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? > > OOM notifier callback should not (directly or indirectly) depend on > __GFP_DIRECT_RECLAIM memory allocation attempt. Can you fix this dependency? -- 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>
