linux-kernel - Re: [PATCH 2/2] drm/i915: Mark pinned shmemfs pages as unevictable

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Date:   Tue, 16 Oct 2018 19:31:06 +0100
From:   Chris Wilson <chris@...is-wilson.co.uk>
To:     Kuo-Hsin Yang <vovoy@...omium.org>,
        Michal Hocko <mhocko@...nel.org>
Cc:     linux-kernel@...r.kernel.org, intel-gfx@...ts.freedesktop.org,
        linux-mm@...ck.org, akpm@...ux-foundation.org,
        peterz@...radead.org, dave.hansen@...el.com, corbet@....net,
        hughd@...gle.com, joonas.lahtinen@...ux.intel.com,
        marcheu@...omium.org, hoegsberg@...omium.org
Subject: Re: [PATCH 2/2] drm/i915: Mark pinned shmemfs pages as unevictable

Quoting Michal Hocko (2018-10-16 19:21:55)
> On Wed 17-10-18 01:43:00, Kuo-Hsin Yang wrote:
> > The i915 driver use shmemfs to allocate backing storage for gem objects.
> > These shmemfs pages can be pinned (increased ref count) by
> > shmem_read_mapping_page_gfp(). When a lot of pages are pinned, vmscan
> > wastes a lot of time scanning these pinned pages. Mark these pinned
> > pages as unevictable to speed up vmscan.
> 
> I would squash the two patches into the single one. One more thing
> though. One more thing to be careful about here. Unless I miss something
> such a page is not migrateable so it shouldn't be allocated from a
> movable zone. Does mapping_gfp_constraint contains __GFP_MOVABLE? If
> yes, we want to drop it as well. Other than that the patch makes sense
> with my very limited knowlege of the i915 code of course.

They are not migrateable today. But we have proposed hooking up
.migratepage and setting __GFP_MOVABLE which would then include unlocking
the mapping at migrate time.

Fwiw, the shmem_unlock_mapping() call feels quite expensive, almost
nullifying the advantage gained from not walking the lists in reclaim.
I'll have better numbers in a couple of days.
-Chris