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
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.
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