Michal Hocko wrote: > On Sat 19-08-17 15:23:19, Tetsuo Handa wrote: > > Tetsuo Handa wrote at http://lkml.kernel.org/r/201708102328.ACD34352.OHFOLJMQVSFOFt@xxxxxxxxxxxxxxxxxxx : > > > Michal Hocko wrote: > > > > On Thu 10-08-17 21:10:30, Tetsuo Handa wrote: > > > > > Michal Hocko wrote: > > > > > > On Tue 08-08-17 11:14:50, Tetsuo Handa wrote: > > > > > > > Michal Hocko wrote: > > > > > > > > On Sat 05-08-17 10:02:55, Tetsuo Handa wrote: > > > > > > > > > Michal Hocko wrote: > > > > > > > > > > On Wed 26-07-17 20:33:21, Tetsuo Handa wrote: > > > > > > > > > > > My question is, how can users know it if somebody was OOM-killed needlessly > > > > > > > > > > > by allowing MMF_OOM_SKIP to race. > > > > > > > > > > > > > > > > > > > > Is it really important to know that the race is due to MMF_OOM_SKIP? > > > > > > > > > > > > > > > > > > Yes, it is really important. Needlessly selecting even one OOM victim is > > > > > > > > > a pain which is difficult to explain to and persuade some of customers. > > > > > > > > > > > > > > > > How is this any different from a race with a task exiting an releasing > > > > > > > > some memory after we have crossed the point of no return and will kill > > > > > > > > something? > > > > > > > > > > > > > > I'm not complaining about an exiting task releasing some memory after we have > > > > > > > crossed the point of no return. > > > > > > > > > > > > > > What I'm saying is that we can postpone "the point of no return" if we ignore > > > > > > > MMF_OOM_SKIP for once (both this "oom_reaper: close race without using oom_lock" > > > > > > > thread and "mm, oom: task_will_free_mem(current) should ignore MMF_OOM_SKIP for > > > > > > > once." thread). These are race conditions we can avoid without crystal ball. > > > > > > > > > > > > If those races are really that common than we can handle them even > > > > > > without "try once more" tricks. Really this is just an ugly hack. If you > > > > > > really care then make sure that we always try to allocate from memory > > > > > > reserves before going down the oom path. In other words, try to find a > > > > > > robust solution rather than tweaks around a problem. > > > > > > > > > > Since your "mm, oom: allow oom reaper to race with exit_mmap" patch removes > > > > > oom_lock serialization from the OOM reaper, possibility of calling out_of_memory() > > > > > due to successful mutex_trylock(&oom_lock) would increase when the OOM reaper set > > > > > MMF_OOM_SKIP quickly. > > > > > > > > > > What if task_is_oom_victim(current) became true and MMF_OOM_SKIP was set > > > > > on current->mm between after __gfp_pfmemalloc_flags() returned 0 and before > > > > > out_of_memory() is called (due to successful mutex_trylock(&oom_lock)) ? > > > > > > > > > > Excuse me? Are you suggesting to try memory reserves before > > > > > task_is_oom_victim(current) becomes true? > > > > > > > > No what I've tried to say is that if this really is a real problem, > > > > which I am not sure about, then the proper way to handle that is to > > > > attempt to allocate from memory reserves for an oom victim. I would be > > > > even willing to take the oom_lock back into the oom reaper path if the > > > > former turnes out to be awkward to implement. But all this assumes this > > > > is a _real_ problem. > > > > > > Aren't we back to square one? My question is, how can users know it if > > > somebody was OOM-killed needlessly by allowing MMF_OOM_SKIP to race. > > > > > > You don't want to call get_page_from_freelist() from out_of_memory(), do you? > > > But without passing a flag "whether get_page_from_freelist() with memory reserves > > > was already attempted if current thread is an OOM victim" to task_will_free_mem() > > > in out_of_memory() and a flag "whether get_page_from_freelist() without memory > > > reserves was already attempted if current thread is not an OOM victim" to > > > test_bit(MMF_OOM_SKIP) in oom_evaluate_task(), we won't be able to know > > > if somebody was OOM-killed needlessly by allowing MMF_OOM_SKIP to race. > > > > Michal, I did not get your answer, and your "mm, oom: do not rely on > > TIF_MEMDIE for memory reserves access" did not help solving this problem. > > (I confirmed it by reverting your "mm, oom: allow oom reaper to race with > > exit_mmap" and applying Andrea's "mm: oom: let oom_reap_task and exit_mmap > > run concurrently" and this patch on top of linux-next-20170817.) > > By "this patch" you probably mean a BUG_ON(tsk_is_oom_victim) somewhere > in task_will_free_mem right? I do not see anything like that in you > email. I wrote You can confirm it by adding "BUG_ON(1);" at "task->oom_kill_free_check_raced = 1;" of this patch. in the patch description. > > > [ 204.413605] Out of memory: Kill process 9286 (a.out) score 930 or sacrifice child > > [ 204.416241] Killed process 9286 (a.out) total-vm:4198476kB, anon-rss:72kB, file-rss:0kB, shmem-rss:3465520kB > > [ 204.419783] oom_reaper: reaped process 9286 (a.out), now anon-rss:0kB, file-rss:0kB, shmem-rss:3465720kB > > [ 204.455864] ------------[ cut here ]------------ > > [ 204.457921] kernel BUG at mm/oom_kill.c:786! > > > > Therefore, I propose this patch for inclusion. > > i've already told you that this is a wrong approach to handle a possible > race and offered you an alternative. I realy fail to see why you keep > reposting it. So to make myself absolutely clear > > Nacked-by: Michal Hocko <mhocko@xxxxxxxx> to the patch below. Where is your alternative? -- 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>
