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Thu, 22 Oct 2026 02:18:34 +0000 X-Mizu-Trace-ID: 4b7912ba18c63ef4 X-Migadu-Flow: FLOW_OUT Content-Type: text/plain; charset=us-ascii X-Archived-By: Mime-Version: 1.0 (Mac OS X Mail 16.0 \(3901.100.1.1.12\)) Subject: Re: [PATCH] mm/page_counter: use WRITE_ONCE() for emin/elow when parent is the root From: Muchun Song In-Reply-To: <20261021204818.3431099-1-shakeel.butt@linux.dev> Date: Thu, 22 Oct 2026 10:18:17 +0800 Cc: Andrew Morton , Johannes Weiner , Michal Hocko , Roman Gushchin , Chris Down , Meta kernel team , linux-mm@kvack.org, cgroups@vger.kernel.org, linux-kernel@vger.kernel.org, Sashiko Content-Transfer-Encoding: quoted-printable Message-Id: References: <20261021204818.3431099-1-shakeel.butt@linux.dev> To: Shakeel Butt X-Mailer: Apple Mail (2.3901.100.1.1.12) X-Rspamd-Server: rspam10 X-Rspamd-Queue-Id: 300FF180002 X-Stat-Signature: g5xfur41xtssk4jm6ark64zmnu3oe3bb X-Spam-Status: No, score=-10.8 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_CR_TRAILER,MAILING_LIST_MULTI,SPF_HELO_PASS,SPF_PASS autolearn=ham autolearn_force=no version=4.0.1 X-Rspam-User: X-HE-Tag: 1792635517-692941 X-HE-Meta: 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 MANBAUG7 t3UAil0hAX+kdD6zB9GiYlJz1eZXM9u9wwLtHonF7Zd6oe9qYycyw/etQli6narfBdhz/oj8uJHE3FkRHBLe12iJTCFLrDWzbguM5PdVMIshJQNEloIV4ocj/+12rismAcyc/4kol8244dGJ5/RI0xatgzQPoBNwYXA69BKubxGA2iWJr9W+m9nfag7gf24L314jjAiLC2R6udhauhWLTwP6ZTuXhFlXBOTCyHxOAv7AsQlpvyEaxJLH5amqxaged7kggG2oPhCs5eeN2e9VJebd2YkVXjIdTrWrLtb09Oe87eeCcywc0Mi7PHfGIDFD51Y2OCYiC283GZrt8cIdtRw+DE0UEcJMbHjDo/A6a4PbhFxkDoCounyvdKg== Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: X-Spam-Checker-Version: SpamAssassin 4.0.1 (2024-03-26) on aws-us-west-2-korg-listserv-alma10-1 > On Oct 22, 2026, at 04:48, Shakeel Butt = wrote: >=20 > page_counter_calculate_protection() runs without locks. Several > reclaimers can update the same counter's emin and elow at the same > time, and readers such as mem_cgroup_below_min(), = mem_cgroup_protection() > and the dmem eviction code load them with READ_ONCE(). >=20 > Commit b3a7822e5e75 ("mm, memcg: prevent mem_cgroup_protected store > tearing") made the main path store emin and elow with WRITE_ONCE(), = but > it missed the early return for a counter whose parent is the reclaim > root. Those two stores are the only plain accesses to emin and elow > left. >=20 > With CONFIG_KCSAN_STRICT=3Dy, a few tasks writing to the same cgroup's > memory.reclaim trigger reports like this within seconds: >=20 > BUG: KCSAN: data-race in page_counter_calculate_protection / = shrink_node >=20 > read (marked) to 0xff1f91b146d40e40 of 8 bytes by task 163 on cpu 4: > shrink_node+0x57a/0x1210 > do_try_to_free_pages+0x194/0x930 > try_to_free_mem_cgroup_pages+0x204/0x3f0 > user_proactive_reclaim+0x28a/0x4d0 > memory_reclaim+0x35/0x50 > ... >=20 > write to 0xff1f91b146d40e40 of 8 bytes by task 167 on cpu 6: > page_counter_calculate_protection+0x1fa/0x360 > mem_cgroup_calculate_protection+0x4e/0x70 > shrink_node+0x53e/0x1210 > ... >=20 > The default KCSAN config does not report it, because it treats aligned > word-sized plain writes as atomic. >=20 > Use WRITE_ONCE() for these two stores as well. >=20 > There is no functional change. Readers already accept stale values, = and > compilers emit a single word-sized store either way: with GCC 14 the > x86-64 object code is the same before and after this change. >=20 > Fixes: b3a7822e5e75 ("mm, memcg: prevent mem_cgroup_protected store = tearing") > Reported-by: Sashiko > Signed-off-by: Shakeel Butt Acked-by: Muchun Song Thanks.