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From:   Chris Wilson <chris@chris-wilson.co.uk>
Subject: Re: [PATCH 1/2] mm/mmu_notifier: Mark up direct reclaim paths with MAYFAIL
To:     Jason Gunthorpe <jgg@ziepe.ca>
Cc:     linux-mm@kvack.org, linux-kernel@vger.kernel.org,
        intel-gfx@lists.freedesktop.org,
        Andrew Morton <akpm@linux-foundation.org>
Message-ID: <159300850942.4527.8335506003268197914@build.alporthouse.com>
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Date:   Wed, 24 Jun 2020 15:21:49 +0100
Sender: linux-kernel-owner@vger.kernel.org
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Quoting Jason Gunthorpe (2020-06-24 15:16:04)
> On Wed, Jun 24, 2020 at 03:12:42PM +0100, Chris Wilson wrote:
> > Quoting Jason Gunthorpe (2020-06-24 13:39:10)
> > > On Wed, Jun 24, 2020 at 01:21:03PM +0100, Chris Wilson wrote:
> > > > Quoting Jason Gunthorpe (2020-06-24 13:10:53)
> > > > > On Wed, Jun 24, 2020 at 09:02:47AM +0100, Chris Wilson wrote:
> > > > > > When direct reclaim enters the shrinker and tries to reclaim pages, it
> > > > > > has to opportunitically unmap them [try_to_unmap_one]. For direct
> > > > > > reclaim, the calling context is unknown and may include attempts to
> > > > > > unmap one page of a dma object while attempting to allocate more pages
> > > > > > for that object. Pass the information along that we are inside an
> > > > > > opportunistic unmap that can allow that page to remain referenced and
> > > > > > mapped, and let the callback opt in to avoiding a recursive wait.
> > > > > 
> > > > > i915 should already not be holding locks shared with the notifiers
> > > > > across allocations that can trigger reclaim. This is already required
> > > > > to use notifiers correctly anyhow - why do we need something in the
> > > > > notifiers?
> > > > 
> > > > for (n = 0; n < num_pages; n++)
> > > >       pin_user_page()
> > > > 
> > > > may call try_to_unmap_page from the lru shrinker for [0, n-1].
> > > 
> > > Yes, of course you can't hold any locks that intersect with notifiers
> > > across pin_user_page()/get_user_page()
> > 
> > What lock though? It's just the page refcount, shrinker asks us to drop
> > it [via mmu], we reply we would like to keep using that page as freeing
> > it for the current allocation is "robbing Peter to pay Paul".
> 
> Maybe I'm unclear what this series is actually trying to fix? 
> 
> You said "avoiding a recursive wait" which sounds like some locking
> deadlock to me.

It's the shrinker being called while we are allocating for/on behalf of
the object. As we are actively using the object, we don't want to free
it -- the partial object allocation being the clearest, if the object
consists of 2 pages, trying to free page 0 in order to allocate page 1
has to fail (and the shrinker should find another candidate to reclaim,
or fail the allocation).
-Chris