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Date: Wed, 25 Mar 2020 14:06:56 +0100
From: Michal Hocko <mhocko@suse.com>
To: Jason Gunthorpe <jgg@mellanox.com>
Cc: linux-mm@kvack.org,
	=?iso-8859-1?B?Suly9G1l?= Glisse <jglisse@redhat.com>,
	Christoph Hellwig <hch@lst.de>
Subject: Re: [PATCH v3] mm/mmu_notifier: prevent unpaired invalidate_start
 and invalidate_end
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On Wed 25-03-20 09:14:07, Jason Gunthorpe wrote:
> On Wed, Mar 25, 2020 at 09:01:17AM +0100, Michal Hocko wrote:
> > > > I'm going to put this in linux-next to see if there are any reports of
> > > > the pr_warn failing.
> > > > 
> > > > Michal, are you happy with this solution now?
> > > 
> > > It's been a month in linux-next now, with no complaints. If there are
> > > no comments I will go ahead to send it in the hmm PR.
> > 
> > I will not block this but it still looks like a wrong approach. A more
> > robust solution would be to allow calling invalidate_range_end even for
> > the failing invalidate_start.
> 
> That requires reliably walking a rcu list backwards under rcu. I don't
> have a good feeling about that algorithm. Do you know of a solution?

No, not really. I would be pushing for it if I had. Maybe we want a
different data structure than the RCU list. But...

> Since we don't actually have any users that care about this any more,
> I felt this testable solution was better.

... you are right that this is probably the best short term workaround.
-- 
Michal Hocko
SUSE Labs