Michal Hocko wrote: > On Wed 19-07-17 05:51:03, Tetsuo Handa wrote: > > Michal Hocko wrote: > > > On Tue 18-07-17 23:06:50, Tetsuo Handa wrote: > > > > Commit e2fe14564d3316d1 ("oom_reaper: close race with exiting task") > > > > guarded whole OOM reaping operations using oom_lock. But there was no > > > > need to guard whole operations. We needed to guard only setting of > > > > MMF_OOM_REAPED flag because get_page_from_freelist() in > > > > __alloc_pages_may_oom() is called with oom_lock held. > > > > > > > > If we change to guard only setting of MMF_OOM_SKIP flag, the OOM reaper > > > > can start reaping operations as soon as wake_oom_reaper() is called. > > > > But since setting of MMF_OOM_SKIP flag at __mmput() is not guarded with > > > > oom_lock, guarding only the OOM reaper side is not sufficient. > > > > > > > > If we change the OOM killer side to ignore MMF_OOM_SKIP flag once, > > > > there is no need to guard setting of MMF_OOM_SKIP flag, and we can > > > > guarantee a chance to call get_page_from_freelist() in > > > > __alloc_pages_may_oom() without depending on oom_lock serialization. > > > > > > > > This patch makes MMF_OOM_SKIP act as if MMF_OOM_REAPED, and adds a new > > > > flag which acts as if MMF_OOM_SKIP, in order to close both race window > > > > (the OOM reaper side and __mmput() side) without using oom_lock. > > > > > > Why do we need this patch when > > > http://lkml.kernel.org/r/20170626130346.26314-1-mho...@kernel.org > > > already removes the lock and solves another problem at once? > > > > We haven't got an answer from Hugh and/or Andrea whether that patch is safe. > > So what? I haven't see anybody disputing the correctness. And to be > honest I really dislike your patch. Yet another round kind of solutions > are just very ugly hacks usually because they are highly timing > sensitive.
Yes, OOM killer is highly timing sensitive. > > > Even if that patch is safe, this patch still helps with CONFIG_MMU=n case. > > Could you explain how? Nothing prevents sequence below. Process-1 Process-2 Takes oom_lock. Fails get_page_from_freelist(). Enters out_of_memory(). Gets SIGKILL. Gets TIF_MEMDIE. Leaves out_of_memory(). Releases oom_lock. Enters do_exit(). Calls __mmput(). Takes oom_lock. Fails get_page_from_freelist(). Releases some memory. Sets MMF_OOM_SKIP. Enters out_of_memory(). Selects next victim because there is no !MMF_OOM_SKIP mm. Sends SIGKILL needlessly. If we ignore MMF_OOM_SKIP once, we can avoid sequence above. Process-1 Process-2 Takes oom_lock. Fails get_page_from_freelist(). Enters out_of_memory(). Get SIGKILL. Get TIF_MEMDIE. Leaves out_of_memory(). Releases oom_lock. Enters do_exit(). Calls __mmput(). Takes oom_lock. Fails get_page_from_freelist(). Releases some memory. Sets MMF_OOM_SKIP. Enters out_of_memory(). Ignores MMF_OOM_SKIP mm once. Leaves out_of_memory(). Releases oom_lock. Succeeds get_page_from_freelist(). Strictly speaking, this patch is independent with OOM reaper. This patch increases possibility of succeeding get_page_from_freelist() without sending SIGKILL. Your patch is trying to drop it silently. Serializing setting of MMF_OOM_SKIP with oom_lock is one approach, and ignoring MMF_OOM_SKIP once without oom_lock is another approach.