linux-kernel - Re: [PATCH] mm/migrate: Split only transparent huge pages when allocation fails

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Message-ID: <20180924143027.GE18685@dhcp22.suse.cz>
Date:   Mon, 24 Sep 2018 16:30:27 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Anshuman Khandual <anshuman.khandual@....com>
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        akpm@...ux-foundation.org
Subject: Re: [PATCH] mm/migrate: Split only transparent huge pages when
 allocation fails

On Mon 24-09-18 19:44:55, Anshuman Khandual wrote:
> When unmap_and_move[_huge_page] function fails due to lack of memory, the
> splitting should happen only for transparent huge pages not for HugeTLB
> pages. PageTransHuge() returns true for both THP and HugeTLB pages. Hence
> the conditonal check should test PagesHuge() flag to make sure that given
> pages is not a HugeTLB one.

Well spotted! Have you actually seen this happening or this is review
driven? I am wondering what would be the real effect of this mismatch?
I have tried to follow to code path but I suspect
split_huge_page_to_list would fail for hugetlbfs pages. If there is a
more serious effect then we should mark the patch for stable as well.

> 
> Fixes: 94723aafb9 ("mm: unclutter THP migration")
> Signed-off-by: Anshuman Khandual <anshuman.khandual@....com>

Acked-by: Michal Hocko <mhocko@...e.com>

> ---
>  mm/migrate.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/mm/migrate.c b/mm/migrate.c
> index d6a2e89..d2297fe 100644
> --- a/mm/migrate.c
> +++ b/mm/migrate.c
> @@ -1411,7 +1411,7 @@ int migrate_pages(struct list_head *from, new_page_t get_new_page,
>  				 * we encounter them after the rest of the list
>  				 * is processed.
>  				 */
> -				if (PageTransHuge(page)) {
> +				if (PageTransHuge(page) && !PageHuge(page)) {
>  					lock_page(page);
>  					rc = split_huge_page_to_list(page, from);
>  					unlock_page(page);
> -- 
> 2.7.4

-- 
Michal Hocko
SUSE Labs