linux-kernel - Re: [PATCH 1/2] mm/mmu_notifier: Mark up direct reclaim paths with MAYFAIL

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Message-ID: <20200624141604.GH6578@ziepe.ca>
Date:   Wed, 24 Jun 2020 11:16:04 -0300
From:   Jason Gunthorpe <jgg@...pe.ca>
To:     Chris Wilson <chris@...is-wilson.co.uk>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        intel-gfx@...ts.freedesktop.org,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH 1/2] mm/mmu_notifier: Mark up direct reclaim paths with
 MAYFAIL

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.

Still, again, notifiers are for tracking, not for influencing MM policy.

Jason