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[213.151.95.130]) by smtp.gmail.com with ESMTPSA id y20sm1499314wmi.23.2020.01.29.00.51.25 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 29 Jan 2020 00:51:25 -0800 (PST) Date: Wed, 29 Jan 2020 09:51:24 +0100 From: Michal Hocko To: Qian Cai Cc: akpm@linux-foundation.org, hannes@cmpxchg.org, elver@google.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH -next] mm/page_counter: mark intentional data races Message-ID: <20200129085124.GF24244@dhcp22.suse.cz> References: <20200129042019.3632-1-cai@lca.pw> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200129042019.3632-1-cai@lca.pw> X-Bogosity: Ham, tests=bogofilter, spamicity=0.000001, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On Tue 28-01-20 23:20:19, Qian Cai wrote: > The commit 3e32cb2e0a12 ("mm: memcontrol: lockless page counters") > had memcg->memsw->failcnt and ->watermark could be accessed concurrently > as reported by KCSAN, > > Reported by Kernel Concurrency Sanitizer on: > BUG: KCSAN: data-race in page_counter_try_charge / page_counter_try_charge > > read to 0xffff8fb18c4cd190 of 8 bytes by task 1081 on cpu 59: > page_counter_try_charge+0x4d/0x150 mm/page_counter.c:138 > try_charge+0x131/0xd50 > __memcg_kmem_charge_memcg+0x58/0x140 > __memcg_kmem_charge+0xcc/0x280 > __alloc_pages_nodemask+0x1e1/0x450 > alloc_pages_current+0xa6/0x120 > pte_alloc_one+0x17/0xd0 > __pte_alloc+0x3a/0x1f0 > copy_p4d_range+0xc36/0x1990 > copy_page_range+0x21d/0x360 > dup_mmap+0x5f5/0x7a0 > dup_mm+0xa2/0x240 > copy_process+0x1b3f/0x3460 > _do_fork+0xaa/0xa20 > __x64_sys_clone+0x13b/0x170 > do_syscall_64+0x91/0xb47 > entry_SYSCALL_64_after_hwframe+0x49/0xbe > > write to 0xffff8fb18c4cd190 of 8 bytes by task 1153 on cpu 120: > page_counter_try_charge+0x5b/0x150 mm/page_counter.c:139 > try_charge+0x131/0xd50 > mem_cgroup_try_charge+0x159/0x460 > mem_cgroup_try_charge_delay+0x3d/0xa0 > wp_page_copy+0x14d/0x930 > do_wp_page+0x107/0x7b0 > __handle_mm_fault+0xce6/0xd40 > handle_mm_fault+0xfc/0x2f0 > do_page_fault+0x263/0x6f9 > page_fault+0x34/0x40 > > Since the failcnt and watermark are tolerant of some inaccuracy, a data > race will not be harmful, thus mark them as intentional data races with > the data_race() macro. I am not familiar with KCSAN and git grep for data_race on the current linux-next doesn't really show any users of this macro. Is there a general consensus that data_race is going to be used to silence all KCSAN false positives? > Signed-off-by: Qian Cai > --- > mm/page_counter.c | 10 +++++----- > 1 file changed, 5 insertions(+), 5 deletions(-) > > diff --git a/mm/page_counter.c b/mm/page_counter.c > index de31470655f6..13934636eafd 100644 > --- a/mm/page_counter.c > +++ b/mm/page_counter.c > @@ -82,8 +82,8 @@ void page_counter_charge(struct page_counter *counter, unsigned long nr_pages) > * This is indeed racy, but we can live with some > * inaccuracy in the watermark. > */ > - if (new > c->watermark) > - c->watermark = new; > + if (data_race(new > c->watermark)) > + data_race(c->watermark = new); > } > } > > @@ -126,7 +126,7 @@ bool page_counter_try_charge(struct page_counter *counter, > * This is racy, but we can live with some > * inaccuracy in the failcnt. > */ > - c->failcnt++; > + data_race(c->failcnt++); > *fail = c; > goto failed; > } > @@ -135,8 +135,8 @@ bool page_counter_try_charge(struct page_counter *counter, > * Just like with failcnt, we can live with some > * inaccuracy in the watermark. > */ > - if (new > c->watermark) > - c->watermark = new; > + if (data_race(new > c->watermark)) > + data_race(c->watermark = new); > } > return true; > > -- > 2.21.0 (Apple Git-122.2) -- Michal Hocko SUSE Labs