linux-kernel - [PATCH 09/14] mm: mremap: move_ptes() use pte_offset_map_maywrite

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Message-Id: <89bd1d48db1986ec83dcae228defa748f38238ff.1724226076.git.zhengqi.arch@bytedance.com>
Date: Wed, 21 Aug 2024 16:18:52 +0800
From: Qi Zheng <zhengqi.arch@...edance.com>
To: david@...hat.com,
	hughd@...gle.com,
	willy@...radead.org,
	muchun.song@...ux.dev,
	vbabka@...nel.org,
	akpm@...ux-foundation.org,
	rppt@...nel.org,
	vishal.moola@...il.com,
	peterx@...hat.com,
	ryan.roberts@....com
Cc: linux-kernel@...r.kernel.org,
	linux-mm@...ck.org,
	linux-arm-kernel@...ts.infradead.org,
	linuxppc-dev@...ts.ozlabs.org,
	Qi Zheng <zhengqi.arch@...edance.com>
Subject: [PATCH 09/14] mm: mremap: move_ptes() use pte_offset_map_maywrite_nolock()

In move_ptes(), we may modify the new_pte after acquiring the new_ptl, so
convert it to using pte_offset_map_maywrite_nolock(). But since we already
hold the exclusive mmap_lock, there is no need to get pmdval to do
pmd_same() check, just pass NULL to pmdvalp parameter.

Signed-off-by: Qi Zheng <zhengqi.arch@...edance.com>
---
 mm/mremap.c | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/mm/mremap.c b/mm/mremap.c
index e7ae140fc6409..33a0ccf79c32d 100644
--- a/mm/mremap.c
+++ b/mm/mremap.c
@@ -175,7 +175,12 @@ static int move_ptes(struct vm_area_struct *vma, pmd_t *old_pmd,
 		err = -EAGAIN;
 		goto out;
 	}
-	new_pte = pte_offset_map_nolock(mm, new_pmd, new_addr, &new_ptl);
+	/*
+	 * Use the maywrite version to indicate that new_pte will be modified,
+	 * but since we hold the exclusive mmap_lock, there is no need to
+	 * recheck pmd_same() after acquiring the new_ptl.
+	 */
+	new_pte = pte_offset_map_maywrite_nolock(mm, new_pmd, new_addr, NULL, &new_ptl);
 	if (!new_pte) {
 		pte_unmap_unlock(old_pte, old_ptl);
 		err = -EAGAIN;
-- 
2.20.1