linux-kernel - Re: [RFC PATCH 1/2] mm,page_alloc: Make alloc_contig

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Date:   Wed, 10 Feb 2021 09:56:37 +0100
From:   David Hildenbrand <david@...hat.com>
To:     Oscar Salvador <osalvador@...e.de>,
        Mike Kravetz <mike.kravetz@...cle.com>
Cc:     Muchun Song <songmuchun@...edance.com>,
        Michal Hocko <mhocko@...nel.org>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH 1/2] mm,page_alloc: Make alloc_contig_range handle
 in-use hugetlb pages

On 08.02.21 11:38, Oscar Salvador wrote:
> alloc_contig_range is not prepared to handle hugetlb pages and will
> fail if it ever sees one, but since they can be migrated as any other
> page (LRU and Movable), it makes sense to also handle them.
> 
> For now, do it only when coming from alloc_contig_range.
> 
> Signed-off-by: Oscar Salvador <osalvador@...e.de>
> ---
>   mm/compaction.c | 17 +++++++++++++++++
>   mm/vmscan.c     |  5 +++--
>   2 files changed, 20 insertions(+), 2 deletions(-)
> 
> diff --git a/mm/compaction.c b/mm/compaction.c
> index e5acb9714436..89cd2e60da29 100644
> --- a/mm/compaction.c
> +++ b/mm/compaction.c
> @@ -940,6 +940,22 @@ isolate_migratepages_block(struct compact_control *cc, unsigned long low_pfn,
>   			goto isolate_fail;
>   		}
>   
> +		/*
> +		 * Handle hugetlb pages only when coming from alloc_contig
> +		 */
> +		if (PageHuge(page) && cc->alloc_contig) {
> +			if (page_count(page)) {

I wonder if we should care about races here. What if someone 
concurrently allocates/frees?

Note that PageHuge() succeeds on tail pages, isolate_huge_page() not, i 
assume we'll have to handle that as well.

I wonder if it would make sense to move some of the magic to hugetlb 
code and handle it there with less chances for races (isolate if used, 
alloc-and-dissolve if not).

> +				/*
> +				 * Hugetlb page in-use. Isolate and migrate.
> +				 */
> +				if (isolate_huge_page(page, &cc->migratepages)) {
> +					low_pfn += compound_nr(page) - 1;
> +					goto isolate_success_no_list;
> +				}
> +			}

-- 
Thanks,

David / dhildenb