linux-kernel - [PATCH v2 11/14] mm: userfaultfd: move_pages_pte() use pte_offset_map_rw

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Message-Id: <d4e0c8970662ecc88352822e6d81a11b4dd72b1e.1724310149.git.zhengqi.arch@bytedance.com>
Date: Thu, 22 Aug 2024 15:13:26 +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 v2 11/14] mm: userfaultfd: move_pages_pte() use pte_offset_map_rw_nolock()

In move_pages_pte(), we may modify the dst_pte and src_pte after acquiring
the ptl, so convert it to using pte_offset_map_rw_nolock(). But since we
already do the pte_same() check, there is no need to get pmdval to do
pmd_same() check, just pass a dummy variable to it.

Signed-off-by: Qi Zheng <zhengqi.arch@...edance.com>
---
 mm/userfaultfd.c | 15 ++++++++++++---
 1 file changed, 12 insertions(+), 3 deletions(-)

diff --git a/mm/userfaultfd.c b/mm/userfaultfd.c
index 6ef42d9cd482e..414ee744257b7 100644
--- a/mm/userfaultfd.c
+++ b/mm/userfaultfd.c
@@ -1135,7 +1135,7 @@ static int move_pages_pte(struct mm_struct *mm, pmd_t *dst_pmd, pmd_t *src_pmd,
 	spinlock_t *src_ptl, *dst_ptl;
 	pte_t *src_pte = NULL;
 	pte_t *dst_pte = NULL;
-
+	pmd_t dummy_pmdval;
 	struct folio *src_folio = NULL;
 	struct anon_vma *src_anon_vma = NULL;
 	struct mmu_notifier_range range;
@@ -1146,7 +1146,14 @@ static int move_pages_pte(struct mm_struct *mm, pmd_t *dst_pmd, pmd_t *src_pmd,
 				src_addr, src_addr + PAGE_SIZE);
 	mmu_notifier_invalidate_range_start(&range);
 retry:
-	dst_pte = pte_offset_map_nolock(mm, dst_pmd, dst_addr, &dst_ptl);
+	/*
+	 * Use the maywrite version to indicate that dst_pte will be modified,
+	 * but since we will use pte_same() to detect the change of the pte
+	 * entry, there is no need to get pmdval, so just pass a dummy variable
+	 * to it.
+	 */
+	dst_pte = pte_offset_map_rw_nolock(mm, dst_pmd, dst_addr, &dummy_pmdval,
+					   &dst_ptl);
 
 	/* Retry if a huge pmd materialized from under us */
 	if (unlikely(!dst_pte)) {
@@ -1154,7 +1161,9 @@ static int move_pages_pte(struct mm_struct *mm, pmd_t *dst_pmd, pmd_t *src_pmd,
 		goto out;
 	}
 
-	src_pte = pte_offset_map_nolock(mm, src_pmd, src_addr, &src_ptl);
+	/* same as dst_pte */
+	src_pte = pte_offset_map_rw_nolock(mm, src_pmd, src_addr, &dummy_pmdval,
+					   &src_ptl);
 
 	/*
 	 * We held the mmap_lock for reading so MADV_DONTNEED
-- 
2.20.1