linux-kernel - Re: [PATCH 2/2] mm,thp,shm: limit gfp mask to no more than specified

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Date:   Mon, 23 Nov 2020 14:39:51 -0500
From:   Rik van Riel <riel@...riel.com>
To:     Michal Hocko <mhocko@...e.com>
Cc:     hughd@...gle.com, xuyu@...ux.alibaba.com,
        akpm@...ux-foundation.org, mgorman@...e.de, aarcange@...hat.com,
        willy@...radead.org, linux-kernel@...r.kernel.org,
        kernel-team@...com, linux-mm@...ck.org, vbabka@...e.cz,
        Andrey Grodzovsky <andrey.grodzovsky@....com>,
        Chris Wilson <chris@...is-wilson.co.uk>
Subject: Re: [PATCH 2/2] mm,thp,shm: limit gfp mask to no more than specified

On Thu, 2020-11-19 at 10:38 +0100, Michal Hocko wrote:
> On Fri 13-11-20 22:40:40, Rik van Riel wrote:
> > On Thu, 2020-11-12 at 12:22 +0100, Michal Hocko wrote:
> > > [Cc Chris for i915 and Andray]
> > > 
> > > On Thu 05-11-20 14:15:08, Rik van Riel wrote:
> > > > Matthew Wilcox pointed out that the i915 driver
> > > > opportunistically
> > > > allocates tmpfs memory, but will happily reclaim some of its
> > > > pool if no memory is available.
> > > 
> > > It would be good to explicitly mention the requested gfp flags
> > > for
> > > those
> > > allocations. i915 uses __GFP_NORETRY | __GFP_NOWARN, or
> > > GFP_KERNEL.
> > > Is
> > > __shmem_rw really meant to not allocate from highmeme/movable
> > > zones?
> > > Can
> > > it be ever backed by THPs?
> > 
> > You are right, I need to copy the zone flags __GFP_DMA
> > through
> > __GFP_MOVABLE straight from the limiting gfp_mask
> > into the gfp_mask used for THP allocations, and not use
> > the default THP zone flags if the caller specifies something
> > else.
> > 
> > I'll send out a new version that fixes that.
> 
> Can we make one step back here and actually check whether all this is
> actually needed for those shmem users before adding more hacks here
> and
> there?

It doesn't look like that is needed, after all.

The i915 driver seems to support having its buffer in
highmem, the shmem_pwrite and shmem_pread functions
both do kmap/kunmap.

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