On Thu 15-06-17 15:42:23, David Rientjes wrote: > On Fri, 16 Jun 2017, Michal Hocko wrote: > > > I am sorry but I have really hard to make the oom reaper a reliable way > > to stop all the potential oom lockups go away. I do not want to > > reintroduce another potential lockup now. > > Please show where this "potential lockup" ever existed in a bug report or > a testcase? I am not aware of any specific bug report. But the main point of the reaper is to close all _possible_ lockups due to oom victim being stuck somewhere. exit_aio waits for all kiocbs. Can we guarantee that none of them will depend on an allocation (directly or via a lock chain) to proceed? Likewise ksm_exit/khugepaged_exit depend on mmap_sem for write to proceed. Are we _guaranteed_ nobody can hold mmap_sem for read at that time and depend on an allocation? Can we guarantee that __mmput path will work without any depency on allocation in future? > I have never seen __mmput() block when trying to free the > memory it maps. > > > I also do not see why any > > solution should be rushed into. I have proposed a way to go and unless > > it is clear that this is not a way forward then I simply do not agree > > with any partial workarounds or shortcuts. > > This is not a shortcut, it is a bug fix. 4.12 kills 1-4 processes > unnecessarily as a result of setting MMF_OOM_SKIP incorrectly before the > mm's memory can be freed. If you have not seen this issue before, which > is why you asked if I ever observed it in practice, then you have not > stress tested oom reaping. It is very observable and reproducible. I am not questioning that it works for your particular test. I just argue that it reduces the robustness of the oom reaper because it allows oom victim to leave the reaper without MMF_OOM_SKIP set and that is the core concept to guarantee a forward progress. So we should think about something more appropriate. > I do > not agree that adding additional and obscure locking into __mmput() is the > solution to what is plainly and obviously fixed with this simple patch. Well, __mmput path already depends on the mmap_sem for write. So this is not a new concept. I am not saying using mmap_sem is the only way. I will think about that more. > 4.12 needs to stop killing 2-5 processes on every oom condition instead of > 1. Believe me, I am not dismissing the issue nor the fact it _has_ to be fixed. I just disagree we should make the oom reaper less robust. -- Michal Hocko SUSE Labs -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>
