On Mon 06-08-18 21:45:53, Michal Hocko wrote: > [CCing Greg - the email thread starts here > http://lkml.kernel.org/r/0000000000005e979605729c1564@xxxxxxxxxx] now for real > > On Mon 06-08-18 12:12:02, syzbot wrote: > > Hello, > > > > syzbot has tested the proposed patch and the reproducer did not trigger > > crash: > > OK, this is reassuring. Btw Greg has pointed out this potential case > http://lkml.kernel.org/r/xr93in62jy8k.fsf@xxxxxxxxxxxxxxxxxxxxxxxxxxx > but I simply didn't get what he meant. He was suggesting MMF_OOM_SKIP > but I didn't get why that matters. I didn't think about a race. > > So how about this patch: > From 74d980f8d066d06ada657ebf9b586dbf5668ed26 Mon Sep 17 00:00:00 2001 > From: Michal Hocko <mhocko@xxxxxxxx> > Date: Mon, 6 Aug 2018 21:21:24 +0200 > Subject: [PATCH] memcg, oom: be careful about races when warning about no > reclaimable task > > "memcg, oom: move out_of_memory back to the charge path" has added a > warning triggered when the oom killer cannot find any eligible task > and so there is no way to reclaim the oom memcg under its hard limit. > Further charges for such a memcg are forced and therefore the hard limit > isolation is weakened. > > The current warning is however too eager to trigger even when we are not > really hitting the above condition. Syzbot and Greg Thelen have noticed > that we can hit this condition even when there is still oom victim > pending. E.g. the following race is possible: > > memcg has two tasks taskA, taskB. > > CPU1 (taskA) CPU2 CPU3 (taskB) > try_charge > mem_cgroup_out_of_memory try_charge > select_bad_process(taskB) > oom_kill_process oom_reap_task > # No real memory reaped > mem_cgroup_out_of_memory > # set taskB -> MMF_OOM_SKIP > # retry charge > mem_cgroup_out_of_memory > oom_lock oom_lock > select_bad_process(self) > oom_kill_process(self) > oom_unlock > # no eligible task > > In fact syzbot test triggered this situation by placing multiple tasks > into a memcg with hard limit set to 0. So no task really had any memory > charged to the memcg > > : Memory cgroup stats for /ile0: cache:0KB rss:0KB rss_huge:0KB shmem:0KB mapped_file:0KB dirty:0KB writeback:0KB swap:0KB inactive_anon:0KB active_anon:0KB inactive_file:0KB active_file:0KB unevictable:0KB > : Tasks state (memory values in pages): > : [ pid ] uid tgid total_vm rss pgtables_bytes swapents oom_score_adj name > : [ 6569] 0 6562 9427 1 53248 0 0 syz-executor0 > : [ 6576] 0 6576 9426 0 61440 0 0 syz-executor6 > : [ 6578] 0 6578 9426 534 61440 0 0 syz-executor4 > : [ 6579] 0 6579 9426 0 57344 0 0 syz-executor5 > : [ 6582] 0 6582 9426 0 61440 0 0 syz-executor7 > : [ 6584] 0 6584 9426 0 57344 0 0 syz-executor1 > > so in principle there is indeed nothing reclaimable in this memcg and > this looks like a misconfiguration. On the other hand we can clearly > kill all those tasks so it is a bit early to warn and scare users. Do > that by checking that the current is the oom victim and bypass the > warning then. The victim is allowed to force charge and terminate to > release its temporal charge along the way. > > Fixes: "memcg, oom: move out_of_memory back to the charge path" > Noticed-by: Greg Thelen <gthelen@xxxxxxxxxx> > Reported-and-tested-by: syzbot+bab151e82a4e973fa325@xxxxxxxxxxxxxxxxxxxxxxxxx > Signed-off-by: Michal Hocko <mhocko@xxxxxxxx> > --- > mm/memcontrol.c | 3 ++- > 1 file changed, 2 insertions(+), 1 deletion(-) > > diff --git a/mm/memcontrol.c b/mm/memcontrol.c > index 4603ad75c9a9..1b6eed1bc404 100644 > --- a/mm/memcontrol.c > +++ b/mm/memcontrol.c > @@ -1703,7 +1703,8 @@ static enum oom_status mem_cgroup_oom(struct mem_cgroup *memcg, gfp_t mask, int > return OOM_ASYNC; > } > > - if (mem_cgroup_out_of_memory(memcg, mask, order)) > + if (mem_cgroup_out_of_memory(memcg, mask, order) || > + tsk_is_oom_victim(current)) > return OOM_SUCCESS; > > WARN(1,"Memory cgroup charge failed because of no reclaimable memory! " > -- > 2.18.0 > > -- > Michal Hocko > SUSE Labs -- Michal Hocko SUSE Labs -- To unsubscribe from this list: send the line "unsubscribe cgroups" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
