On Mon 16-07-18 16:04:26, Tetsuo Handa wrote: > On 2018/07/16 15:13, Michal Hocko wrote: > > On Sat 14-07-18 06:18:58, Tetsuo Handa wrote: > >>> @@ -3073,9 +3073,7 @@ void exit_mmap(struct mm_struct *mm) > >>> * which clears VM_LOCKED, otherwise the oom reaper cannot > >>> * reliably test it. > >>> */ > >>> - mutex_lock(&oom_lock); > >>> __oom_reap_task_mm(mm); > >>> - mutex_unlock(&oom_lock); > >>> > >>> set_bit(MMF_OOM_SKIP, &mm->flags); > >> > >> David and Michal are using different version as a baseline here. > >> David is making changes using timeout based back off (in linux-next.git) > >> which is inappropriately trying to use MMF_UNSTABLE for two purposes. > >> > >> Michal is making changes using current code (in linux.git) which does not > >> address David's concern. > > > > Yes I have based it on top of Linus tree because the point of this patch > > is to get rid of the locking which is no longer needed. I do not see > > what concern are you talking about. > > I'm saying that applying your patch does not work on linux-next.git > because David's patch already did s/MMF_OOM_SKIP/MMF_UNSTABLE/ . This patch has been nacked by me AFAIR so I assume it should be dropped from the mmotm tree. > >> My version ( https://marc.info/?l=linux-mm&m=153119509215026 ) is > >> making changes using current code which also provides oom-badness > >> based back off in order to address David's concern. > >> > >>> down_write(&mm->mmap_sem); > >> > >> Anyway, I suggest doing > >> > >> mutex_lock(&oom_lock); > >> set_bit(MMF_OOM_SKIP, &mm->flags); > >> mutex_unlock(&oom_lock); > > > > Why do we need it? > > > >> like I mentioned at > >> http://lkml.kernel.org/r/201807130620.w6D6KiAJ093010@xxxxxxxxxxxxxxxxxxx > >> even if we make changes on top of linux-next's timeout based back off. > > > > says > > : (3) Prevent from selecting new OOM victim when there is an !MMF_OOM_SKIP mm > > : which current thread should wait for. > > [...] > > : Regarding (A), we can reduce the range oom_lock serializes from > > : "__oom_reap_task_mm()" to "setting MMF_OOM_SKIP", for oom_lock is useful for (3). > > > > But why there is a lock needed for this? This doesn't make much sense to > > me. If we do not have MMF_OOM_SKIP set we still should have mm_is_oom_victim > > so no new task should be selected. If we race with the oom reaper than > > ok, we would just not select a new victim and retry later. > > > > How mm_is_oom_victim() helps? mm_is_oom_victim() is used by exit_mmap() whether > current thread should call __oom_reap_task_mm(). > > I'm talking about below sequence (i.e. after returning from __oom_reap_task_mm()). > > CPU 0 CPU 1 > > mutex_trylock(&oom_lock) in __alloc_pages_may_oom() succeeds. > get_page_from_freelist() fails. > Enters out_of_memory(). > > __oom_reap_task_mm() reclaims some memory. > Sets MMF_OOM_SKIP. > > select_bad_process() selects new victim because MMF_OOM_SKIP is already set. > Kills a new OOM victim without retrying last second allocation attempt. > Leaves out_of_memory(). > mutex_unlock(&oom_lock) in __alloc_pages_may_oom() is called. OK, that wasn't clear from your above wording. As you explicitly mentioned !MMF_OOM_SKIP mm. > If setting MMF_OOM_SKIP is guarded by oom_lock, we can enforce > last second allocation attempt like below. > > CPU 0 CPU 1 > > mutex_trylock(&oom_lock) in __alloc_pages_may_oom() succeeds. > get_page_from_freelist() fails. > Enters out_of_memory(). > > __oom_reap_task_mm() reclaims some memory. > mutex_lock(&oom_lock); > > select_bad_process() does not select new victim because MMF_OOM_SKIP is not yet set. > Leaves out_of_memory(). > mutex_unlock(&oom_lock) in __alloc_pages_may_oom() is called. > > Sets MMF_OOM_SKIP. > mutex_unlock(&oom_lock); > > get_page_from_freelist() likely succeeds before reaching __alloc_pages_may_oom() again. > Saved one OOM victim from being needlessly killed. > > That is, guarding setting MMF_OOM_SKIP works as if synchronize_rcu(); it waits for anybody > who already acquired (or started waiting for) oom_lock to release oom_lock, in order to > prevent select_bad_process() from needlessly selecting new OOM victim. Hmm, is this a practical problem though? Do we really need to have a broader locking context just to defeat this race? How about this goes into a separate patch with some data justifying it? -- Michal Hocko SUSE Labs
