On Wed 20-10-21 18:46:56, Vasily Averin wrote: > On 20.10.2021 16:02, Michal Hocko wrote: > > On Wed 20-10-21 15:14:27, Vasily Averin wrote: > >> mem_cgroup_oom() can fail if current task was marked unkillable > >> and oom killer cannot find any victim. > >> > >> Currently we force memcg charge for such allocations, > >> however it allow memcg-limited userspace task in to overuse assigned limits > >> and potentially trigger the global memory shortage. > > > > You should really go into more details whether that is a practical > > problem to handle. OOM_FAILED means that the memcg oom killer couldn't > > find any oom victim so it cannot help with a forward progress. There are > > not that many situations when that can happen. Naming that would be > > really useful. > > I've pointed above: > "if current task was marked unkillable and oom killer cannot find any victim." > This may happen when current task cannot be oom-killed because it was marked > unkillable i.e. it have p->signal->oom_score_adj == OOM_SCORE_ADJ_MIN > and other processes in memcg are either dying, or are kernel threads or are marked unkillable > by the same way. Or when memcg have this process only. > > If we always approve such kind of allocation, it can be misused. > Process can mmap a lot of memory, > ant then touch it and generate page fault and make overcharged memory allocations. > Finally it can consume all node memory and trigger global memory shortage on the host. Yes, this is true but a) OOM_SCORE_ADJ_MIN tasks are excluded from the OOM handling so they have to be careful with the memory consumption and b) is this a theoretical or a practical concern. This is mostly what I wanted to make sure you describe in the changelog. > >> Let's fail the memory charge in such cases. > >> > >> This failure should be somehow recognised in #PF context, > > > > explain why > > When #PF cannot allocate memory (due to reason described above), handle_mm_fault returns VM_FAULT_OOM, > then its caller executes pagefault_out_of_memory(). If last one cannot recognize the real reason of this fail, > it expect it was global memory shortage and executed global out_ouf_memory() that can kill random process > or just crash node if sysctl vm.panic_on_oom is set to 1. > > Currently pagefault_out_of_memory() knows about possible async memcg OOM and handles it correctly. > However it is not aware that memcg can reject some other allocations, do not recognize the fault > as memcg-related and allows to run global OOM. Again something to be added to the changelog. > >> so let's use current->memcg_in_oom == (struct mem_cgroup *)OOM_FAILED > >> > >> ToDo: what is the best way to notify pagefault_out_of_memory() about > >> mem_cgroup_out_of_memory failure ? > > > > why don't you simply remove out_of_memory from pagefault_out_of_memory > > and leave it only with the blocking memcg OOM handling? Wouldn't that be a > > more generic solution? Your first patch already goes that way partially. > > I clearly understand that global out_of_memory should not be trggired by memcg restrictions. > I clearly understand that dying task will release some memory soon and we can do not run global oom if current task is dying. > > However I'm not sure that I can remove out_of_memory at all. At least I do not have good arguments to do it. I do understand that handling a very specific case sounds easier but it would be better to have a robust fix even if that requires some more head scratching. So far we have collected several reasons why the it is bad to trigger oom killer from the #PF path. There is no single argument to keep it so it sounds like a viable path to pursue. Maybe there are some very well hidden reasons but those should be documented and this is a great opportunity to do either of the step. Moreover if it turns out that there is a regression then this can be easily reverted and a different, maybe memcg specific, solution can be implemented. -- Michal Hocko SUSE Labs
