linux-kernel - Re: [PATCH] mm/hmm: replace hmm_update with mmu_notifier

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Date:   Wed, 24 Jul 2019 20:59:10 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Jason Gunthorpe <jgg@...pe.ca>
Cc:     Christoph Hellwig <hch@....de>,
        Ralph Campbell <rcampbell@...dia.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, nouveau@...ts.freedesktop.org,
        dri-devel@...ts.freedesktop.org,
        Jérôme Glisse <jglisse@...hat.com>,
        Ben Skeggs <bskeggs@...hat.com>
Subject: Re: [PATCH] mm/hmm: replace hmm_update with mmu_notifier_range

On Wed 24-07-19 20:56:17, Michal Hocko wrote:
> On Wed 24-07-19 15:08:37, Jason Gunthorpe wrote:
> > On Wed, Jul 24, 2019 at 07:58:58PM +0200, Michal Hocko wrote:
> [...]
> > > Maybe new users have started relying on a new semantic in the meantime,
> > > back then, none of the notifier has even started any action in blocking
> > > mode on a EAGAIN bailout. Most of them simply did trylock early in the
> > > process and bailed out so there was nothing to do for the range_end
> > > callback.
> > 
> > Single notifiers are not the problem. I tried to make this clear in
> > the commit message, but lets be more explicit.
> > 
> > We have *two* notifiers registered to the mm, A and B:
> > 
> > A invalidate_range_start: (has no blocking)
> >     spin_lock()
> >     counter++
> >     spin_unlock()
> > 
> > A invalidate_range_end:
> >     spin_lock()
> >     counter--
> >     spin_unlock()
> > 
> > And this one:
> > 
> > B invalidate_range_start: (has blocking)
> >     if (!try_mutex_lock())
> >         return -EAGAIN;
> >     counter++
> >     mutex_unlock()
> > 
> > B invalidate_range_end:
> >     spin_lock()
> >     counter--
> >     spin_unlock()
> > 
> > So now the oom path does:
> > 
> > invalidate_range_start_non_blocking:
> >  for each mn:
> >    a->invalidate_range_start
> >    b->invalidate_range_start
> >    rc = EAGAIN
> > 
> > Now we SKIP A's invalidate_range_end even though A had no idea this
> > would happen has state that needs to be unwound. A is broken.
> > 
> > B survived just fine.
> > 
> > A and B *alone* work fine, combined they fail.
> 
> But that requires that they share some state, right?
> 
> > When the commit was landed you can use KVM as an example of A and RDMA
> > ODP as an example of B
> 
> Could you point me where those two share the state please? KVM seems to
> be using kvm->mmu_notifier_count but I do not know where to look for the
> RDMA...

Scratch that. ELONGDAY... I can see your point. It is all or nothing
that doesn't really work here. Looking back at your patch it seems
reasonable but I am not sure what is supposed to be a behavior for
notifiers that failed.
-- 
Michal Hocko
SUSE Labs