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[2a03:2880:30ff:3::face:b00c]) by smtp.gmail.com with ESMTPSA id lu10-20020a170906faca00b00a52299d8eecsm8214771ejb.135.2024.04.24.05.59.56 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 24 Apr 2024 05:59:56 -0700 (PDT) From: Breno Leitao To: Johannes Weiner , Michal Hocko , Roman Gushchin , Shakeel Butt , Muchun Song , Andrew Morton Cc: leit@meta.com, cgroups@vger.kernel.org (open list:CONTROL GROUP - MEMORY RESOURCE CONTROLLER (MEMCG)), linux-mm@kvack.org (open list:CONTROL GROUP - MEMORY RESOURCE CONTROLLER (MEMCG)), linux-kernel@vger.kernel.org (open list) Subject: [PATCH] memcg: Fix data-race KCSAN bug in rstats Date: Wed, 24 Apr 2024 05:59:39 -0700 Message-ID: <20240424125940.2410718-1-leitao@debian.org> X-Mailer: git-send-email 2.43.0 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: quoted-printable Content-Type: text/plain; charset="utf-8" A data-race issue in memcg rstat occurs when two distinct code paths access the same 4-byte region concurrently. KCSAN detection triggers the following BUG as a result. BUG: KCSAN: data-race in __count_memcg_events / mem_cgroup_css_rstat_flush write to 0xffffe8ffff98e300 of 4 bytes by task 5274 on cpu 17: mem_cgroup_css_rstat_flush (mm/memcontrol.c:5850) cgroup_rstat_flush_locked (kernel/cgroup/rstat.c:243 (discriminator 7)) cgroup_rstat_flush (./include/linux/spinlock.h:401 kernel/cgroup/rstat.c:2= 78) mem_cgroup_flush_stats.part.0 (mm/memcontrol.c:767) memory_numa_stat_show (mm/memcontrol.c:6911) read to 0xffffe8ffff98e300 of 4 bytes by task 410848 on cpu 27: __count_memcg_events (mm/memcontrol.c:725 mm/memcontrol.c:962) count_memcg_event_mm.part.0 (./include/linux/memcontrol.h:1097 ./include/l= inux/memcontrol.h:1120) handle_mm_fault (mm/memory.c:5483 mm/memory.c:5622) value changed: 0x00000029 -> 0x00000000 The race occurs because two code paths access the same "stats_updates" location. Although "stats_updates" is a per-CPU variable, it is remotely accessed by another CPU at cgroup_rstat_flush_locked()->mem_cgroup_css_rstat_flush(), leading to the data race mentioned. Considering that memcg_rstat_updated() is in the hot code path, adding a lock to protect it may not be desirable, especially since this variable pertains solely to statistics. Therefore, annotating accesses to stats_updates with READ/WRITE_ONCE() can prevent KCSAN splats and potential partial reads/writes. Suggested-by: Shakeel Butt Signed-off-by: Breno Leitao Acked-by: Johannes Weiner Acked-by: Michal Hocko Acked-by: Shakeel Butt Reviewed-by: Yosry Ahmed --- mm/memcontrol.c | 12 +++++++----- 1 file changed, 7 insertions(+), 5 deletions(-) diff --git a/mm/memcontrol.c b/mm/memcontrol.c index fabce2b50c69..3c99457b36a1 100644 --- a/mm/memcontrol.c +++ b/mm/memcontrol.c @@ -715,6 +715,7 @@ static inline void memcg_rstat_updated(struct mem_cgrou= p *memcg, int val) { struct memcg_vmstats_percpu *statc; int cpu =3D smp_processor_id(); + unsigned int stats_updates; =20 if (!val) return; @@ -722,8 +723,9 @@ static inline void memcg_rstat_updated(struct mem_cgrou= p *memcg, int val) cgroup_rstat_updated(memcg->css.cgroup, cpu); statc =3D this_cpu_ptr(memcg->vmstats_percpu); for (; statc; statc =3D statc->parent) { - statc->stats_updates +=3D abs(val); - if (statc->stats_updates < MEMCG_CHARGE_BATCH) + stats_updates =3D READ_ONCE(statc->stats_updates) + abs(val); + WRITE_ONCE(statc->stats_updates, stats_updates); + if (stats_updates < MEMCG_CHARGE_BATCH) continue; =20 /* @@ -731,9 +733,9 @@ static inline void memcg_rstat_updated(struct mem_cgrou= p *memcg, int val) * redundant. Avoid the overhead of the atomic update. */ if (!memcg_vmstats_needs_flush(statc->vmstats)) - atomic64_add(statc->stats_updates, + atomic64_add(stats_updates, &statc->vmstats->stats_updates); - statc->stats_updates =3D 0; + WRITE_ONCE(statc->stats_updates, 0); } } =20 @@ -5845,7 +5847,7 @@ static void mem_cgroup_css_rstat_flush(struct cgroup_= subsys_state *css, int cpu) } } } - statc->stats_updates =3D 0; + WRITE_ONCE(statc->stats_updates, 0); /* We are in a per-cpu loop here, only do the atomic write once */ if (atomic64_read(&memcg->vmstats->stats_updates)) atomic64_set(&memcg->vmstats->stats_updates, 0); --=20 2.43.0