[PATCH v4 1/2] memcg: acquire peaks_lock when reading memory.peak

[Ridong Chen <ridong.chen@linux.dev>]

From: Ridong Chen <chenridong@xiaomi.com>

Sashiko reported that a reader can transiently observe a lower peak
within a race window [1]. peak_show() returns
max(local_watermark, ofp->value), but peak_write() updates those two
under peaks_lock while the reader takes no lock. The interleaving is:

  writer (reset on fd A)                 reader (fd B)
  ----------------------                 -------------
  usage = page_counter_read(pc)
  WRITE_ONCE(local_watermark, usage)
  // watermark lowered to usage
                                         lw  = READ_ONCE(local_watermark)
                                         // sees the lowered usage
                                         val = READ_ONCE(ofp->value)
                                         // B's value not updated yet
                                         return max(lw, val)
                                         // both low -> low peak
  WRITE_ONCE(peer_ctx->value, usage)
  // B updated, but too late

Fix it by acquiring peaks_lock when reading the peak, so the reader sees
a consistent snapshot of local_watermark and the per-fd values. The same
race applies to memory.swap.peak, which shares peaks_lock and the
peak_write() path, so take the lock there as well.

[1] https://sashiko.dev/#/patchset/20260730115314.1069089-1-ridong.chen@linux.dev?part=1
Fixes: c6f53ed8f213 ("mm, memcg: cg2 memory{.swap,}.peak write handlers")
Assisted-by: Claude:claude-opus-4-8
Signed-off-by: Ridong Chen <chenridong@xiaomi.com>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Acked-by: Shakeel Butt <shakeel.butt@linux.dev>
---
 mm/memcontrol.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 17da1f43b7d3..ceb43b855122 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -4714,6 +4714,7 @@ static int memory_peak_show(struct seq_file *sf, void *v)
 {
 	struct mem_cgroup *memcg = mem_cgroup_from_css(seq_css(sf));
 
+	guard(spinlock)(&memcg->peaks_lock);
 	return peak_show(sf, v, &memcg->memory);
 }
 
@@ -5859,6 +5860,7 @@ static int swap_peak_show(struct seq_file *sf, void *v)
 {
 	struct mem_cgroup *memcg = mem_cgroup_from_css(seq_css(sf));
 
+	guard(spinlock)(&memcg->peaks_lock);
 	return peak_show(sf, v, &memcg->swap);
 }
 
-- 
2.34.1