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[37.188.174.201]) by smtp.gmail.com with ESMTPSA id d9sm24638967wre.28.2020.06.15.08.01.29 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 15 Jun 2020 08:01:29 -0700 (PDT) Date: Mon, 15 Jun 2020 17:01:28 +0200 From: Michal Hocko To: Johannes Weiner Cc: Andrew Morton , Tejun Heo , Roman Gushchin , Chris Down , linux-mm@kvack.org, cgroups@vger.kernel.org, linux-kernel@vger.kernel.org, kernel-team@fb.com Subject: Re: [PATCH] mm: memcontrol: handle div0 crash race condition in memory.low Message-ID: <20200615150128.GL25296@dhcp22.suse.cz> References: <20200615140658.601684-1-hannes@cmpxchg.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200615140658.601684-1-hannes@cmpxchg.org> Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon 15-06-20 10:06:58, Johannes Weiner wrote: > Tejun reports seeing rare div0 crashes in memory.low stress testing: > > [37228.504582] RIP: 0010:mem_cgroup_calculate_protection+0xed/0x150 > [37228.505059] Code: 0f 46 d1 4c 39 d8 72 57 f6 05 16 d6 42 01 40 74 1f 4c 39 d8 76 1a 4c 39 d1 76 15 4c 29 d1 4c 29 d8 4d 29 d9 31 d2 48 0f af c1 <49> f7 f1 49 01 c2 4c 89 96 38 01 00 00 5d c3 48 0f af c7 31 d2 49 > [37228.506254] RSP: 0018:ffffa14e01d6fcd0 EFLAGS: 00010246 > [37228.506769] RAX: 000000000243e384 RBX: 0000000000000000 RCX: 0000000000008f4b > [37228.507319] RDX: 0000000000000000 RSI: ffff8b89bee84000 RDI: 0000000000000000 > [37228.507869] RBP: ffffa14e01d6fcd0 R08: ffff8b89ca7d40f8 R09: 0000000000000000 > [37228.508376] R10: 0000000000000000 R11: 00000000006422f7 R12: 0000000000000000 > [37228.508881] R13: ffff8b89d9617000 R14: ffff8b89bee84000 R15: ffffa14e01d6fdb8 > [37228.509397] FS: 0000000000000000(0000) GS:ffff8b8a1f1c0000(0000) knlGS:0000000000000000 > [37228.509917] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033 > [37228.510442] CR2: 00007f93b1fc175b CR3: 000000016100a000 CR4: 0000000000340ea0 > [37228.511076] Call Trace: > [37228.511561] shrink_node+0x1e5/0x6c0 > [37228.512044] balance_pgdat+0x32d/0x5f0 > [37228.512521] kswapd+0x1d7/0x3d0 > [37228.513346] ? wait_woken+0x80/0x80 > [37228.514170] kthread+0x11c/0x160 > [37228.514983] ? balance_pgdat+0x5f0/0x5f0 > [37228.515797] ? kthread_park+0x90/0x90 > [37228.516593] ret_from_fork+0x1f/0x30 > > This happens when parent_usage == siblings_protected. We check that > usage is bigger than protected, which should imply parent_usage being > bigger than siblings_protected. However, we don't read (or even > update) these values atomically, and they can be out of sync as the > memory state changes under us. A bit of fluctuation around the target > protection isn't a big deal, but we need to handle the div0 case. > > Check the parent state explicitly to make sure we have a reasonable > positive value for the divisor. > > Fixes: 8a931f801340 ("mm: memcontrol: recursive memory.low protection") > Reported-by: Tejun Heo > Cc: > Signed-off-by: Johannes Weiner Acked-by: Michal Hocko > --- > mm/memcontrol.c | 9 +++++++-- > 1 file changed, 7 insertions(+), 2 deletions(-) > > diff --git a/mm/memcontrol.c b/mm/memcontrol.c > index 0b38b6ad547d..5de0a9035b5f 100644 > --- a/mm/memcontrol.c > +++ b/mm/memcontrol.c > @@ -6360,11 +6360,16 @@ static unsigned long effective_protection(unsigned long usage, > * We're using unprotected memory for the weight so that if > * some cgroups DO claim explicit protection, we don't protect > * the same bytes twice. > + * > + * Check both usage and parent_usage against the respective > + * protected values. One should imply the other, but they > + * aren't read atomically - make sure the division is sane. > */ > if (!(cgrp_dfl_root.flags & CGRP_ROOT_MEMORY_RECURSIVE_PROT)) > return ep; > - > - if (parent_effective > siblings_protected && usage > protected) { > + if (parent_effective > siblings_protected && > + parent_usage > siblings_protected && > + usage > protected) { > unsigned long unclaimed; > > unclaimed = parent_effective - siblings_protected; > -- > 2.26.2 -- Michal Hocko SUSE Labs