linux-kernel - [PATCH v3 09/14] mm: mremap: move_ptes() use pte_offset_map_rw

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Message-Id: <20240904084022.32728-10-zhengqi.arch@bytedance.com>
Date: Wed,  4 Sep 2024 16:40:17 +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 v3 09/14] 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(). Since we may free the PTE
page in retract_page_tables() without holding the read lock of mmap_lock,
so we still need to do a pmd_same() check after holding the PTL.

Signed-off-by: Qi Zheng <zhengqi.arch@...edance.com>
---
 mm/mremap.c | 20 ++++++++++++++++++--
 1 file changed, 18 insertions(+), 2 deletions(-)

diff --git a/mm/mremap.c b/mm/mremap.c
index 24712f8dbb6b5..16e54151395ad 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 pmdval;
 	int err = 0;
 
 	/*
@@ -175,14 +176,29 @@ 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);
+	/*
+	 * Since we may free the PTE page in retract_page_tables() without
+	 * holding the read lock of mmap_lock, so we still need to do a
+	 * pmd_same() check after holding the PTL.
+	 */
+	new_pte = pte_offset_map_rw_nolock(mm, new_pmd, new_addr, &pmdval,
+					   &new_ptl);
 	if (!new_pte) {
 		pte_unmap_unlock(old_pte, old_ptl);
 		err = -EAGAIN;
 		goto out;
 	}
-	if (new_ptl != old_ptl)
+	if (new_ptl != old_ptl) {
 		spin_lock_nested(new_ptl, SINGLE_DEPTH_NESTING);
+
+		if (unlikely(!pmd_same(pmdval, pmdp_get_lockless(new_pmd)))) {
+			pte_unmap_unlock(new_pte, new_ptl);
+			pte_unmap_unlock(old_pte, old_ptl);
+			err = -EAGAIN;
+			goto out;
+		}
+	}
+
 	flush_tlb_batched_pending(vma->vm_mm);
 	arch_enter_lazy_mmu_mode();
 
-- 
2.20.1