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Sat, 15 Aug 2026 00:38:35 +0000 X-Migadu-Flow: FLOW_OUT Date: Fri, 14 Aug 2026 17:38:30 -0700 From: Shakeel Butt To: Ridong Chen Cc: Johannes Weiner , Michal Hocko , Roman Gushchin , Andrew Morton , Muchun Song , David Finkel , Tejun Heo , "open list:CONTROL GROUP - MEMORY RESOURCE CONTROLLER (MEMCG)" , "open list:CONTROL GROUP - MEMORY RESOURCE CONTROLLER (MEMCG)" , linux-kernel@vger.kernel.org, Tao Cui , Ridong Chen Subject: Re: [PATCH v4 2/2] mm, memcg: fix memory.peak reset clobbering other fds' watermark Message-ID: References: <20260814033005.2481920-1-ridong.chen@linux.dev> <20260814033005.2481920-3-ridong.chen@linux.dev> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20260814033005.2481920-3-ridong.chen@linux.dev> X-Rspamd-Server: rspam02 X-Rspamd-Queue-Id: AC456C0004 X-Stat-Signature: de8c5rqcgaeegbnzp534zgwe1e514m7c X-Rspam-User: X-HE-Tag: 1786754316-804006 X-HE-Meta: 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 Ir/djLPY A2I4iHa7OJpE/T/JpVF2+qZ6+uASzRcaIm08FUvH35uvhClGfxRU0OD1qYQhs/JzLmu0yl8iXMLTlU7U7AQtFpcYW9m3f5/SawvOLz5iyvh7gFQfFcemdIrrd3uhqJvfKhc9wT7olF7SsJnV0cWrzz5by3Z62UlclRRHKwToEypVc0EWhF0CxqlP5AwLXol9hw6F/QA59M1bWhnlJWDXAywQMF4B2xW2nY5OLwzIlS8OLBjdqiXBrr40Qod/r3Szl9ZM8d+/XaW8jnlg/B2RMZd4PfiiUq73bZk4EiUpEUBR6+nnA0e5GI+HQt2slBFof+yy0/9ysvRswa09+quxkb73XWcak5eiUx/KBJdMfB8YYvpmMT/OakdlQkcAV/5aR7aJubcLn57XXifFzElaGSOhv8UmTZR41xyiNNdxmiF/n9bRG74EXypD/G/7ADvtCEuyCWsGT5KaXLgnmXl/45c22aA== Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Fri, Aug 14, 2026 at 11:30:05AM +0800, Ridong Chen wrote: > 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") > Closes: https://sashiko.dev/#/patchset/20260807090000.1532495-1-ridong.chen@linux.dev > Assisted-by: Claude:claude-opus-4-8 > Acked-by: Tao Cui > Acked-by: Johannes Weiner > Signed-off-by: Ridong Chen Acked-by: Shakeel Butt