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DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.dev; s=key1; t=1785412460; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version: content-transfer-encoding:content-transfer-encoding; bh=ti2bSmazGTeJeWuqEsKxELAbB2h/8DFLycOObfhZA2c=; b=ltZxcP7t3+OjBynxx5eUHF4NPsMADw77gEwHiFN27N1wa/6uj5b0KtDS09qnl70esX3g/P OfpcZ1n9roz4V4PhzDlvoAlEoque5p/Xel7rhYs3tlBwdc+xDsCo56T6y4Y0NrQbh5VrYA p0n+Zb1zGpsF+8rNSKmuYp3ZeaJq6fU= From: Ridong To: Johannes Weiner , Michal Hocko , Roman Gushchin , Shakeel Butt , Andrew Morton Cc: Muchun Song , David Finkel , =?UTF-8?q?Michal=20Koutn=C3=BD?= , Tejun Heo , cgroups@vger.kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org, Ridong Chen , Ridong Chen Subject: [PATCH] mm, memcg: fix memory.peak reset clobbering other fds' watermark Date: Thu, 30 Jul 2026 19:53:14 +0800 Message-Id: <20260730115314.1069089-1-ridong.chen@linux.dev> MIME-Version: 1.0 Content-Transfer-Encoding: 8bit X-Migadu-Flow: FLOW_OUT X-Rspam-User: X-Rspamd-Server: rspam09 X-Rspamd-Queue-Id: 7AC9B1C0003 X-Stat-Signature: e4txxsociwkypkh74ot76fshxwgmwkjr X-HE-Tag: 1785412464-925062 X-HE-Meta: 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 FvJfnnvm iTR4Vi91j2f2+MOBX+pDPVAwEQhLPtP4YJNJgcFfSriZhzRUfD0ZyElqMkdpu/KrNQJFTGmvzRPxmzp/l8HuCSgIEWMxtx1iq7UCmkHLa0a0r+IisJis+qetLN9la4iejvPSX13zydvdj5YP1ADr4lUQueVjbjA9/j6Q4/Gn1UlltHLROYlEvIds1gJarTUr00mTDo5Ksm0gE42BvRQN9ulMgySsj4MS4QY0e Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: From: Ridong Chen 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") Assisted-by: Claude:claude-opus-4-8 Signed-off-by: Ridong Chen --- mm/memcontrol.c | 7 ++++--- 1 file changed, 4 insertions(+), 3 deletions(-) diff --git a/mm/memcontrol.c b/mm/memcontrol.c index 60145aadfc5e..881e7c459c64 100644 --- a/mm/memcontrol.c +++ b/mm/memcontrol.c @@ -4692,7 +4692,7 @@ static ssize_t peak_write(struct kernfs_open_file *of, char *buf, size_t nbytes, loff_t off, struct page_counter *pc, struct list_head *watchers) { - unsigned long usage; + unsigned long usage, old_watermark; struct cgroup_of_peak *peer_ctx; struct mem_cgroup *memcg = mem_cgroup_from_css(of_css(of)); struct cgroup_of_peak *ofp = of_peak(of); @@ -4700,11 +4700,12 @@ static ssize_t peak_write(struct kernfs_open_file *of, char *buf, size_t nbytes, spin_lock(&memcg->peaks_lock); usage = page_counter_read(pc); + old_watermark = READ_ONCE(pc->local_watermark); WRITE_ONCE(pc->local_watermark, usage); list_for_each_entry(peer_ctx, watchers, list) - if (usage > peer_ctx->value) - WRITE_ONCE(peer_ctx->value, usage); + if (peer_ctx != ofp && old_watermark > peer_ctx->value) + WRITE_ONCE(peer_ctx->value, old_watermark); /* initial write, register watcher */ if (ofp->value == OFP_PEAK_UNSET) -- 2.34.1