mm, memcg: fix memory.peak reset clobbering other fds' watermark

From: Ridong Chen <[email protected]>

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/[email protected]?part=1
[2] https://lore.kernel.org/all/[email protected]/

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