(Reducing recipients in a hope not to be filtered at the servers.) Michal Hocko wrote: > On Mon 02-10-17 20:33:52, Tetsuo Handa wrote: > > > On Mon 02-10-17 06:59:12, Michael S. Tsirkin wrote: > > > > On Sun, Oct 01, 2017 at 02:44:34PM +0900, Tetsuo Handa wrote: > > > > > Tetsuo Handa wrote: > > > > > > Michael S. Tsirkin wrote: > > > > > > > On Mon, Sep 11, 2017 at 07:27:19PM +0900, Tetsuo Handa wrote: > > > > > > > > Hello. > > > > > > > > > > > > > > > > I noticed that virtio_balloon is using register_oom_notifier() and > > > > > > > > leak_balloon() from virtballoon_oom_notify() might depend on > > > > > > > > __GFP_DIRECT_RECLAIM memory allocation. > > > > > > > > > > > > > > > > In leak_balloon(), mutex_lock(&vb->balloon_lock) is called in order to > > > > > > > > serialize against fill_balloon(). But in fill_balloon(), > > > > > > > > alloc_page(GFP_HIGHUSER[_MOVABLE] | __GFP_NOMEMALLOC | __GFP_NORETRY) is > > > > > > > > called with vb->balloon_lock mutex held. Since GFP_HIGHUSER[_MOVABLE] implies > > > > > > > > __GFP_DIRECT_RECLAIM | __GFP_IO | __GFP_FS, this allocation attempt might > > > > > > > > depend on somebody else's __GFP_DIRECT_RECLAIM | !__GFP_NORETRY memory > > > > > > > > allocation. Such __GFP_DIRECT_RECLAIM | !__GFP_NORETRY allocation can reach > > > > > > > > __alloc_pages_may_oom() and hold oom_lock mutex and call out_of_memory(). > > > > > > > > And leak_balloon() is called by virtballoon_oom_notify() via > > > > > > > > blocking_notifier_call_chain() callback when vb->balloon_lock mutex is already > > > > > > > > held by fill_balloon(). As a result, despite __GFP_NORETRY is specified, > > > > > > > > fill_balloon() can indirectly get stuck waiting for vb->balloon_lock mutex > > > > > > > > at leak_balloon(). > > > > > > This is really nasty! And I would argue that this is an abuse of the oom > > > notifier interface from the virtio code. OOM notifiers are an ugly hack > > > on its own but all its users have to be really careful to not depend on > > > any allocation request because that is a straight deadlock situation. > > > > > I do not think that making oom notifier API more complex is the way to > > > go. Can we simply change the lock to try_lock? > > > > Using mutex_trylock(&vb->balloon_lock) alone is not sufficient. Inside the > > mutex, __GFP_DIRECT_RECLAIM && !__GFP_NORETRY allocation attempt is used > > which will fail to make progress due to oom_lock already held. Therefore, > > virtballoon_oom_notify() needs to guarantee that all allocation attempts use > > GFP_NOWAIT when called from virtballoon_oom_notify(). > > Ohh, I missed your point and thought the dependency is indirect and some > other call path is allocating while holding the lock. But you seem to be > right and > leak_balloon > tell_host > virtqueue_add_outbuf > virtqueue_add > > can do GFP_KERNEL allocation and this is clearly wrong. Nobody should > try to allocate while we are in the OOM path. Michael, is there any way > to drop this? Michael already said That would be tricky to fix. I guess we'll need to drop the lock while allocating memory - not an easy fix. and I think that it would be possible for virtio to locally offload virtballoon_oom_notify() using this patch's approach, if you don't like globally offloading at the OOM notifier API level. -- 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>
