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Date:   Wed, 24 Jul 2019 20:56:17 +0200
From:   Michal Hocko <mhocko@kernel.org>
To:     Jason Gunthorpe <jgg@ziepe.ca>
Cc:     Christoph Hellwig <hch@lst.de>,
        Ralph Campbell <rcampbell@nvidia.com>, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org, nouveau@lists.freedesktop.org,
        dri-devel@lists.freedesktop.org,
        =?iso-8859-1?B?Suly9G1l?= Glisse <jglisse@redhat.com>,
        Ben Skeggs <bskeggs@redhat.com>
Subject: Re: [PATCH] mm/hmm: replace hmm_update with mmu_notifier_range
Message-ID: <20190724185617.GE6410@dhcp22.suse.cz>
References: <20190723210506.25127-1-rcampbell@nvidia.com>
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 <20190724152858.GB28493@ziepe.ca>
 <20190724175858.GC6410@dhcp22.suse.cz>
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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...
-- 
Michal Hocko
SUSE Labs