From mboxrd@z Thu Jan 1 00:00:00 1970 From: Michal Hocko Subject: Re: [PATCH] virtio: avoid possible OOM lockup at virtballoon_oom_notify() Date: Tue, 10 Oct 2017 13:47:06 +0200 Message-ID: <20171010114706.mp3hpuulze75av43__24713.0694952377$1507636058$gmane$org@dhcp22.suse.cz> References: <1507632457-4611-1-git-send-email-penguin-kernel@I-love.SAKURA.ne.jp> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Return-path: Content-Disposition: inline In-Reply-To: <1507632457-4611-1-git-send-email-penguin-kernel@I-love.SAKURA.ne.jp> List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: virtualization-bounces@lists.linux-foundation.org Errors-To: virtualization-bounces@lists.linux-foundation.org To: Tetsuo Handa Cc: linux-mm@kvack.org, virtualization@lists.linux-foundation.org, mst@redhat.com List-Id: virtualization@lists.linuxfoundation.org On Tue 10-10-17 19:47:37, Tetsuo Handa wrote: > 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 indirectly depend on somebody > else's __GFP_DIRECT_RECLAIM memory allocation. And such indirect > __GFP_DIRECT_RECLAIM memory allocation might call leak_balloon() via > virtballoon_oom_notify() via blocking_notifier_call_chain() callback via > out_of_memory() when it reached __alloc_pages_may_oom() and held oom_lock > mutex. Since vb->balloon_lock mutex is already held by fill_balloon(), it > will cause OOM lockup. Thus, do not wait for vb->balloon_lock mutex if > leak_balloon() is called from out_of_memory(). > > Thread1 Thread2 > fill_balloon() > takes a balloon_lock > balloon_page_enqueue() > alloc_page(GFP_HIGHUSER_MOVABLE) > direct reclaim (__GFP_FS context) takes a fs lock > waits for that fs lock alloc_page(GFP_NOFS) > __alloc_pages_may_oom() > takes the oom_lock > out_of_memory() > blocking_notifier_call_chain() > leak_balloon() > tries to take that balloon_lock and deadlocks > > Signed-off-by: Tetsuo Handa FWIW this looks good to me from the deadlock POV. I cannot judge virtio internals and I would appreciate if it could move away from the oom notifier API to a more generic reclaim mechanism (e.g. shrinkers). Reviewed-by: Michal Hocko > --- > drivers/virtio/virtio_balloon.c | 16 +++++++++++----- > 1 file changed, 11 insertions(+), 5 deletions(-) > > diff --git a/drivers/virtio/virtio_balloon.c b/drivers/virtio/virtio_balloon.c > index f0b3a0b..03e6078 100644 > --- a/drivers/virtio/virtio_balloon.c > +++ b/drivers/virtio/virtio_balloon.c > @@ -192,7 +192,7 @@ static void release_pages_balloon(struct virtio_balloon *vb, > } > } > > -static unsigned leak_balloon(struct virtio_balloon *vb, size_t num) > +static unsigned leak_balloon(struct virtio_balloon *vb, size_t num, bool wait) > { > unsigned num_freed_pages; > struct page *page; > @@ -202,7 +202,13 @@ static unsigned leak_balloon(struct virtio_balloon *vb, size_t num) > /* We can only do one array worth at a time. */ > num = min(num, ARRAY_SIZE(vb->pfns)); > > - mutex_lock(&vb->balloon_lock); > + if (wait) > + mutex_lock(&vb->balloon_lock); > + else if (!mutex_trylock(&vb->balloon_lock)) { > + pr_info("virtio_balloon: Unable to release %lu pages due to lock contention.\n", > + (unsigned long) min(num, (size_t)vb->num_pages)); > + return 0; > + } > /* We can't release more pages than taken */ > num = min(num, (size_t)vb->num_pages); > for (vb->num_pfns = 0; vb->num_pfns < num; > @@ -367,7 +373,7 @@ static int virtballoon_oom_notify(struct notifier_block *self, > return NOTIFY_OK; > > freed = parm; > - num_freed_pages = leak_balloon(vb, oom_pages); > + num_freed_pages = leak_balloon(vb, oom_pages, false); > update_balloon_size(vb); > *freed += num_freed_pages; > > @@ -395,7 +401,7 @@ static void update_balloon_size_func(struct work_struct *work) > if (diff > 0) > diff -= fill_balloon(vb, diff); > else if (diff < 0) > - diff += leak_balloon(vb, -diff); > + diff += leak_balloon(vb, -diff, true); > update_balloon_size(vb); > > if (diff) > @@ -597,7 +603,7 @@ static void remove_common(struct virtio_balloon *vb) > { > /* There might be pages left in the balloon: free them. */ > while (vb->num_pages) > - leak_balloon(vb, vb->num_pages); > + leak_balloon(vb, vb->num_pages, true); > update_balloon_size(vb); > > /* Now we reset the device so we can clean up the queues. */ > -- > 1.8.3.1 -- Michal Hocko SUSE Labs