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Date:   Mon, 15 Jun 2020 16:09:11 +0100
From:   Chris Down <chris@chrisdown.name>
To:     Johannes Weiner <hannes@cmpxchg.org>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        Tejun Heo <tj@kernel.org>, Michal Hocko <mhocko@suse.com>,
        Roman Gushchin <guro@fb.com>, 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: <20200615150911.GD157916@chrisdown.name>
References: <20200615140658.601684-1-hannes@cmpxchg.org>
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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 <tj@kernel.org>

Acked-by: Chris Down <chris@chrisdown.name>