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Wed, 12 Aug 2026 23:46:15 +0000 X-Migadu-Flow: FLOW_OUT Date: Wed, 12 Aug 2026 16:46:09 -0700 From: Shakeel Butt To: Ridong Cc: Johannes Weiner , Michal Hocko , Roman Gushchin , Andrew Morton , Muchun Song , Tejun Heo , Michal =?utf-8?Q?Koutn=C3=BD?= , cgroups@vger.kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org, cui.tao@linux.dev, Ridong Chen Subject: Re: [PATCH v2 1/2] memcg: acquire peaks_lock when reading memory.peak Message-ID: References: <20260807090000.1532495-1-ridong.chen@linux.dev> <20260807090000.1532495-2-ridong.chen@linux.dev> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20260807090000.1532495-2-ridong.chen@linux.dev> X-Rspam-User: X-Stat-Signature: hbiwwrnfz5h4dr49xuazwms8eqzaw6oo X-Rspamd-Server: rspam01 X-Rspamd-Queue-Id: F0360140005 X-HE-Tag: 1786578376-296477 X-HE-Meta: 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 ZIOKKy7h T89ayoGPtYfz3clMXKjx8ZMckIuPhNM8oyBb+c3yzYQYDEKsf+pUA8patjzU0rUJrIMMDeQ/PDd6rBZEH4+Me5yGRy+8qzc+wek437yf/wFC12xezkGwXD+rtkjGVwGGjjqGA5IX0w8j+cDQug+C5WyV8FUpj4Syp4qACsziscKRJxdBRQVfR4S0hw+IhM+akS4b6Vw4HV6gWEsQPROY7rn5TQg6bwFHWUsdjVRbo4bO+NJ5gXsGuTsX2CIfUGcQv4jtX5mINQZcd/jbfndD+KRXQKmKxd13dBaee92uq3u8bSec7Dw1ML6iBLRVnwtVI0G9mxHHlrLujnvNEZ31BwYSY50bV44qZjgxJKrTyMgnFUu/bVUlf9HRvrBXhtGEbdnhbJFns8mIs2ahNjFdhbqMLt4PNYIvHFbSFRTrAM2VaXg2XhW2Rl3W6T6Xd20dMWfV2s1l23SSI9sIa71BkhLvs/alGkY1I5woo Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Fri, Aug 07, 2026 at 04:59:59PM +0800, Ridong wrote: > From: Ridong Chen > > Sashiko reported that a reader can transiently observe a lower peak > within a race window [1]. peak_show() returns > max(local_watermark, ofp->value), but peak_write() updates those two > under peaks_lock while the reader takes no lock. The interleaving is: > > writer (reset on fd A) reader (fd B) > ---------------------- ------------- > usage = page_counter_read(pc) > WRITE_ONCE(local_watermark, usage) > // watermark lowered to usage > lw = READ_ONCE(local_watermark) > // sees the lowered usage > val = READ_ONCE(ofp->value) > // B's value not updated yet > return max(lw, val) > // both low -> low peak > WRITE_ONCE(peer_ctx->value, usage) > // B updated, but too late > > Fix it by acquiring peaks_lock when reading the peak, so the reader sees > a consistent snapshot of local_watermark and the per-fd values. The same > race applies to memory.swap.peak, which shares peaks_lock and the > peak_write() path, so take the lock there as well. > > [1] https://sashiko.dev/#/patchset/20260730115314.1069089-1-ridong.chen@linux.dev?part=1 > Fixes: c6f53ed8f213 ("mm, memcg: cg2 memory{.swap,}.peak write handlers") > Assisted-by: Claude:claude-opus-4-8 > Signed-off-by: Ridong Chen Acked-by: Shakeel Butt