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

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 b3a7822e5e ("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.

Link: https://lore.kernel.org/20261021204818.3431099-1-shakeel.butt@linux.dev
Fixes: b3a7822e5e ("mm, memcg: prevent mem_cgroup_protected store tearing")
Signed-off-by: Shakeel Butt <shakeel.butt@linux.dev>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Reported-by: Sashiko <sashiko-bot@kernel.org>
Reviewed-by: Gregory Price (Meta) <gourry@gourry.net>
Reviewed-by: SJ Park <sj@kernel.org>
Acked-by: Muchun Song <muchun.song@linux.dev>
Reviewed-by: Anshuman Khandual <anshuman.khandual@arm.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Roman Gushchin <roman.gushchin@linux.dev>
Cc: Chris Down <chris@chrisdown.name>

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;
 	}