linux-kernel - [PATCH] mm/mremap: Honour writable bit in mremap pte batching

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Message-Id: <20251028063952.90313-1-dev.jain@arm.com>
Date: Tue, 28 Oct 2025 12:09:52 +0530
From: Dev Jain <dev.jain@....com>
To: linux-kernel@...r.kernel.org
Cc: Dev Jain <dev.jain@....com>,
	stable@...r.kernel.org,
	David Hildenbrand <david@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"Liam R. Howlett" <Liam.Howlett@...cle.com>,
	Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
	Vlastimil Babka <vbabka@...e.cz>,
	Jann Horn <jannh@...gle.com>,
	Pedro Falcato <pfalcato@...e.de>,
	Barry Song <baohua@...nel.org>,
	linux-mm@...ck.org (open list:MEMORY MAPPING)
Subject: [PATCH] mm/mremap: Honour writable bit in mremap pte batching

Currently mremap folio pte batch ignores the writable bit during figuring
out a set of similar ptes mapping the same folio. Suppose that the first
pte of the batch is writable while the others are not - set_ptes will
end up setting the writable bit on the other ptes, which is a violation
of mremap semantics. Therefore, use FPB_RESPECT_WRITE to check the writable
bit while determining the pte batch.

Cc: stable@...r.kernel.org #6.17
Fixes: f822a9a81a31 ("mm: optimize mremap() by PTE batching")
Reported-by: David Hildenbrand <david@...hat.com>
Debugged-by: David Hildenbrand <david@...hat.com>
Signed-off-by: Dev Jain <dev.jain@....com>
---
mm-selftests pass. Based on mm-new. Need David H. to confirm whether
the repro passes.

 mm/mremap.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/mm/mremap.c b/mm/mremap.c
index a7f531c17b79..8ad06cf50783 100644
--- a/mm/mremap.c
+++ b/mm/mremap.c
@@ -187,7 +187,7 @@ static int mremap_folio_pte_batch(struct vm_area_struct *vma, unsigned long addr
 	if (!folio || !folio_test_large(folio))
 		return 1;
 
-	return folio_pte_batch(folio, ptep, pte, max_nr);
+	return folio_pte_batch_flags(folio, NULL, ptep, &pte, max_nr, FPB_RESPECT_WRITE);
 }
 
 static int move_ptes(struct pagetable_move_control *pmc,
-- 
2.30.2