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Subject: Re: [PATCH v2] mm: cma: support sysfs
To:     Minchan Kim <minchan@kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>
CC:     linux-mm <linux-mm@kvack.org>, LKML <linux-kernel@vger.kernel.org>,
        <gregkh@linuxfoundation.org>, <surenb@google.com>,
        <joaodias@google.com>, <willy@infradead.org>
References: <20210208180142.2765456-1-minchan@kernel.org>
From:   John Hubbard <jhubbard@nvidia.com>
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Date:   Mon, 8 Feb 2021 13:34:06 -0800
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On 2/8/21 10:01 AM, Minchan Kim wrote:
> Since CMA is getting used more widely, it's more important to
> keep monitoring CMA statistics for system health since it's
> directly related to user experience.
> 
> This patch introduces sysfs for the CMA and exposes stats below
> to keep monitor for telemetric in the system.

Or:

This patch introduces sysfs statistics for CMA, in order to provide
some basic monitoring of the CMA allocator.

> 
>   * the number of CMA page allocation attempts
>   * the number of CMA page allocation failures
> 
> With those per-CMA statistics, we could know how CMA allocadtion
> failure rate for each usecases.

Maybe:

These two values allow the user to calcuate the allocation
failure rate for each CMA area.

> 
> e.g.)
>    /sys/kernel/mm/cma/WIFI/cma_alloc_pages_[attempt|fail]
>    /sys/kernel/mm/cma/SENSOR/cma_alloc_pages_[attempt|fail]
>    /sys/kernel/mm/cma/BLUETOOTH/cma_alloc_pages_[attempt|fail]
> 
> Signed-off-by: Minchan Kim <minchan@kernel.org>
> ---
> 
>  From v1 - https://lore.kernel.org/linux-mm/20210203155001.4121868-1-minchan@kernel.org/
>   * fix sysfs build and refactoring - willy
>   * rename and drop some attributes - jhubbard
> 
>   Documentation/ABI/testing/sysfs-kernel-mm-cma |  25 ++++
>   mm/Kconfig                                    |   7 ++
>   mm/Makefile                                   |   1 +
>   mm/cma.c                                      |   6 +-
>   mm/cma.h                                      |  18 +++
>   mm/cma_sysfs.c                                | 114 ++++++++++++++++++
>   6 files changed, 170 insertions(+), 1 deletion(-)
>   create mode 100644 Documentation/ABI/testing/sysfs-kernel-mm-cma
>   create mode 100644 mm/cma_sysfs.c
> 
> diff --git a/Documentation/ABI/testing/sysfs-kernel-mm-cma b/Documentation/ABI/testing/sysfs-kernel-mm-cma
> new file mode 100644
> index 000000000000..68bdcc8c7681
> --- /dev/null
> +++ b/Documentation/ABI/testing/sysfs-kernel-mm-cma
> @@ -0,0 +1,25 @@
> +What:		/sys/kernel/mm/cma/
> +Date:		Feb 2021
> +Contact:	Minchan Kim <minchan@kernel.org>
> +Description:
> +		/sys/kernel/mm/cma/ contains a number of subdirectories by
> +		cma-heap name. The subdirectory contains a number of files
> +		to represent cma allocation statistics.
> +
> +		There are number of files under
> +				/sys/kernel/mm/cma/<cma-heap-name> directory
> +
> +			- cma_alloc_pages_attempt
> +			- cma_alloc_pages_fail

How about this instead:
Description:
		/sys/kernel/mm/cma/ contains a subdirectory for each CMA heap name (also
         sometimes called CMA areas).

         Each CMA heap subdirectory (that is, each
         /sys/kernel/mm/cma/<cma-heap-name> directory) contains the following
         items:

			cma_alloc_pages_attempt
			cma_alloc_pages_fail


> +
> +What:		/sys/kernel/mm/cma/<cma-heap-name>/cma_alloc_pages_attempt

Actually, shall we change that from "attempt" to "attempts"? Otherwise, the
language is a little odd there.

> +Date:		Feb 2021
> +Contact:	Minchan Kim <minchan@kernel.org>
> +Description:
> +		the number of pages CMA API tried to allocate
> +
> +What:		/sys/kernel/mm/cma/<cma-heap-name>/cma_alloc_pages_fail
> +Date:		Feb 2021
> +Contact:	Minchan Kim <minchan@kernel.org>
> +Description:
> +		the number of pages CMA API failed to allocate
> diff --git a/mm/Kconfig b/mm/Kconfig
> index ec35bf406439..ad7e9c065657 100644
> --- a/mm/Kconfig
> +++ b/mm/Kconfig
> @@ -513,6 +513,13 @@ config CMA_DEBUGFS
>   	help
>   	  Turns on the DebugFS interface for CMA.
>   
> +config CMA_SYSFS
> +	bool "CMA information through sysfs interface"
> +	depends on CMA && SYSFS
> +	help
> +	  This option exposes some sysfs attributes to get information
> +	  from CMA.
> +
>   config CMA_AREAS
>   	int "Maximum count of the CMA areas"
>   	depends on CMA
> diff --git a/mm/Makefile b/mm/Makefile
> index b2a564eec27f..0ae764e5b1a8 100644
> --- a/mm/Makefile
> +++ b/mm/Makefile
> @@ -109,6 +109,7 @@ obj-$(CONFIG_CMA)	+= cma.o
>   obj-$(CONFIG_MEMORY_BALLOON) += balloon_compaction.o
>   obj-$(CONFIG_PAGE_EXTENSION) += page_ext.o
>   obj-$(CONFIG_CMA_DEBUGFS) += cma_debug.o
> +obj-$(CONFIG_CMA_SYSFS)	+= cma_sysfs.o

Remove the unnecessary tab there, none of the other nearby lines have one.

>   obj-$(CONFIG_USERFAULTFD) += userfaultfd.o
>   obj-$(CONFIG_IDLE_PAGE_TRACKING) += page_idle.o
>   obj-$(CONFIG_DEBUG_PAGE_REF) += debug_page_ref.o
> diff --git a/mm/cma.c b/mm/cma.c
> index 23d4a97c834a..0611202d6e7d 100644
> --- a/mm/cma.c
> +++ b/mm/cma.c
> @@ -447,9 +447,10 @@ struct page *cma_alloc(struct cma *cma, size_t count, unsigned int align,
>   	offset = cma_bitmap_aligned_offset(cma, align);
>   	bitmap_maxno = cma_bitmap_maxno(cma);
>   	bitmap_count = cma_bitmap_pages_to_bits(cma, count);
> +	cma_sysfs_alloc_count(cma, count);
>   
>   	if (bitmap_count > bitmap_maxno)
> -		return NULL;
> +		goto out;
>   
>   	for (;;) {
>   		mutex_lock(&cma->lock);
> @@ -504,6 +505,9 @@ struct page *cma_alloc(struct cma *cma, size_t count, unsigned int align,
>   			__func__, count, ret);
>   		cma_debug_show_areas(cma);
>   	}
> +out:
> +	if (!page)
> +		cma_sysfs_fail(cma, count);
>   
>   	pr_debug("%s(): returned %p\n", __func__, page);
>   	return page;
> diff --git a/mm/cma.h b/mm/cma.h
> index 42ae082cb067..49a8ceddd9e8 100644
> --- a/mm/cma.h
> +++ b/mm/cma.h
> @@ -3,6 +3,14 @@
>   #define __MM_CMA_H__
>   
>   #include <linux/debugfs.h>
> +#include <linux/kobject.h>
> +
> +struct cma_stat {
> +	spinlock_t lock;
> +	unsigned long pages_attempt;	/* the number of CMA page allocation attempts */
> +	unsigned long pages_fail;	/* the number of CMA page allocation failures */
> +	struct kobject kobj;
> +};
>   
>   struct cma {
>   	unsigned long   base_pfn;
> @@ -16,6 +24,9 @@ struct cma {
>   	struct debugfs_u32_array dfs_bitmap;
>   #endif
>   	char name[CMA_MAX_NAME];
> +#ifdef CONFIG_CMA_SYSFS
> +	struct cma_stat	*stat;

This should not be a pointer. By making it a pointer, you've added a bunch of pointless
extra code to the implementation.

Here's a diff to implement the non-pointer way, and also to fix a build error in this
patch (missing semicolon):

diff --git a/Documentation/ABI/testing/sysfs-kernel-mm-cma 
b/Documentation/ABI/testing/sysfs-kernel-mm-cma
index 68bdcc8c7681..f3769b4e1a3c 100644
--- a/Documentation/ABI/testing/sysfs-kernel-mm-cma
+++ b/Documentation/ABI/testing/sysfs-kernel-mm-cma
@@ -2,15 +2,15 @@ What:		/sys/kernel/mm/cma/
  Date:		Feb 2021
  Contact:	Minchan Kim <minchan@kernel.org>
  Description:
-		/sys/kernel/mm/cma/ contains a number of subdirectories by
-		cma-heap name. The subdirectory contains a number of files
-		to represent cma allocation statistics.
+		/sys/kernel/mm/cma/ contains a subdirectory for each CMA heap name (also
+        sometimes called CMA areas).

-		There are number of files under
-				/sys/kernel/mm/cma/<cma-heap-name> directory
+        Each CMA heap subdirectory (that is, each
+        /sys/kernel/mm/cma/<cma-heap-name> directory) contains the following
+        items:

-			- cma_alloc_pages_attempt
-			- cma_alloc_pages_fail
+			cma_alloc_pages_attempt
+			cma_alloc_pages_fail

  What:		/sys/kernel/mm/cma/<cma-heap-name>/cma_alloc_pages_attempt
  Date:		Feb 2021
diff --git a/mm/cma.h b/mm/cma.h
index 49a8ceddd9e8..1e109830f553 100644
--- a/mm/cma.h
+++ b/mm/cma.h
@@ -25,7 +25,7 @@ struct cma {
  #endif
  	char name[CMA_MAX_NAME];
  #ifdef CONFIG_CMA_SYSFS
-	struct cma_stat	*stat;
+	struct cma_stat	stat;
  #endif
  };

diff --git a/mm/cma_sysfs.c b/mm/cma_sysfs.c
index 1f6b9f785825..52905694b6b7 100644
--- a/mm/cma_sysfs.c
+++ b/mm/cma_sysfs.c
@@ -11,20 +11,18 @@

  #include "cma.h"

-static struct cma_stat *cma_stats;
-
  void cma_sysfs_alloc_count(struct cma *cma, size_t count)
  {
-	spin_lock(&cma->stat->lock);
-	cma->stat->pages_attempt += count;
-	spin_unlock(&cma->stat->lock);
+	spin_lock(&cma->stat.lock);
+	cma->stat.pages_attempt += count;
+	spin_unlock(&cma->stat.lock);
  }

  void cma_sysfs_fail(struct cma *cma, size_t count)
  {
-	spin_lock(&cma->stat->lock);
-	cma->stat->pages_fail += count;
-	spin_unlock(&cma->stat->lock);
+	spin_lock(&cma->stat.lock);
+	cma->stat.pages_fail += count;
+	spin_unlock(&cma->stat.lock);
  }

  #define CMA_ATTR_RO(_name) \
@@ -50,13 +48,6 @@ static ssize_t cma_alloc_pages_fail_show(struct kobject *kobj,
  }
  CMA_ATTR_RO(cma_alloc_pages_fail);

-static void cma_kobj_release(struct kobject *kobj)
-{
-	struct cma_stat *stat = container_of(kobj, struct cma_stat, kobj);
-
-	kfree(stat);
-}
-
  static struct attribute *cma_attrs[] = {
  	&cma_alloc_pages_attempt_attr.attr,
  	&cma_alloc_pages_fail_attr.attr,
@@ -65,7 +56,6 @@ static struct attribute *cma_attrs[] = {
  ATTRIBUTE_GROUPS(cma);

  static struct kobj_type cma_ktype = {
-	.release = cma_kobj_release,
  	.sysfs_ops = &kobj_sysfs_ops,
  	.default_groups = cma_groups
  };
@@ -81,32 +71,23 @@ static int __init cma_sysfs_init(void)
  		return -ENOMEM;
  	}

-	cma_stats = kzalloc(array_size(sizeof(struct cma_stat),
-				cma_area_count), GFP_KERNEL);
-	if (!cma_stats) {
-		pr_err("failed to create cma_stats\n");
-		goto out;
-	}
-
  	do {
  		cma = &cma_areas[i];
-		cma->stat = &cma_stats[i];
-		spin_lock_init(&cma->stat->lock);
-		if (kobject_init_and_add(&cma->stat->kobj, &cma_ktype,
+		spin_lock_init(&cma->stat.lock);
+		if (kobject_init_and_add(&cma->stat.kobj, &cma_ktype,
  					cma_kobj, "%s", cma->name)) {
-			kobject_put(&cma->stat->kobj);
+			kobject_put(&cma->stat.kobj);
  			goto out;
  		}
-	} while (++i < cma_area_count)
+	} while (++i < cma_area_count);

  	return 0;
  out:
  	while (--i >= 0) {
  		cma = &cma_areas[i];
-		kobject_put(&cma->stat->kobj);
+		kobject_put(&cma->stat.kobj);
  	}

-	kfree(cma_stats);
  	kobject_put(cma_kobj);

  	return -ENOMEM;

> +#endif
>   };
>   
>   extern struct cma cma_areas[MAX_CMA_AREAS];
> @@ -26,4 +37,11 @@ static inline unsigned long cma_bitmap_maxno(struct cma *cma)
>   	return cma->count >> cma->order_per_bit;
>   }
>   
> +#ifdef CONFIG_CMA_SYSFS
> +void cma_sysfs_alloc_count(struct cma *cma, size_t count);
> +void cma_sysfs_fail(struct cma *cma, size_t count);
> +#else
> +static inline void cma_sysfs_alloc_count(struct cma *cma, size_t count) {};
> +static inline void cma_sysfs_fail(struct cma *cma, size_t count) {};
> +#endif
>   #endif
> diff --git a/mm/cma_sysfs.c b/mm/cma_sysfs.c
> new file mode 100644
> index 000000000000..1f6b9f785825
> --- /dev/null
> +++ b/mm/cma_sysfs.c
> @@ -0,0 +1,114 @@
> +// SPDX-License-Identifier: GPL-2.0
> +/*
> + * CMA SysFS Interface
> + *
> + * Copyright (c) 2021 Minchan Kim <minchan@kernel.org>
> + */
> +
> +#include <linux/cma.h>
> +#include <linux/kernel.h>
> +#include <linux/slab.h>
> +
> +#include "cma.h"
> +
> +static struct cma_stat *cma_stats;

I don't know what that's for but it definitely is not needed if you make cma.stat
not a pointer, and not in any other case either.

> +
> +void cma_sysfs_alloc_count(struct cma *cma, size_t count)
> +{
> +	spin_lock(&cma->stat->lock);
> +	cma->stat->pages_attempt += count;
> +	spin_unlock(&cma->stat->lock);
> +}
> +
> +void cma_sysfs_fail(struct cma *cma, size_t count)
> +{
> +	spin_lock(&cma->stat->lock);
> +	cma->stat->pages_fail += count;
> +	spin_unlock(&cma->stat->lock);
> +}
> +
> +#define CMA_ATTR_RO(_name) \
> +	static struct kobj_attribute _name##_attr = __ATTR_RO(_name)
> +
> +static struct kobject *cma_kobj;
> +
> +static ssize_t cma_alloc_pages_attempt_show(struct kobject *kobj,
> +			struct kobj_attribute *attr, char *buf)
> +{
> +	struct cma_stat *stat = container_of(kobj, struct cma_stat, kobj);
> +
> +	return sysfs_emit(buf, "%lu\n", stat->pages_attempt);
> +}
> +CMA_ATTR_RO(cma_alloc_pages_attempt);
> +
> +static ssize_t cma_alloc_pages_fail_show(struct kobject *kobj,
> +			struct kobj_attribute *attr, char *buf)
> +{
> +	struct cma_stat *stat = container_of(kobj, struct cma_stat, kobj);
> +
> +	return sysfs_emit(buf, "%lu\n", stat->pages_fail);
> +}
> +CMA_ATTR_RO(cma_alloc_pages_fail);
> +
> +static void cma_kobj_release(struct kobject *kobj)
> +{
> +	struct cma_stat *stat = container_of(kobj, struct cma_stat, kobj);
> +
> +	kfree(stat);
> +}
> +
> +static struct attribute *cma_attrs[] = {
> +	&cma_alloc_pages_attempt_attr.attr,
> +	&cma_alloc_pages_fail_attr.attr,
> +	NULL,
> +};
> +ATTRIBUTE_GROUPS(cma);
> +
> +static struct kobj_type cma_ktype = {
> +	.release = cma_kobj_release,
> +	.sysfs_ops = &kobj_sysfs_ops,
> +	.default_groups = cma_groups
> +};
> +
> +static int __init cma_sysfs_init(void)
> +{
> +	int i = 0;
> +	struct cma *cma;
> +
> +	cma_kobj = kobject_create_and_add("cma", mm_kobj);
> +	if (!cma_kobj) {
> +		pr_err("failed to create cma kobject\n");
> +		return -ENOMEM;
> +	}
> +
> +	cma_stats = kzalloc(array_size(sizeof(struct cma_stat),
> +				cma_area_count), GFP_KERNEL);
> +	if (!cma_stats) {
> +		pr_err("failed to create cma_stats\n");
> +		goto out;
> +	}
> +
> +	do {
> +		cma = &cma_areas[i];
> +		cma->stat = &cma_stats[i];
> +		spin_lock_init(&cma->stat->lock);
> +		if (kobject_init_and_add(&cma->stat->kobj, &cma_ktype,
> +					cma_kobj, "%s", cma->name)) {
> +			kobject_put(&cma->stat->kobj);
> +			goto out;
> +		}
> +	} while (++i < cma_area_count)

This clearly is not going to compile! Don't forget to build and test the
patches.

> +
> +	return 0;
> +out:
> +	while (--i >= 0) {
> +		cma = &cma_areas[i];
> +		kobject_put(&cma->stat->kobj);
> +	}
> +
> +	kfree(cma_stats);
> +	kobject_put(cma_kobj);
> +
> +	return -ENOMEM;
> +}
> +subsys_initcall(cma_sysfs_init);
> 

thanks,
-- 
John Hubbard
NVIDIA