mm/cma: add sysfs file 'release_pages_success'

This adds the following new sysfs file tracking the number of successfully
released pages from a given CMA heap area.  This file will be available
via CONFIG_CMA_SYSFS and help in determining active CMA pages available on
the CMA heap area.  This adds a new 'nr_pages_released' (CONFIG_CMA_SYSFS)
into 'struct cma' which gets updated during cma_release().

/sys/kernel/mm/cma/<cma-heap-area>/release_pages_success

After this change, an user will be able to find active CMA pages available
in a given CMA heap area via the following method.

Active pages = alloc_pages_success - release_pages_success

That's valuable information for both software designers, and system admins
as it allows them to tune the number of CMA pages available in the system.
This increases user visibility for allocated CMA area and its
utilization.

Link: https://lkml.kernel.org/r/20240206045731.472759-1-anshuman.khandual@arm.comSigned-off-by: Anshuman Khandual <anshuman.khandual@arm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

Documentation/ABI/testing/sysfs-kernel-mm-cma

@@ -23,3 +23,9 @@ Date:		Feb 2021
 Contact:	Minchan Kim <minchan@kernel.org>
 Description:
 		the number of pages CMA API failed to allocate
+
+What:		/sys/kernel/mm/cma/<cma-heap-name>/release_pages_success
+Date:		Feb 2024
+Contact:	Anshuman Khandual <anshuman.khandual@arm.com>
+Description:
+		the number of pages CMA API succeeded to release

mm/cma.c

@@ -562,6 +562,7 @@ bool cma_release(struct cma *cma, const struct page *pages,
 
 	free_contig_range(pfn, count);
 	cma_clear_bitmap(cma, pfn, count);
+	cma_sysfs_account_release_pages(cma, count);
 	trace_cma_release(cma->name, pfn, pages, count);
 
 	return true;

mm/cma.h

@@ -27,6 +27,8 @@ struct cma {
 	atomic64_t nr_pages_succeeded;
 	/* the number of CMA page allocation failures */
 	atomic64_t nr_pages_failed;
+	/* the number of CMA page released */
+	atomic64_t nr_pages_released;
 	/* kobject requires dynamic object */
 	struct cma_kobject *cma_kobj;
 #endif
@@ -44,10 +46,13 @@ static inline unsigned long cma_bitmap_maxno(struct cma *cma)
 #ifdef CONFIG_CMA_SYSFS
 void cma_sysfs_account_success_pages(struct cma *cma, unsigned long nr_pages);
 void cma_sysfs_account_fail_pages(struct cma *cma, unsigned long nr_pages);
+void cma_sysfs_account_release_pages(struct cma *cma, unsigned long nr_pages);
 #else
 static inline void cma_sysfs_account_success_pages(struct cma *cma,
 						   unsigned long nr_pages) {};
 static inline void cma_sysfs_account_fail_pages(struct cma *cma,
 						unsigned long nr_pages) {};
+static inline void cma_sysfs_account_release_pages(struct cma *cma,
+						   unsigned long nr_pages) {};
 #endif
 #endif

mm/cma_sysfs.c

@@ -24,6 +24,11 @@ void cma_sysfs_account_fail_pages(struct cma *cma, unsigned long nr_pages)
 	atomic64_add(nr_pages, &cma->nr_pages_failed);
 }
 
+void cma_sysfs_account_release_pages(struct cma *cma, unsigned long nr_pages)
+{
+	atomic64_add(nr_pages, &cma->nr_pages_released);
+}
+
 static inline struct cma *cma_from_kobj(struct kobject *kobj)
 {
 	return container_of(kobj, struct cma_kobject, kobj)->cma;
@@ -48,6 +53,15 @@ static ssize_t alloc_pages_fail_show(struct kobject *kobj,
 }
 CMA_ATTR_RO(alloc_pages_fail);
 
+static ssize_t release_pages_success_show(struct kobject *kobj,
+					  struct kobj_attribute *attr, char *buf)
+{
+	struct cma *cma = cma_from_kobj(kobj);
+
+	return sysfs_emit(buf, "%llu\n", atomic64_read(&cma->nr_pages_released));
+}
+CMA_ATTR_RO(release_pages_success);
+
 static void cma_kobj_release(struct kobject *kobj)
 {
 	struct cma *cma = cma_from_kobj(kobj);
@@ -60,6 +74,7 @@ static void cma_kobj_release(struct kobject *kobj)
 static struct attribute *cma_attrs[] = {
 	&alloc_pages_success_attr.attr,
 	&alloc_pages_fail_attr.attr,
+	&release_pages_success_attr.attr,
 	NULL,
 };
 ATTRIBUTE_GROUPS(cma);