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

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Date:   Fri, 26 Jun 2020 09:23:24 +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 4/8] mm/hugetlb: make hugetlb migration callback CMA
 aware

On Fri 26-06-20 13:49:15, Joonsoo Kim wrote:
> 2020년 6월 25일 (목) 오후 8:54, Michal Hocko <mhocko@...nel.org>님이 작성:
> >
> > On Tue 23-06-20 15:13:44, Joonsoo Kim wrote:
> > > From: Joonsoo Kim <iamjoonsoo.kim@....com>
> > >
> > > new_non_cma_page() in gup.c which try to allocate migration target page
> > > requires to allocate the new page that is not on the CMA area.
> > > new_non_cma_page() implements it by removing __GFP_MOVABLE flag. This way
> > > works well for THP page or normal page but not for hugetlb page.
> >
> > Could you explain why? I mean why cannot you simply remove __GFP_MOVABLE
> > flag when calling alloc_huge_page_nodemask and check for it in dequeue
> > path?
> 
> If we remove __GFP_MOVABLE when calling alloc_huge_page_nodemask, we cannot
> use the page in ZONE_MOVABLE on dequeing.
> 
> __GFP_MOVABLE is not only used for CMA selector but also used for zone
> selector.  If we clear it, we cannot use the page in the ZONE_MOVABLE
> even if it's not CMA pages.  For THP page or normal page allocation,
> there is no way to avoid this weakness without introducing another
> flag or argument. For me, introducing another flag or argument for
> these functions looks over-engineering so I don't change them and
> leave them as they are (removing __GFP_MOVABLE).
> 
> But, for alloc_huge_page_nodemask(), introducing a new argument
> doesn't seem to be a problem since it is not a general function but
> just a migration target allocation function.

I really do not see why hugetlb and only the dequeing part should be
special. This just leads to a confusion. From the code point of view it
makes perfect sense to opt out CMA regions for !__GFP_MOVABLE when
dequeing. So I would rather see a consistent behavior than a special
case deep in the hugetlb allocator layer.
-- 
Michal Hocko
SUSE Labs