>>
>> static int alloc_unpopulated_pages(unsigned int nr_pages, struct page **pages,
>>          bool contiguous)
>> {
>>
>> [snip]
>>
>>      if (contiguous) {
>>          while (!(vaddr = (void *)gen_pool_alloc(dma_pool, nr_pages *
>> PAGE_SIZE))) {
>>              if (filled)
>>                  ret = -ENOMEM;
>>              else {
>>                  ret = fill_list(nr_pages);
>>                  filled = true;
>>              }
>>              if (ret)
>>                  goto out;
>>          }
>>
>>          for (i = 0; i < nr_pages; i++)
>>              pages[i] = virt_to_page(vaddr + PAGE_SIZE * i);
>>      } else {
>>          if (gen_pool_avail(dma_pool) < nr_pages) {
>>              ret = fill_list(nr_pages - gen_pool_avail(dma_pool));
>>              if (ret)
>>                  goto out;
>>          }
>>
>>          for (i = 0; i < nr_pages; i++) {
>>              vaddr = (void *)gen_pool_alloc(dma_pool, PAGE_SIZE);
>>              if (!vaddr) {
>>                  while (i--)
>>                      gen_pool_free(dma_pool, (unsigned
>> long)page_to_virt(pages[i]),
>>                              PAGE_SIZE);
>>
>>                  ret = -ENOMEM;
>>                  goto out;
>>              }
>>
>>              pages[i] = virt_to_page(vaddr);
>>          }
>>      }
>>
>> [snip]
>>
>> }
> Basically, if we allocate (and free) page-by-page it leads to more
> efficient resource utilization but it is slower.

yes, I think the same


>   If we allocate larger
> contiguous chunks it is faster but it leads to less efficient resource
> utilization.

yes, I think the same


>
> Given that on both x86 and ARM the unpopulated memory resource is
> arbitrarily large, I don't think we need to worry about resource
> utilization. It is not backed by real memory. The only limitation is the
> address space size which is very large.

I agree


>
> So I would say optimize for speed and use larger contiguous chunks even
> when continuity is not strictly required.

thank you for the clarification, will do


-- 
Regards,

Oleksandr Tyshchenko

Re: [RFC PATCH 1/2] xen/unpopulated-alloc: Introduce helpers for DMA allocations

[Roger Pau Monné]

On Tue, Jun 14, 2022 at 05:45:53PM -0700, Stefano Stabellini wrote:
> Basically, if we allocate (and free) page-by-page it leads to more
> efficient resource utilization but it is slower. If we allocate larger
> contiguous chunks it is faster but it leads to less efficient resource
> utilization.
> 
> Given that on both x86 and ARM the unpopulated memory resource is
> arbitrarily large, I don't think we need to worry about resource
> utilization. It is not backed by real memory. The only limitation is the
> address space size which is very large.

Well, unpopulated memory will consume memory to populate the related
metadata (ie: struct page and realted tracking information) for the
unpopulated region, so it's not completely free.

Thanks, Roger.