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

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Date:   Fri, 26 Jun 2020 13:49:15 +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 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.

If you agree with this argument, I will add more description to the patch.

Thanks.