linux-kernel - [PATCH v4 09/13] mm: mremap: move_ptes() use pte_offset_map_rw

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Message-Id: <c7b3ba9e8e58efe7bba9d52ce30a8ff4f4298e2f.1727148662.git.zhengqi.arch@bytedance.com>
Date: Tue, 24 Sep 2024 14:10:01 +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,
	christophe.leroy2@...soprasteria.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 v4 09/13] mm: mremap: move_ptes() use pte_offset_map_rw_nolock()

In move_ptes(), we may modify the new_pte after acquiring the new_ptl, so
convert it to using pte_offset_map_rw_nolock(). Now new_pte is none, so
hpage_collapse_scan_file() path can not find this by traversing
file->f_mapping, so there is no concurrency with retract_page_tables(). In
addition, we already hold the exclusive mmap_lock, so this new_pte page is
stable, so there is no need to get pmdval and do pmd_same() check.

Signed-off-by: Qi Zheng <zhengqi.arch@...edance.com>
---
 mm/mremap.c | 11 ++++++++++-
 1 file changed, 10 insertions(+), 1 deletion(-)

diff --git a/mm/mremap.c b/mm/mremap.c
index 24712f8dbb6b5..9dffd4a5b4d18 100644
--- a/mm/mremap.c
+++ b/mm/mremap.c
@@ -143,6 +143,7 @@ static int move_ptes(struct vm_area_struct *vma, pmd_t *old_pmd,
 	spinlock_t *old_ptl, *new_ptl;
 	bool force_flush = false;
 	unsigned long len = old_end - old_addr;
+	pmd_t dummy_pmdval;
 	int err = 0;
 
 	/*
@@ -175,7 +176,15 @@ 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);
+	/*
+	 * Now new_pte is none, so hpage_collapse_scan_file() path can not find
+	 * this by traversing file->f_mapping, so there is no concurrency with
+	 * retract_page_tables(). In addition, we already hold the exclusive
+	 * mmap_lock, so this new_pte page is stable, so there is no need to get
+	 * pmdval and do pmd_same() check.
+	 */
+	new_pte = pte_offset_map_rw_nolock(mm, new_pmd, new_addr, &dummy_pmdval,
+					   &new_ptl);
 	if (!new_pte) {
 		pte_unmap_unlock(old_pte, old_ptl);
 		err = -EAGAIN;
-- 
2.20.1