Re: [PATCH v4 2/2] mm, memcg: fix memory.peak reset clobbering other fds' watermark

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

On Fri, Aug 14, 2026 at 11:30:05AM +0800, Ridong Chen wrote:
> From: Ridong Chen <chenridong@xiaomi.com>
> 
> Writing to memory.peak resets the peak for that fd only. Each fd is a
> watcher and reads back max(its own value, the shared local_watermark).
> 
> peak_write() resets by lowering local_watermark to the current usage.
> To keep the other watchers' peaks it then walks the watcher list, but it
> stores the current usage into them instead of the old watermark. So once
> usage has dropped from a peak, a reset on one fd wrongly drags every
> other fd's peak down too, even fds that never reset.
> 
> Reproduced on 7.2.0-rc5-next under QEMU, two fds A and B on one cgroup:
> B sees the peak (410624 KB), usage drops, then A resets -- and B's peak
> collapses to 1060 KB although B never reset. With this patch B keeps
> reading 410624 KB.
> 
> Fix: save the old watermark before lowering it and use that to floor the
> other watchers, so a reset only affects the fd that issued it.
> 
> Fixes: c6f53ed8f213 ("mm, memcg: cg2 memory{.swap,}.peak write handlers")
> Closes: https://sashiko.dev/#/patchset/20260807090000.1532495-1-ridong.chen@linux.dev
> Assisted-by: Claude:claude-opus-4-8
> Acked-by: Tao Cui <cuitao@kylinos.cn>
> Acked-by: Johannes Weiner <hannes@cmpxchg.org>
> Signed-off-by: Ridong Chen <chenridong@xiaomi.com>

Acked-by: Shakeel Butt <shakeel.butt@linux.dev>