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

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Date:   Fri, 06 Nov 2020 12:53:33 -0500
From:   Rik van Riel <riel@...riel.com>
To:     Hillf Danton <hdanton@...a.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,
        mhocko@...e.com
Subject: Re: [PATCH 2/2] mm,thp,shm: limit gfp mask to no more than specified

On Fri, 2020-11-06 at 11:05 +0800, Hillf Danton wrote:
> On Thu,  5 Nov 2020 14:15:08 -0500
> > 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.
> > 
> > Make sure the gfp mask used to opportunistically allocate a THP
> > is always at least as restrictive as the original gfp mask.
> > 
> > Signed-off-by: Rik van Riel <riel@...riel.com>
> > Suggested-by: Matthew Wilcox <willy@...radead.org>
> > ---
> >  mm/shmem.c | 21 +++++++++++++++++++++
> >  1 file changed, 21 insertions(+)
> > 
> > diff --git a/mm/shmem.c b/mm/shmem.c
> > index 6c3cb192a88d..ee3cea10c2a4 100644
> > --- a/mm/shmem.c
> > +++ b/mm/shmem.c
> > @@ -1531,6 +1531,26 @@ static struct page *shmem_swapin(swp_entry_t
> > swap, gfp_t gfp,
> >  	return page;
> >  }
> >  
> > +/*
> > + * Make sure huge_gfp is always more limited than limit_gfp.
> > + * Some of the flags set permissions, while others set
> > limitations.
> > + */
> > +static gfp_t limit_gfp_mask(gfp_t huge_gfp, gfp_t limit_gfp)
> > +{
> > +	gfp_t allowflags = __GFP_IO | __GFP_FS | __GFP_RECLAIM;
> > +	gfp_t denyflags = __GFP_NOWARN | __GFP_NORETRY;
> > +	gfp_t result = huge_gfp & ~allowflags;
> > +
> > +	/*
> > +	 * Minimize the result gfp by taking the union with the deny
> > flags,
> > +	 * and the intersection of the allow flags.
> > +	 */
> > +	result |= (limit_gfp & denyflags);
> 
> Currently NORETRY is always set regardless of i915 and if it's
> determined in 1/2 then the i915 thing can be done like
> 
> 	return huge_gfp | (limit_gfp & __GFP_RECLAIM);

No, if __GFP_KSWAPD_RECLAIM or __GFP_DIRECT_RECLAIM are
not set in either huge_gfp or limit_gfp, we want to ensure
the resulting gfp does not have it set, either.

Your suggested change
would result in __GFP_KSWAPD_RECLAIM
or __GFP_DIRECT_RECLAIM getting set if it was set in either
of the input gfp variables, which is probably not the desired
behavior.


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