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/ . >> >> 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. 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.
