On 2018/07/13 23:26, Michal Hocko wrote: > On Thu 12-07-18 14:34:00, David Rientjes wrote: > [...] >> diff --git a/mm/oom_kill.c b/mm/oom_kill.c >> index 0fe4087d5151..e6328cef090f 100644 >> --- a/mm/oom_kill.c >> +++ b/mm/oom_kill.c >> @@ -488,9 +488,11 @@ void __oom_reap_task_mm(struct mm_struct *mm) >> * Tell all users of get_user/copy_from_user etc... that the content >> * is no longer stable. No barriers really needed because unmapping >> * should imply barriers already and the reader would hit a page fault >> - * if it stumbled over a reaped memory. >> + * if it stumbled over a reaped memory. If MMF_UNSTABLE is already set, >> + * reaping as already occurred so nothing left to do. >> */ >> - set_bit(MMF_UNSTABLE, &mm->flags); >> + if (test_and_set_bit(MMF_UNSTABLE, &mm->flags)) >> + return; > > This could lead to pre mature oom victim selection > oom_reaper exiting victim > oom_reap_task exit_mmap > __oom_reap_task_mm __oom_reap_task_mm > test_and_set_bit(MMF_UNSTABLE) # wins the race > test_and_set_bit(MMF_UNSTABLE) > set_bit(MMF_OOM_SKIP) # new victim can be selected now. > > Besides that, why should we back off in the first place. We can > race the two without any problems AFAICS. We already do have proper > synchronization between the two due to mmap_sem and MMF_OOM_SKIP. > > diff --git a/mm/mmap.c b/mm/mmap.c > index fc41c0543d7f..4642964f7741 100644 > --- a/mm/mmap.c > +++ b/mm/mmap.c > @@ -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. 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); 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.
