On Mon, Oct 09, 2017 at 03:37:34PM +0200, Michal Hocko wrote: > On Mon 09-10-17 22:31:18, Tetsuo Handa wrote: > > Michal Hocko wrote: > > > On Mon 09-10-17 17:06:51, Tetsuo Handa wrote: > > > > Michal Hocko wrote: > > > > > On Sat 07-10-17 20:30:19, Tetsuo Handa wrote: > > > > > [...] > > > > > > >From 6a0fd8a5e013ac63a6bcd06bd2ae6fdb25a4f3de Mon Sep 17 00:00:00 2001 > > > > > > From: Tetsuo Handa <penguin-kernel@xxxxxxxxxxxxxxxxxxx> > > > > > > Date: Sat, 7 Oct 2017 19:29:21 +0900 > > > > > > Subject: [PATCH] virtio: avoid possible OOM lockup at virtballoon_oom_notify() > > > > > > > > > > > > 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, despite __GFP_NORETRY > > > > > > is specified, this allocation attempt might depend on somebody else's > > > > > > __GFP_DIRECT_RECLAIM memory allocation. > > > > > > > > > > How would that dependency look like? Is the holder of the lock doing > > > > > only __GFP_NORETRY? > > > > > > > > __GFP_NORETRY makes difference only after reclaim attempt failed. > > > > > > > > Reclaim attempt of __GFP_DIRECT_RECLAIM | __GFP_IO | __GFP_FS request can > > > > indirectly wait for somebody else's GFP_NOFS and/or GFP_NOIO request (e.g. > > > > blocked on filesystem's fs lock). And such indirect GFP_NOFS and/or > > > > GFP_NOIO request can reach __alloc_pages_may_oom() unless they also have > > > > __GFP_NORETRY. And such indirect GFP_NOFS and/or GFP_NOIO request can call > > > > OOM notifier callback and try to hold balloon_lock at leak_balloon() which > > > > fill_balloon() has already held before doing > > > > GFP_HIGHUSER[_MOVABLE] | __GFP_NOMEMALLOC | __GFP_NORETRY request. > > > > > > OK, so let me decipher. > > > Thread1 Thread2 Thread3 > > > alloc_pages(GFP_KERNEL) fill_balloon fs_lock #1 > > > out_of_memory balloon_lock #2 alloc_page(GFP_NOFS) > > > blocking_notifier_call_chain balloon_page_enqueue # keep retrying > > > leak_balloon alloc_page(GFP_HIGHUSER_MOVABLE) > > > balloon_lock #2 direct_reclaim (__GFP_FS context) > > > fs_lock #1 > > > > > > in other words, let's make the description understandable even for > > > somebody not really familiar with the allocation&reclaim internals. > > > The whole point is that the dependency is indirect and it requires > > > more actors and an example call grapg should be easier to follow. > > > > > > Yes. But it is more simple. Only two threads are needed. > > > > Thread1 Thread2 > > fill_balloon > > balloon_lock #1 > > balloon_page_enqueue > > alloc_page(GFP_HIGHUSER_MOVABLE) > > direct reclaim (__GFP_FS context) fs lock #2 > > fs lock #2 alloc_page(GFP_NOFS) > > __alloc_pages_may_oom() > > oom_lock > > out_of_memory() > > blocking_notifier_call_chain() > > leak_balloon > > balloon_lock #1 # dead lock > > Oh, right. I forgot we are allowed oom notifiers from NOFS context. > > > And other __GFP_DIRECT_RECLAIM && !__GFP_NORETRY allocations (if any) will keep > > retrying forever because oom_lock is held by Thread2. > > > > > > > > One more nit. If there is a way to estimate how much memory could be > > > freed by the notifier when the trylock would succeed I would print that > > > value for debugging purposes. > > > > I don't know internal of virtio-balloon. > > Maybe Michael can help here. num_pages is a way to estimate that. > -- > Michal Hocko > SUSE Labs -- 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>
