Re: [RFC] mm,drm/i915: Mark pinned shmemfs pages as unevictable

[Chris Wilson <chris@chris-wilson.co.uk>]

Quoting Vlastimil Babka (2017-06-06 13:30:15)
> On 06/06/2017 02:14 PM, Michal Hocko wrote:
> > On Tue 06-06-17 13:04:36, Chris Wilson wrote:
> >> Similar in principle to the treatment of get_user_pages, pages that
> >> i915.ko acquires from shmemfs are not immediately reclaimable and so
> >> should be excluded from the mm accounting and vmscan until they have
> >> been returned to the system via shrink_slab/i915_gem_shrink. By moving
> >> the unreclaimable pages off the inactive anon lru, not only should
> >> vmscan be improved by avoiding walking unreclaimable pages, but the
> >> system should also have a better idea of how much memory it can reclaim
> >> at that moment in time.
> > 
> > That is certainly desirable. Peter has proposed a generic pin_page (or
> > similar) API. What happened with it? I think it would be a better
> > approach than (ab)using mlock API. I am also not familiar with the i915
> > code to be sure that using lock_page is really safe here. I think that
> > all we need is to simply move those pages in/out to/from unevictable LRU
> > list on pin/unpining.
> 
> Hmm even when on unevictable list, the pages were still allocated as
> MOVABLE, while pinning prevents them from being migrated, so it doesn't
> play well with compaction/grouping by mobility/CMA etc. Addressing that
> would be more useful IMHO, and e.g. one of the features envisioned for
> the pinning API was to first migrate the pinned pages out of movable
> zones and CMA/MOVABLE pageblocks.

Whilst today i915 doesn't take part in compaction, we do have
plans/patches for enabling migratepage. It would be nice not to nip that
in the bud.
-Chris

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