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

[Baolin Wang <baolin.wang@linux.alibaba.com>]

On 2025/9/19 20:40, Kiryl Shutsemau wrote:
> From: Kiryl Shutsemau <kas@kernel.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@kernel.org>
> Acked-by: David Hildenbrand <david@redhat.com>
> Acked-by: Johannes Weiner <hannes@cmpxchg.org>
> Acked-by: Shakeel Butt <shakeel.butt@linux.dev>
> Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
> ---

LGTM.
Reviewed-by: Baolin Wang <baolin.wang@linux.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);
>   }
>