[PATCH mm-new v2] mm/memcontrol: Flush stats when write stat file

[Leon Huang Fu <leon.huangfu@shopee.com>]

On high-core count systems, memory cgroup statistics can become stale
due to per-CPU caching and deferred aggregation. Monitoring tools and
management applications sometimes need guaranteed up-to-date statistics
at specific points in time to make accurate decisions.

This patch adds write handlers to both memory.stat and memory.numa_stat
files to allow userspace to explicitly force an immediate flush of
memory statistics. When "1" is written to either file, it triggers
__mem_cgroup_flush_stats(memcg, true), which unconditionally flushes
all pending statistics for the cgroup and its descendants.

The write operation validates the input and only accepts the value "1",
returning -EINVAL for any other input.

Usage example:
  # Force immediate flush before reading critical statistics
  echo 1 > /sys/fs/cgroup/mygroup/memory.stat
  cat /sys/fs/cgroup/mygroup/memory.stat

This provides several benefits:

1. On-demand accuracy: Tools can flush only when needed, avoiding
   continuous overhead

2. Targeted flushing: Allows flushing specific cgroups when precision
   is required for particular workloads

3. Integration flexibility: Monitoring scripts can decide when to pay
   the flush cost based on their specific accuracy requirements

The implementation is shared between cgroup v1 and v2 interfaces,
with memory_stat_write() providing the common validation and flush
logic. Both memory.stat and memory.numa_stat use the same write
handler since they both benefit from forcing accurate statistics.

Documentation is updated to reflect that these files are now read-write
instead of read-only, with clear explanation of the write behavior.

Signed-off-by: Leon Huang Fu <leon.huangfu@shopee.com>
---
v1 -> v2:
  - Flush stats when write the file (per Michal).
  - https://lore.kernel.org/linux-mm/20251104031908.77313-1-leon.huangfu@shopee.com/

 Documentation/admin-guide/cgroup-v2.rst | 31 +++++++++++++++++--------
 mm/memcontrol-v1.c                      |  2 ++
 mm/memcontrol-v1.h                      |  1 +
 mm/memcontrol.c                         | 13 +++++++++++
 4 files changed, 37 insertions(+), 10 deletions(-)

diff --git a/Documentation/admin-guide/cgroup-v2.rst b/Documentation/admin-guide/cgroup-v2.rst
index 3345961c30ac..2a4a81d2cc2f 100644
--- a/Documentation/admin-guide/cgroup-v2.rst
+++ b/Documentation/admin-guide/cgroup-v2.rst
@@ -1337,7 +1337,7 @@ PAGE_SIZE multiple when read back.
 	cgroup is within its effective low boundary, the cgroup's
 	memory won't be reclaimed unless there is no reclaimable
 	memory available in unprotected cgroups.
-	Above the effective low	boundary (or
+	Above the effective low	boundary (or
 	effective min boundary if it is higher), pages are reclaimed
 	proportionally to the overage, reducing reclaim pressure for
 	smaller overages.
@@ -1525,11 +1525,17 @@ The following nested keys are defined.
 	generated on this file reflects only the local events.

   memory.stat
-	A read-only flat-keyed file which exists on non-root cgroups.
+	A read-write flat-keyed file which exists on non-root cgroups.

-	This breaks down the cgroup's memory footprint into different
-	types of memory, type-specific details, and other information
-	on the state and past events of the memory management system.
+	Reading this file breaks down the cgroup's memory footprint into
+	different types of memory, type-specific details, and other
+	information on the state and past events of the memory management
+	system.
+
+	Writing the value "1" to this file forces an immediate flush of
+	memory statistics for this cgroup and its descendants, improving
+	the accuracy of subsequent reads. Any other value will result in
+	an error.

 	All memory amounts are in bytes.

@@ -1786,11 +1792,16 @@ The following nested keys are defined.
 		cgroup is mounted with the memory_hugetlb_accounting option).

   memory.numa_stat
-	A read-only nested-keyed file which exists on non-root cgroups.
+	A read-write nested-keyed file which exists on non-root cgroups.
+
+	Reading this file breaks down the cgroup's memory footprint into
+	different types of memory, type-specific details, and other
+	information per node on the state of the memory management system.

-	This breaks down the cgroup's memory footprint into different
-	types of memory, type-specific details, and other information
-	per node on the state of the memory management system.
+	Writing the value "1" to this file forces an immediate flush of
+	memory statistics for this cgroup and its descendants, improving
+	the accuracy of subsequent reads. Any other value will result in
+	an error.

 	This is useful for providing visibility into the NUMA locality
 	information within an memcg since the pages are allowed to be
@@ -2173,7 +2184,7 @@ of the two is enforced.

 cgroup writeback requires explicit support from the underlying
 filesystem.  Currently, cgroup writeback is implemented on ext2, ext4,
-btrfs, f2fs, and xfs.  On other filesystems, all writeback IOs are
+btrfs, f2fs, and xfs.  On other filesystems, all writeback IOs are
 attributed to the root cgroup.

 There are inherent differences in memory and writeback management
diff --git a/mm/memcontrol-v1.c b/mm/memcontrol-v1.c
index 6eed14bff742..8cab6b52424b 100644
--- a/mm/memcontrol-v1.c
+++ b/mm/memcontrol-v1.c
@@ -2040,6 +2040,7 @@ struct cftype mem_cgroup_legacy_files[] = {
 	{
 		.name = "stat",
 		.seq_show = memory_stat_show,
+		.write_u64 = memory_stat_write,
 	},
 	{
 		.name = "force_empty",
@@ -2078,6 +2079,7 @@ struct cftype mem_cgroup_legacy_files[] = {
 	{
 		.name = "numa_stat",
 		.seq_show = memcg_numa_stat_show,
+		.write_u64 = memory_stat_write,
 	},
 #endif
 	{
diff --git a/mm/memcontrol-v1.h b/mm/memcontrol-v1.h
index 6358464bb416..1c92d58330aa 100644
--- a/mm/memcontrol-v1.h
+++ b/mm/memcontrol-v1.h
@@ -29,6 +29,7 @@ void drain_all_stock(struct mem_cgroup *root_memcg);
 unsigned long memcg_events(struct mem_cgroup *memcg, int event);
 unsigned long memcg_page_state_output(struct mem_cgroup *memcg, int item);
 int memory_stat_show(struct seq_file *m, void *v);
+int memory_stat_write(struct cgroup_subsys_state *css, struct cftype *cft, u64 val);

 void mem_cgroup_id_get_many(struct mem_cgroup *memcg, unsigned int n);
 struct mem_cgroup *mem_cgroup_id_get_online(struct mem_cgroup *memcg);
diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index c34029e92bab..d6a5d872fbcb 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -4531,6 +4531,17 @@ int memory_stat_show(struct seq_file *m, void *v)
 	return 0;
 }

+int memory_stat_write(struct cgroup_subsys_state *css, struct cftype *cft, u64 val)
+{
+	if (val != 1)
+		return -EINVAL;
+
+	if (css)
+		css_rstat_flush(css);
+
+	return 0;
+}
+
 #ifdef CONFIG_NUMA
 static inline unsigned long lruvec_page_state_output(struct lruvec *lruvec,
 						     int item)
@@ -4666,11 +4677,13 @@ static struct cftype memory_files[] = {
 	{
 		.name = "stat",
 		.seq_show = memory_stat_show,
+		.write_u64 = memory_stat_write,
 	},
 #ifdef CONFIG_NUMA
 	{
 		.name = "numa_stat",
 		.seq_show = memory_numa_stat_show,
+		.write_u64 = memory_stat_write,
 	},
 #endif
 	{
--
2.51.2