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

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Message-ID: <20250918112157.410172-3-kirill@shutemov.name>
Date: Thu, 18 Sep 2025 12:21:57 +0100
From: kirill@...temov.name
To: 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: [PATCH 2/2] mm/rmap: Improve mlock tracking for large folios

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>
---
 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);
 }
 
@@ -1620,8 +1615,8 @@ 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 */
+	if (folio_nr_pages(folio) == nr_pages)
 		mlock_vma_folio(folio, vma);
 }
 
-- 
2.50.1