Re: [RFC] [PATCH] mm,oom: Offload OOM notify callback to a kernel thread.

[Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>]

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@I-love.SAKURA.ne.jp>
> > > > 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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