[PATCH] mm/page_counter: use WRITE_ONCE() for emin/elow when parent is the root

[Shakeel Butt <shakeel.butt@linux.dev>]

page_counter_calculate_protection() runs without locks. Several
reclaimers can update the same counter's emin and elow at the same
time, and readers such as mem_cgroup_below_min(), mem_cgroup_protection()
and the dmem eviction code load them with READ_ONCE().

Commit b3a7822e5e75 ("mm, memcg: prevent mem_cgroup_protected store
tearing") made the main path store emin and elow with WRITE_ONCE(), but
it missed the early return for a counter whose parent is the reclaim
root. Those two stores are the only plain accesses to emin and elow
left.

With CONFIG_KCSAN_STRICT=y, a few tasks writing to the same cgroup's
memory.reclaim trigger reports like this within seconds:

  BUG: KCSAN: data-race in page_counter_calculate_protection / shrink_node

  read (marked) to 0xff1f91b146d40e40 of 8 bytes by task 163 on cpu 4:
   shrink_node+0x57a/0x1210
   do_try_to_free_pages+0x194/0x930
   try_to_free_mem_cgroup_pages+0x204/0x3f0
   user_proactive_reclaim+0x28a/0x4d0
   memory_reclaim+0x35/0x50
   ...

  write to 0xff1f91b146d40e40 of 8 bytes by task 167 on cpu 6:
   page_counter_calculate_protection+0x1fa/0x360
   mem_cgroup_calculate_protection+0x4e/0x70
   shrink_node+0x53e/0x1210
   ...

The default KCSAN config does not report it, because it treats aligned
word-sized plain writes as atomic.

Use WRITE_ONCE() for these two stores as well.

There is no functional change. Readers already accept stale values, and
compilers emit a single word-sized store either way: with GCC 14 the
x86-64 object code is the same before and after this change.

Fixes: b3a7822e5e75 ("mm, memcg: prevent mem_cgroup_protected store tearing")
Reported-by: Sashiko <sashiko-bot@kernel.org>
Signed-off-by: Shakeel Butt <shakeel.butt@linux.dev>
---
 mm/page_counter.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/mm/page_counter.c b/mm/page_counter.c
index 9167ffd1380c..5293460831f8 100644
--- a/mm/page_counter.c
+++ b/mm/page_counter.c
@@ -472,8 +472,8 @@ void page_counter_calculate_protection(struct page_counter *root,
 		return;
 
 	if (parent == root) {
-		counter->emin = READ_ONCE(counter->min);
-		counter->elow = READ_ONCE(counter->low);
+		WRITE_ONCE(counter->emin, READ_ONCE(counter->min));
+		WRITE_ONCE(counter->elow, READ_ONCE(counter->low));
 		return;
 	}
 
-- 
2.53.0-Meta