linux-kernel - [PATCH] mm: move rmap of mTHP upon CoW reuse

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Message-Id: <20250925085429.41607-1-dev.jain@arm.com>
Date: Thu, 25 Sep 2025 14:24:29 +0530
From: Dev Jain <dev.jain@....com>
To: akpm@...ux-foundation.org,
	david@...hat.com
Cc: lorenzo.stoakes@...cle.com,
	Liam.Howlett@...cle.com,
	vbabka@...e.cz,
	rppt@...nel.org,
	surenb@...gle.com,
	mhocko@...e.com,
	linux-mm@...ck.org,
	linux-kernel@...r.kernel.org,
	Dev Jain <dev.jain@....com>
Subject: [PATCH] mm: move rmap of mTHP upon CoW reuse

At wp-fault time, when we find that a folio is exclusively mapped, we move
folio->mapping to the faulting VMA's anon_vma, so that rmap overhead
reduces. This is currently done for small folios (base pages) and
PMD-mapped THPs. Do this for mTHP too.

Signed-off-by: Dev Jain <dev.jain@....com>
---
mm-selftests pass.

 mm/memory.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/mm/memory.c b/mm/memory.c
index 7e32eb79ba99..ec04d2cec6b1 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -4014,6 +4014,11 @@ static bool __wp_can_reuse_large_anon_folio(struct folio *folio,
 	 * an additional folio reference and never ended up here.
 	 */
 	exclusive = true;
+
+	if (folio_trylock(folio)) {
+		folio_move_anon_rmap(folio, vma);
+		folio_unlock(folio);
+	}
 unlock:
 	folio_unlock_large_mapcount(folio);
 	return exclusive;
-- 
2.30.2