From: Ridong Chen 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 Acked-by: Johannes Weiner Acked-by: Shakeel Butt --- 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