Re: [Xen-devel] [PATCH] mm, oom: distinguish blockable mode for mmu notifiers

[Jerome Glisse <jglisse@xxxxxxxxxx>]

On Fri, Aug 24, 2018 at 02:33:41PM +0200, Michal Hocko wrote:
> On Fri 24-08-18 14:18:44, Christian König wrote:
> > Am 24.08.2018 um 14:03 schrieb Michal Hocko:
> > > On Fri 24-08-18 13:57:52, Christian König wrote:
> > > > Am 24.08.2018 um 13:52 schrieb Michal Hocko:
> > > > > On Fri 24-08-18 13:43:16, Christian König wrote:
> > > [...]
> > > > > > That won't work like this there might be multiple
> > > > > > invalidate_range_start()/invalidate_range_end() pairs open at the 
> > > > > > same time.
> > > > > > E.g. the lock might be taken recursively and that is illegal for a
> > > > > > rw_semaphore.
> > > > > I am not sure I follow. Are you saying that one invalidate_range might
> > > > > trigger another one from the same path?
> > > > No, but what can happen is:
> > > > 
> > > > invalidate_range_start(A,B);
> > > > invalidate_range_start(C,D);
> > > > ...
> > > > invalidate_range_end(C,D);
> > > > invalidate_range_end(A,B);
> > > > 
> > > > Grabbing the read lock twice would be illegal in this case.
> > > I am sorry but I still do not follow. What is the context the two are
> > > called from?
> > 
> > I don't have the slightest idea.
> > 
> > > Can you give me an example. I simply do not see it in the
> > > code, mostly because I am not familiar with it.
> > 
> > I'm neither.
> > 
> > We stumbled over that by pure observation and after discussing the problem
> > with Jerome came up with this solution.
> > 
> > No idea where exactly that case comes from, but I can confirm that it indeed
> > happens.
> 
> Thiking about it some more, I can imagine that a notifier callback which
> performs an allocation might trigger a memory reclaim and that in turn
> might trigger a notifier to be invoked and recurse. But notifier
> shouldn't really allocate memory. They are called from deep MM code
> paths and this would be extremely deadlock prone. Maybe Jerome can come
> up some more realistic scenario. If not then I would propose to simplify
> the locking here. We have lockdep to catch self deadlocks and it is
> always better to handle a specific issue rather than having a code
> without a clear indication how it can recurse.

Multiple concurrent mmu notifier, for overlapping range or not, is
common (each concurrent threads can trigger some). So you might have
multiple invalidate_range_start() in flight for same mm and thus might
complete in different order (invalidate_range_end()). IIRC this is
what this lock was trying to protect against.

I can't think of a reason for recursive mmu notifier call right now.
I will ponder see if i remember something about it.

Cheers,
Jérôme

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