Re: [PATCH v9 7/8] PCI/P2PDMA: Allow userspace VMA allocations through sysfs

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

On Thu, Aug 25, 2022 at 09:24:24AM -0600, Logan Gunthorpe wrote:
> Create a sysfs bin attribute called "allocate" under the existing
> "p2pmem" group. The only allowable operation on this file is the mmap()
> call.
> 
> When mmap() is called on this attribute, the kernel allocates a chunk of
> memory from the genalloc and inserts the pages into the VMA. The
> dev_pagemap .page_free callback will indicate when these pages are no
> longer used and they will be put back into the genalloc.
> 
> On device unbind, remove the sysfs file before the memremap_pages are
> cleaned up. This ensures unmap_mapping_range() is called on the files
> inode and no new mappings can be created.
> 
> Signed-off-by: Logan Gunthorpe <logang@deltatee.com>
> ---
>  drivers/pci/p2pdma.c | 124 +++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 124 insertions(+)
> 
> diff --git a/drivers/pci/p2pdma.c b/drivers/pci/p2pdma.c
> index 4496a7c5c478..a6ed6bbca214 100644
> --- a/drivers/pci/p2pdma.c
> +++ b/drivers/pci/p2pdma.c
> @@ -89,6 +89,90 @@ static ssize_t published_show(struct device *dev, struct device_attribute *attr,
>  }
>  static DEVICE_ATTR_RO(published);
>  
> +static int p2pmem_alloc_mmap(struct file *filp, struct kobject *kobj,
> +		struct bin_attribute *attr, struct vm_area_struct *vma)
> +{
> +	struct pci_dev *pdev = to_pci_dev(kobj_to_dev(kobj));
> +	size_t len = vma->vm_end - vma->vm_start;
> +	struct pci_p2pdma *p2pdma;
> +	struct percpu_ref *ref;
> +	unsigned long vaddr;
> +	void *kaddr;
> +	int ret;
> +
> +	/* prevent private mappings from being established */
> +	if ((vma->vm_flags & VM_MAYSHARE) != VM_MAYSHARE) {
> +		pci_info_ratelimited(pdev,
> +				     "%s: fail, attempted private mapping\n",
> +				     current->comm);
> +		return -EINVAL;
> +	}
> +
> +	if (vma->vm_pgoff) {
> +		pci_info_ratelimited(pdev,
> +				     "%s: fail, attempted mapping with non-zero offset\n",
> +				     current->comm);
> +		return -EINVAL;
> +	}
> +
> +	rcu_read_lock();
> +	p2pdma = rcu_dereference(pdev->p2pdma);
> +	if (!p2pdma) {
> +		ret = -ENODEV;
> +		goto out;
> +	}
> +
> +	kaddr = (void *)gen_pool_alloc_owner(p2pdma->pool, len, (void **)&ref);
> +	if (!kaddr) {
> +		ret = -ENOMEM;
> +		goto out;
> +	}
> +
> +	/*
> +	 * vm_insert_page() can sleep, so a reference is taken to mapping
> +	 * such that rcu_read_unlock() can be done before inserting the
> +	 * pages
> +	 */
> +	if (unlikely(!percpu_ref_tryget_live_rcu(ref))) {
> +		ret = -ENODEV;
> +		goto out_free_mem;
> +	}
> +	rcu_read_unlock();
> +
> +	for (vaddr = vma->vm_start; vaddr < vma->vm_end; vaddr += PAGE_SIZE) {
> +		ret = vm_insert_page(vma, vaddr, virt_to_page(kaddr));
> +		if (ret) {
> +			gen_pool_free(p2pdma->pool, (uintptr_t)kaddr, len);
> +			return ret;
> +		}
> +		percpu_ref_get(ref);
> +		put_page(virt_to_page(kaddr));
> +		kaddr += PAGE_SIZE;
> +		len -= PAGE_SIZE;
> +	}
> +
> +	percpu_ref_put(ref);
> +
> +	return 0;
> +out_free_mem:
> +	gen_pool_free(p2pdma->pool, (uintptr_t)kaddr, len);
> +out:
> +	rcu_read_unlock();
> +	return ret;
> +}
> +
> +static struct bin_attribute p2pmem_alloc_attr = {
> +	.attr = { .name = "allocate", .mode = 0660 },
> +	.mmap = p2pmem_alloc_mmap,
> +	/*
> +	 * Some places where we want to call mmap (ie. python) will check
> +	 * that the file size is greater than the mmap size before allowing
> +	 * the mmap to continue. To work around this, just set the size
> +	 * to be very large.
> +	 */
> +	.size = SZ_1T,
> +};
> +
>  static struct attribute *p2pmem_attrs[] = {
>  	&dev_attr_size.attr,
>  	&dev_attr_available.attr,
> @@ -96,11 +180,32 @@ static struct attribute *p2pmem_attrs[] = {
>  	NULL,
>  };
>  
> +static struct bin_attribute *p2pmem_bin_attrs[] = {
> +	&p2pmem_alloc_attr,
> +	NULL,
> +};
> +
>  static const struct attribute_group p2pmem_group = {
>  	.attrs = p2pmem_attrs,
> +	.bin_attrs = p2pmem_bin_attrs,
>  	.name = "p2pmem",
>  };
>  
> +static void p2pdma_page_free(struct page *page)
> +{
> +	struct pci_p2pdma_pagemap *pgmap = to_p2p_pgmap(page->pgmap);
> +	struct percpu_ref *ref;
> +
> +	gen_pool_free_owner(pgmap->provider->p2pdma->pool,
> +			    (uintptr_t)page_to_virt(page), PAGE_SIZE,
> +			    (void **)&ref);
> +	percpu_ref_put(ref);
> +}
> +
> +static const struct dev_pagemap_ops p2pdma_pgmap_ops = {
> +	.page_free = p2pdma_page_free,
> +};
> +
>  static void pci_p2pdma_release(void *data)
>  {
>  	struct pci_dev *pdev = data;
> @@ -152,6 +257,19 @@ static int pci_p2pdma_setup(struct pci_dev *pdev)
>  	return error;
>  }
>  
> +static void pci_p2pdma_unmap_mappings(void *data)
> +{
> +	struct pci_dev *pdev = data;
> +
> +	/*
> +	 * Removing the alloc attribute from sysfs will call
> +	 * unmap_mapping_range() on the inode, teardown any existing userspace
> +	 * mappings and prevent new ones from being created.
> +	 */
> +	sysfs_remove_file_from_group(&pdev->dev.kobj, &p2pmem_alloc_attr.attr,
> +				     p2pmem_group.name);

Wait, why are you manually removing the sysfs file here?  It's part of
the group, if you do this then it is gone for forever, right?  Why
manually do this the sysfs core should handle this for you if the device
is removed.

And worst case, just pass in the device, not the pci device.

thanks,

greg k-h