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[109.81.32.216]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-49b0dd3e148sm44345705e9.1.2026.08.27.07.16.08 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 27 Aug 2026 07:16:08 -0700 (PDT) Date: Thu, 27 Aug 2026 16:16:07 +0200 From: Michal Hocko To: Jiayuan Chen Cc: linux-mm@kvack.org, stable@vger.kernel.org, syzbot+ced4d9a8cadb5ef3adae@syzkaller.appspotmail.com, Johannes Weiner , Roman Gushchin , Shakeel Butt , Muchun Song , Andrew Morton , Yosry Ahmed , Chris Li , Jens Axboe , Jan Kara , Tejun Heo , cgroups@vger.kernel.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH] mm/memcontrol: fix data-race on reading jiffies_64 Message-ID: References: <20260827025457.116191-1-jiayuan.chen@linux.dev> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20260827025457.116191-1-jiayuan.chen@linux.dev> On Thu 27-08-26 10:54:56, Jiayuan Chen wrote: > KCSAN reported a data-race between tick_do_update_jiffies64() updating > jiffies_64 and mem_cgroup_flush_stats_ratelimited() reading it directly. > > Unlike jiffies, jiffies_64 is not volatile, so raw reads are plain > accesses and can even be torn on 32-bit. Use get_jiffies_64() instead, > and fix the same pattern in mem_cgroup_flush_foreign(). Yes, this looks more correct. I do not expect this to have any practically visible effects, right? > Cc: > Fixes: 508bed884767 ("mm: memcg: change flush_next_time to flush_last_time") > Fixes: 97b27821b485 ("writeback, memcg: Implement foreign dirty flushing") > Reported-by: syzbot+ced4d9a8cadb5ef3adae@syzkaller.appspotmail.com > Signed-off-by: Jiayuan Chen Acked-by: Michal Hocko > --- > feel free to drop Fixes tag. > --- > mm/memcontrol.c | 6 +++--- > 1 file changed, 3 insertions(+), 3 deletions(-) > > diff --git a/mm/memcontrol.c b/mm/memcontrol.c > index 11b85f4b6828..5a3fe057debc 100644 > --- a/mm/memcontrol.c > +++ b/mm/memcontrol.c > @@ -757,7 +757,7 @@ static void __mem_cgroup_flush_stats(struct mem_cgroup *memcg, bool force) > return; > > if (mem_cgroup_is_root(memcg)) > - WRITE_ONCE(flush_last_time, jiffies_64); > + WRITE_ONCE(flush_last_time, get_jiffies_64()); > > css_rstat_flush(&memcg->css); > } > @@ -785,7 +785,7 @@ void mem_cgroup_flush_stats(struct mem_cgroup *memcg) > void mem_cgroup_flush_stats_ratelimited(struct mem_cgroup *memcg) > { > /* Only flush if the periodic flusher is one full cycle late */ > - if (time_after64(jiffies_64, READ_ONCE(flush_last_time) + 2*FLUSH_TIME)) > + if (time_after64(get_jiffies_64(), READ_ONCE(flush_last_time) + 2 * FLUSH_TIME)) > mem_cgroup_flush_stats(memcg); > } > > @@ -3945,7 +3945,7 @@ void mem_cgroup_flush_foreign(struct bdi_writeback *wb) > { > struct mem_cgroup *memcg = mem_cgroup_from_css(wb->memcg_css); > unsigned long intv = msecs_to_jiffies(dirty_expire_interval * 10); > - u64 now = jiffies_64; > + u64 now = get_jiffies_64(); > int i; > > for (i = 0; i < MEMCG_CGWB_FRN_CNT; i++) { > -- > 2.43.0 > -- Michal Hocko SUSE Labs