linux-kernel - Re: [PATCH v4 04/11] mm/hugetlb: make hugetlb migration callback CMA aware

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Date:   Tue, 7 Jul 2020 13:22:31 +0200
From:   Vlastimil Babka <vbabka@...e.cz>
To:     js1304@...il.com, Andrew Morton <akpm@...ux-foundation.org>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        kernel-team@....com, Christoph Hellwig <hch@...radead.org>,
        Roman Gushchin <guro@...com>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Naoya Horiguchi <n-horiguchi@...jp.nec.com>,
        Michal Hocko <mhocko@...e.com>,
        Joonsoo Kim <iamjoonsoo.kim@....com>
Subject: Re: [PATCH v4 04/11] mm/hugetlb: make hugetlb migration callback CMA
 aware

On 7/7/20 9:44 AM, js1304@...il.com 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.
> 
> hugetlb page allocation process consists of two steps.  First is dequeing
> from the pool.  Second is, if there is no available page on the queue,
> allocating from the page allocator.
> 
> new_non_cma_page() can control allocation from the page allocator by
> specifying correct gfp flag.  However, dequeing cannot be controlled until
> now, so, new_non_cma_page() skips dequeing completely.  It is a suboptimal
> since new_non_cma_page() cannot utilize hugetlb pages on the queue so this
> patch tries to fix this situation.
> 
> This patch makes the deque function on hugetlb CMA aware and skip CMA
> pages if newly added skip_cma argument is passed as true.

Hmm, can't you instead change dequeue_huge_page_node_exact() to test the PF_
flag and avoid adding bool skip_cma everywhere?

I think that's what Michal suggested [1] except he said "the code already does
by memalloc_nocma_{save,restore} API". It needs extending a bit though, AFAICS.
__gup_longterm_locked() indeed does the save/restore, but restore comes before
check_and_migrate_cma_pages() and thus new_non_cma_page() is called, so an
adjustment is needed there, but that's all?

Hm the adjustment should be also done because save/restore is done around
__get_user_pages_locked(), but check_and_migrate_cma_pages() also calls
__get_user_pages_locked(), and that call not being between nocma save and
restore is thus also a correctness issue?

[1] https://lore.kernel.org/r/20200629075510.GA32461@dhcp22.suse.cz

> Acked-by: Mike Kravetz <mike.kravetz@...cle.com>
> Signed-off-by: Joonsoo Kim <iamjoonsoo.kim@....com>
> ---
>  include/linux/hugetlb.h |  6 ++----
>  mm/gup.c                |  3 ++-
>  mm/hugetlb.c            | 46 ++++++++++++++++++++++++++++++----------------
>  mm/mempolicy.c          |  2 +-
>  mm/migrate.c            |  2 +-
>  5 files changed, 36 insertions(+), 23 deletions(-)
>