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

[Jason Gunthorpe <jgg@ziepe.ca>]

On Wed, Jun 24, 2020 at 03:21:49PM +0100, Chris Wilson wrote:
> 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).

mmu notifiers are not for influencing policy of the mm.

Jason