Re: [PATCH v4 0/2] mm, memcg: fix memory.peak reset clobbering other fds' watermark

[Ridong Chen]

On 8/14/2026 11:30 AM, Ridong Chen wrote:
> From: Ridong Chen <chenridong@xxxxxxxxxx>
>
> The memory.peak / memory.swap.peak per-fd watermark tracking has two
> issues. Each open fd is a watcher and reads back max(its own value, the
> shared local_watermark); both bugs live in that scheme.
>
> Worst case for both is the same and is userspace-visible: a reader of
> memory.peak (or memory.swap.peak) gets a value lower than the true peak,
> so a tool that sizes or bills a cgroup by its peak usage under-reports it.
>
> Patch 1 (read side) fixes the race Sashiko pointed out in the v1 review
> [1]: peak_show() inspects local_watermark and the per-fd values without
> holding peaks_lock, so a reader that races an unrelated peak_write()
> reset briefly observes the lowered value. Transient. It takes peaks_lock
> in the show path.
>
> Patch 2 (write side) fixes peak_write(): on a reset it stores the
> current usage into the other watchers instead of the old watermark, so
> once usage has dropped from a peak a reset on one fd drags every other
> fd's peak down too, even fds that never reset.
>
> ---
> Changes since v3:
> - Switch to guard(spinlock) in the peak readers, suggested by Muchun.
>
> Changes since v2:
> - Spell out the worst-case userspace-visible effect, per Andrew's
>    Go back to v1 [2].
>
> Changes since v1:
>   - New patch 1: hold peaks_lock in the peak readers (Sashiko).
>   - Patch 2: floor the peers with max(usage, local_watermark), mirroring
>     peak_show(), and skip the writing fd (Johannes Weiner).
>
> [1] https://sashiko.dev/#/patchset/20260730115314.1069089-1-ridong.chen@xxxxxxxxx?part=1
> [2] https://lore.kernel.org/all/20260730115314.1069089-1-ridong.chen@xxxxxxxxx/
>
> Ridong Chen (2):
>    memcg: acquire peaks_lock when reading memory.peak
>    mm, memcg: fix memory.peak reset clobbering other fds' watermark
>
>   mm/memcontrol.c | 9 ++++++---
>   1 file changed, 6 insertions(+), 3 deletions(-)

Hi all,

Just a quick reminder on this series, could it be applied?
Sending this note in case it got buried.

--
Best regards
Ridong