linux-kernel - Re: [PATCHv2 5/5] mm/rmap: Improve mlock tracking for large folios

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Message-ID: <29d9fa8a-5ff2-47f3-8503-fe3032097e5e@linux.alibaba.com>
Date: Mon, 22 Sep 2025 11:20:57 +0800
From: Baolin Wang <baolin.wang@...ux.alibaba.com>
To: Kiryl Shutsemau <kirill@...temov.name>,
 Andrew Morton <akpm@...ux-foundation.org>,
 David Hildenbrand <david@...hat.com>, Hugh Dickins <hughd@...gle.com>,
 Matthew Wilcox <willy@...radead.org>
Cc: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
 "Liam R. Howlett" <Liam.Howlett@...cle.com>, Vlastimil Babka
 <vbabka@...e.cz>, Mike Rapoport <rppt@...nel.org>,
 Suren Baghdasaryan <surenb@...gle.com>, Michal Hocko <mhocko@...e.com>,
 Rik van Riel <riel@...riel.com>, Harry Yoo <harry.yoo@...cle.com>,
 Johannes Weiner <hannes@...xchg.org>, Shakeel Butt <shakeel.butt@...ux.dev>,
 linux-mm@...ck.org, linux-kernel@...r.kernel.org,
 Kiryl Shutsemau <kas@...nel.org>
Subject: Re: [PATCHv2 5/5] mm/rmap: Improve mlock tracking for large folios



On 2025/9/19 20:40, Kiryl Shutsemau wrote:
> From: Kiryl Shutsemau <kas@...nel.org>
> 
> The kernel currently does not mlock large folios when adding them to
> rmap, stating that it is difficult to confirm that the folio is fully
> mapped and safe to mlock it.
> 
> This leads to a significant undercount of Mlocked in /proc/meminfo,
> causing problems in production where the stat was used to estimate
> system utilization and determine if load shedding is required.
> 
> However, nowadays the caller passes a number of pages of the folio that
> are getting mapped, making it easy to check if the entire folio is
> mapped to the VMA.
> 
> mlock the folio on rmap if it is fully mapped to the VMA.
> 
> Mlocked in /proc/meminfo can still undercount, but the value is closer
> the truth and is useful for userspace.
> 
> Signed-off-by: Kiryl Shutsemau <kas@...nel.org>
> Acked-by: David Hildenbrand <david@...hat.com>
> Acked-by: Johannes Weiner <hannes@...xchg.org>
> Acked-by: Shakeel Butt <shakeel.butt@...ux.dev>
> Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
> ---

LGTM.
Reviewed-by: Baolin Wang <baolin.wang@...ux.alibaba.com>

>   mm/rmap.c | 19 ++++++++++++-------
>   1 file changed, 12 insertions(+), 7 deletions(-)
> 
> diff --git a/mm/rmap.c b/mm/rmap.c
> index 482e6504fa88..6e09956670f4 100644
> --- a/mm/rmap.c
> +++ b/mm/rmap.c
> @@ -1462,12 +1462,12 @@ static __always_inline void __folio_add_anon_rmap(struct folio *folio,
>   	}
>   
>   	/*
> -	 * For large folio, only mlock it if it's fully mapped to VMA. It's
> -	 * not easy to check whether the large folio is fully mapped to VMA
> -	 * here. Only mlock normal 4K folio and leave page reclaim to handle
> -	 * large folio.
> +	 * Only mlock it if the folio is fully mapped to the VMA.
> +	 *
> +	 * Partially mapped folios can be split on reclaim and part outside
> +	 * of mlocked VMA can be evicted or freed.
>   	 */
> -	if (!folio_test_large(folio))
> +	if (folio_nr_pages(folio) == nr_pages)
>   		mlock_vma_folio(folio, vma);
>   }
>   
> @@ -1603,8 +1603,13 @@ static __always_inline void __folio_add_file_rmap(struct folio *folio,
>   	nr = __folio_add_rmap(folio, page, nr_pages, vma, level, &nr_pmdmapped);
>   	__folio_mod_stat(folio, nr, nr_pmdmapped);
>   
> -	/* See comments in folio_add_anon_rmap_*() */
> -	if (!folio_test_large(folio))
> +	/*
> +	 * Only mlock it if the folio is fully mapped to the VMA.
> +	 *
> +	 * Partially mapped folios can be split on reclaim and part outside
> +	 * of mlocked VMA can be evicted or freed.
> +	 */
> +	if (folio_nr_pages(folio) == nr_pages)
>   		mlock_vma_folio(folio, vma);
>   }
>