From: Johannes Weiner <hannes@xxxxxxxxxxx> Subject: mm: memcontrol: handle div0 crash race condition in memory.low 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. Link: http://lkml.kernel.org/r/20200615140658.601684-1-hannes@xxxxxxxxxxx Fixes: 8a931f801340 ("mm: memcontrol: recursive memory.low protection") Signed-off-by: Johannes Weiner <hannes@xxxxxxxxxxx> Reported-by: Tejun Heo <tj@xxxxxxxxxx> Acked-by: Michal Hocko <mhocko@xxxxxxxx> Acked-by: Chris Down <chris@xxxxxxxxxxxxxx> Cc: Roman Gushchin <guro@xxxxxx> Cc: <stable@xxxxxxxxxxxxxxx> Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> --- mm/memcontrol.c | 9 +++++++-- 1 file changed, 7 insertions(+), 2 deletions(-) --- a/mm/memcontrol.c~mm-memcontrol-handle-div0-crash-race-condition-in-memorylow +++ a/mm/memcontrol.c @@ -6360,11 +6360,16 @@ static unsigned long effective_protectio * 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; _
