On 23.10.2021 18:01, Tetsuo Handa wrote: > On 2021/10/23 22:20, Vasily Averin wrote: >> /* >> - * The pagefault handler calls here because it is out of memory, so kill a >> - * memory-hogging task. If oom_lock is held by somebody else, a parallel oom >> - * killing is already in progress so do nothing. >> + * The pagefault handler calls here because some allocation has failed. We have >> + * to take care of the memcg OOM here because this is the only safe context without >> + * any locks held but let the oom killer triggered from the allocation context care >> + * about the global OOM. >> */ > > Excuse me for a stupid question. I consider > > if (!mutex_trylock(&oom_lock)) > return; > out_of_memory(&oc); > mutex_unlock(&oom_lock); > > here as the last resort (safeguard) when neither __alloc_pages_may_oom() > nor mem_cgroup_out_of_memory() can make progress. This patch says > > let the oom killer triggered from the allocation context care > about the global OOM. > > but what if the OOM killer cannot be invoked from the allocation context? > Is there a guarantee that all memory allocations which might result in > VM_FAULT_OOM can invoke the OOM killer? I don't think this question is stupid, since I asked it myself :) You can find this discussion here: https://lkml.org/lkml/2021/10/21/900 Let me quote it here too :> 1) VM_FAULT_OOM may be triggered w/o execution of out_of_memory() :> for exampel it can be caused by incorrect vm fault operations, :> (a) which can return this error without calling allocator at all. : :I would argue this to be a bug. How can that particular code tell :whether the system is OOM and the oom killer is the a reasonable measure :to take? : :> (b) or which can provide incorrect gfp flags and allocator can fail without execution of out_of_memory. : : I am not sure I can see any sensible scenario where pagefault oom killer : would be an appropriate fix for that. : :> (c) This may happen on stable/LTS kernels when successful allocation was failed by hit into limit of legacy memcg-kmem contoller. :> We'll drop it in upstream kernels, however how to handle it in old kenrels? : :Triggering the global oom killer for legacy kmem charge failure is :clearly wrong. Removing oom killer from #PF would fix that problem. I would note: (c) is not theoretical but real life problem, in this case allocation was failed without execution of OOM, however, it is in this case that execution out_of_memory() from pagefault_out_of_memory() leads to a disaster. Thank you, Vasily Averin
