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(). > > That would be tricky to fix. I guess we'll need to drop the lock > while allocating memory - not an easy fix. > > > Also, in leak_balloon(), virtqueue_add_outbuf(GFP_KERNEL) is called via > > tell_host(). Reaching __alloc_pages_may_oom() from this virtqueue_add_outbuf() > > request from leak_balloon() from virtballoon_oom_notify() from > > blocking_notifier_call_chain() from out_of_memory() leads to OOM lockup > > because oom_lock mutex is already held before calling out_of_memory(). > > I guess we should just do > > GFP_KERNEL & ~__GFP_DIRECT_RECLAIM there then? Yes, but GFP_KERNEL & ~__GFP_DIRECT_RECLAIM will effectively be GFP_NOWAIT, for __GFP_IO and __GFP_FS won't make sense without __GFP_DIRECT_RECLAIM. It might significantly increases possibility of memory allocation failure. > > > > > > OOM notifier callback should not (directly or indirectly) depend on > > __GFP_DIRECT_RECLAIM memory allocation attempt. Can you fix this dependency? > Another idea would be to use a kernel thread (or workqueue) so that virtballoon_oom_notify() can wait with timeout. We could offload entire blocking_notifier_call_chain(&oom_notify_list, 0, &freed) call to a kernel thread (or workqueue) with timeout if MM folks agree. _______________________________________________ Virtualization mailing list Virtualization@xxxxxxxxxxxxxxxxxxxxxxxxxx https://lists.linuxfoundation.org/mailman/listinfo/virtualization
