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 <penguin-kernel@xxxxxxxxxxxxxxxxxxx>
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 <mhocko@xxxxxxxx>
> ---
> 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
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