From mboxrd@z Thu Jan 1 00:00:00 1970 From: Chris Down Subject: Re: [PATCH] mm: memcontrol: handle div0 crash race condition in memory.low Date: Mon, 15 Jun 2020 16:09:11 +0100 Message-ID: <20200615150911.GD157916@chrisdown.name> References: <20200615140658.601684-1-hannes@cmpxchg.org> Mime-Version: 1.0 Return-path: DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=chrisdown.name; s=google; h=date:from:to:cc:subject:message-id:references:mime-version :content-disposition:in-reply-to:user-agent; bh=I3pYBTJCH1fWxzUVivyIxWA31KHjVQXjnfV1c/iHSQ8=; b=aPeEzEq8Bmncf5GDxG4bKMbLMmpAo3LrvR3mDRIGWrMYi8m0weo1We6uJeEyp/OCdf /K4tB062Vr1+uzaRvdUPls1EMMG+6mqPlTcGG+J0AVYmUhkuQd4uuhW8AEwLZcc4EZoB CSGBuXP88a9JMP/xuTmLWTkbgT6ick7fAsj5k= Content-Disposition: inline In-Reply-To: <20200615140658.601684-1-hannes-druUgvl0LCNAfugRpC6u6w@public.gmane.org> Sender: cgroups-owner-u79uwXL29TY76Z2rM5mHXA@public.gmane.org List-ID: Content-Type: text/plain; charset="us-ascii"; format="flowed" Content-Transfer-Encoding: 7bit To: Johannes Weiner Cc: Andrew Morton , Tejun Heo , Michal Hocko , Roman Gushchin , linux-mm-Bw31MaZKKs3YtjvyW6yDsg@public.gmane.org, cgroups-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, linux-kernel-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, kernel-team-b10kYP2dOMg@public.gmane.org Johannes Weiner writes: >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 Acked-by: Chris Down