linux-kernel - Re: [PATCH 2/2] mm/rmap: Improve mlock tracking for large folios

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Message-ID: <20250918143833.GD828739@cmpxchg.org>
Date: Thu, 18 Sep 2025 10:38:33 -0400
From: Johannes Weiner <hannes@...xchg.org>
To: kirill@...temov.name
Cc: Andrew Morton <akpm@...ux-foundation.org>,
	David Hildenbrand <david@...hat.com>,
	Hugh Dickins <hughd@...gle.com>,
	Matthew Wilcox <willy@...radead.org>,
	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>,
	Shakeel Butt <shakeel.butt@...ux.dev>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org, Kiryl Shutsemau <kas@...nel.org>
Subject: Re: [PATCH 2/2] mm/rmap: Improve mlock tracking for large folios

On Thu, Sep 18, 2025 at 12:21:57PM +0100, kirill@...temov.name 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. 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.
> 
> Signed-off-by: Kiryl Shutsemau <kas@...nel.org>

Acked-by: Johannes Weiner <hannes@...xchg.org>

> ---
>  mm/rmap.c | 13 ++++---------
>  1 file changed, 4 insertions(+), 9 deletions(-)
> 
> diff --git a/mm/rmap.c b/mm/rmap.c
> index 568198e9efc2..ca8d4ef42c2d 100644
> --- a/mm/rmap.c
> +++ b/mm/rmap.c
> @@ -1478,13 +1478,8 @@ static __always_inline void __folio_add_anon_rmap(struct folio *folio,
>  				 PageAnonExclusive(cur_page), 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.
> -	 */
> -	if (!folio_test_large(folio))
> +	/* Only mlock it if the folio is fully mapped to the VMA */
> +	if (folio_nr_pages(folio) == nr_pages)
>  		mlock_vma_folio(folio, vma);

Minor nit, but it might be useful to still mention in the comment that
partially mapped folios are punted to reclaim.