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

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

From: Ridong Chen <chenridong@xiaomi.com>

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@linux.dev?part=1
[2] https://lore.kernel.org/all/20260730115314.1069089-1-ridong.chen@linux.dev/

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(-)

-- 
2.34.1