linux-kernel - Re: [PATCH v3] mm/hugetlb: update nr_huge_pages and surplus_huge

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Message-ID: <Z8lntpQLn9RQMq0G@localhost.localdomain>
Date: Thu, 6 Mar 2025 10:15:34 +0100
From: Oscar Salvador <osalvador@...e.de>
To: Liu Shixin <liushixin2@...wei.com>
Cc: Muchun Song <muchun.song@...ux.dev>,
	Andrew Morton <akpm@...ux-foundation.org>,
	David Hildenbrand <david@...hat.com>,
	Kefeng Wang <wangkefeng.wang@...wei.com>,
	Peter Xu <peterx@...hat.com>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3] mm/hugetlb: update nr_huge_pages and
 surplus_huge_pages together

On Wed, Mar 05, 2025 at 11:54:09AM +0800, Liu Shixin wrote:
> In alloc_surplus_hugetlb_folio(), we increase nr_huge_pages and
> surplus_huge_pages separately. In the middle window, if we set
> nr_hugepages to smaller and satisfy count < persistent_huge_pages(h),
> the surplus_huge_pages will be increased by adjust_pool_surplus().
> 
> After adding delay in the middle window, we can reproduce the problem
> easily by following step:
> 
>  1. echo 3 > /proc/sys/vm/nr_overcommit_hugepages
>  2. mmap two hugepages. When nr_huge_pages=2 and surplus_huge_pages=1,
>     goto step 3.
>  3. echo 0 > /proc/sys/vm/nr_huge_pages
> 
> Finally, nr_huge_pages is less than surplus_huge_pages.
> 
> To fix the problem, call only_alloc_fresh_hugetlb_folio() instead and
> move down __prep_account_new_huge_page() into the hugetlb_lock.
> 
> Fixes: 0c397daea1d4 ("mm, hugetlb: further simplify hugetlb allocation API")
> Signed-off-by: Liu Shixin <liushixin2@...wei.com>

Acked-by: Oscar Salvador <osalvador@...e.de>


-- 
Oscar Salvador
SUSE Labs