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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: <159300796224.4527.2014771396582759689@build.alporthouse.com>
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Date:   Wed, 24 Jun 2020 15:12:42 +0100
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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".
-Chris