linux-kernel - Re: [PATCH v3 5/8] mm/migrate: make a standard migration target allocation function

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Date:   Fri, 26 Jun 2020 09:33:42 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Joonsoo Kim <js1304@...il.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Linux Memory Management List <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>, kernel-team@....com,
        Vlastimil Babka <vbabka@...e.cz>,
        Christoph Hellwig <hch@...radead.org>,
        Roman Gushchin <guro@...com>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Naoya Horiguchi <n-horiguchi@...jp.nec.com>,
        Joonsoo Kim <iamjoonsoo.kim@....com>
Subject: Re: [PATCH v3 5/8] mm/migrate: make a standard migration target
 allocation function

On Fri 26-06-20 14:02:49, Joonsoo Kim wrote:
> 2020년 6월 25일 (목) 오후 9:05, Michal Hocko <mhocko@...nel.org>님이 작성:
> >
> > On Tue 23-06-20 15:13:45, Joonsoo Kim wrote:
[...]
> > > -struct page *new_page_nodemask(struct page *page,
> > > -                             int preferred_nid, nodemask_t *nodemask)
> > > +struct page *alloc_migration_target(struct page *page, unsigned long private)
> > >  {
> > > -     gfp_t gfp_mask = GFP_USER | __GFP_MOVABLE | __GFP_RETRY_MAYFAIL;
> > > +     struct migration_target_control *mtc;
> > > +     gfp_t gfp_mask;
> > >       unsigned int order = 0;
> > >       struct page *new_page = NULL;
> > > +     int zidx;
> > > +
> > > +     mtc = (struct migration_target_control *)private;
> > > +     gfp_mask = mtc->gfp_mask;
> > >
> > >       if (PageHuge(page)) {
> > >               return alloc_huge_page_nodemask(
> > > -                             page_hstate(compound_head(page)),
> > > -                             preferred_nid, nodemask, 0, false);
> > > +                             page_hstate(compound_head(page)), mtc->nid,
> > > +                             mtc->nmask, gfp_mask, false);
> > >       }
> > >
> > >       if (PageTransHuge(page)) {
> > > +             gfp_mask &= ~__GFP_RECLAIM;
> >
> > What's up with this gfp_mask modification?
> 
> THP page allocation uses a standard gfp masks, GFP_TRANSHUGE_LIGHT and
> GFP_TRANHUGE. __GFP_RECLAIM flags is a big part of this standard mask design.
> So, I clear it here so as not to disrupt the THP gfp mask.

Why this wasn't really needed before? I guess I must be missing
something here. This patch should be mostly mechanical convergence of
existing migration callbacks but this change adds a new behavior AFAICS.
It would effectively drop __GFP_RETRY_MAYFAIL and __GFP_KSWAPD_RECLAIM
from the mask so the allocation would "lighter". If that is your
intention then this should be a separate patch with an explanation
rather than hiding it into this patch.
-- 
Michal Hocko
SUSE Labs