linux-kernel - Re: [PATCH v3 4/8] mm/hugetlb: make hugetlb migration callback CMA aware

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Date:   Tue, 30 Jun 2020 16:22:22 +0900
From:   Joonsoo Kim <js1304@...il.com>
To:     Michal Hocko <mhocko@...nel.org>
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 4/8] mm/hugetlb: make hugetlb migration callback CMA aware

2020년 6월 30일 (화) 오후 3:42, Michal Hocko <mhocko@...nel.org>님이 작성:
>
> On Tue 30-06-20 15:30:04, Joonsoo Kim wrote:
> > 2020년 6월 29일 (월) 오후 4:55, Michal Hocko <mhocko@...nel.org>님이 작성:
> [...]
> > > diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> > > index 57ece74e3aae..c1595b1d36f3 100644
> > > --- a/mm/hugetlb.c
> > > +++ b/mm/hugetlb.c
> > > @@ -1092,10 +1092,14 @@ static struct page *dequeue_huge_page_nodemask(struct hstate *h, gfp_t gfp_mask,
> > >  /* Movability of hugepages depends on migration support. */
> > >  static inline gfp_t htlb_alloc_mask(struct hstate *h)
> > >  {
> > > +       gfp_t gfp;
> > > +
> > >         if (hugepage_movable_supported(h))
> > > -               return GFP_HIGHUSER_MOVABLE;
> > > +               gfp = GFP_HIGHUSER_MOVABLE;
> > >         else
> > > -               return GFP_HIGHUSER;
> > > +               gfp = GFP_HIGHUSER;
> > > +
> > > +       return current_gfp_context(gfp);
> > >  }
> > >
> > >  static struct page *dequeue_huge_page_vma(struct hstate *h,
> > >
> > > If we even fix this general issue for other allocations and allow a
> > > better CMA exclusion then it would be implemented consistently for
> > > everybody.
> >
> > Yes, I have reviewed the memalloc_nocma_{} APIs and found the better way
> > for CMA exclusion. I will do it after this patch is finished.
> >
> > > Does this make more sense to you are we still not on the same page wrt
> > > to the actual problem?
> >
> > Yes, but we have different opinions about it. As said above, I will make
> > a patch for better CMA exclusion after this patchset. It will make
> > code consistent.
> > I'd really appreciate it if you wait until then.
>
> As I've said I would _prefer_ simplicity over "correctness" if it is only
> partial and hard to reason about from the userspace experience but this
> is not something I would _insist_ on. If Mike as a maintainer of the
> code is ok with that then I will not stand in the way.

Okay.

> But please note that a missing current_gfp_context inside
> htlb_alloc_mask is a subtle bug. I do not think it matters right now but
> with a growing use of scoped apis this might actually hit some day so I
> believe we want to have it in place.

Okay. I will keep in mind and consider it when fixing CMA exclusion on the
other patchset.

Thanks.