On Fri, 06 Nov 2020 12:53:33 -0500 Rik van Riel wrote:
> 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@xxxxxxxxxxx>
> > > Suggested-by: Matthew Wilcox <willy@xxxxxxxxxxxxx>
> > > ---
> > > 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.
That means huge_gfp can play game without i915 considered if
__GFP_RECLAIM is determined in 1/2 too. Then things become
simpler because we have no need to check limit_gfp from the
begining.
>
> 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.
It makes sense on if we could not determine __GFP_RECLAIM
without i915 considered. Now it is safe to ignore it.
