linux-kernel - [PATCH v4 2/4] mm: Clear vmf->pte after pte_unmap

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Date:   Wed, 15 Sep 2021 14:15:33 -0400
From:   Peter Xu <peterx@...hat.com>
To:     linux-kernel@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org
Cc:     Hugh Dickins <hughd@...gle.com>,
        David Hildenbrand <david@...hat.com>,
        Liam Howlett <liam.howlett@...cle.com>,
        Mike Rapoport <rppt@...ux.vnet.ibm.com>,
        Alistair Popple <apopple@...dia.com>,
        Matthew Wilcox <willy@...radead.org>, peterx@...hat.com,
        Yang Shi <shy828301@...il.com>,
        "Kirill A . Shutemov" <kirill@...temov.name>,
        Jerome Glisse <jglisse@...hat.com>,
        Miaohe Lin <linmiaohe@...wei.com>,
        Andrea Arcangeli <aarcange@...hat.com>
Subject: [PATCH v4 2/4] mm: Clear vmf->pte after pte_unmap_same() returns

pte_unmap_same() will always unmap the pte pointer.  After the unmap, vmf->pte
will not be valid any more, we should clear it.

It was safe only because no one is accessing vmf->pte after pte_unmap_same()
returns, since the only caller of pte_unmap_same() (so far) is do_swap_page(),
where vmf->pte will in most cases be overwritten very soon.

Directly pass in vmf into pte_unmap_same() and then we can also avoid the long
parameter list too, which should be a nice cleanup.

Reviewed-by: Miaohe Lin <linmiaohe@...wei.com>
Reviewed-by: David Hildenbrand <david@...hat.com>
Reviewed-by: Liam Howlett <liam.howlett@...cle.com>
Signed-off-by: Peter Xu <peterx@...hat.com>
---
 mm/memory.c | 12 ++++++------
 1 file changed, 6 insertions(+), 6 deletions(-)

diff --git a/mm/memory.c b/mm/memory.c
index 25fc46e87214..7b095f07c4ef 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -2724,19 +2724,19 @@ EXPORT_SYMBOL_GPL(apply_to_existing_page_range);
  * proceeding (but do_wp_page is only called after already making such a check;
  * and do_anonymous_page can safely check later on).
  */
-static inline int pte_unmap_same(struct mm_struct *mm, pmd_t *pmd,
-				pte_t *page_table, pte_t orig_pte)
+static inline int pte_unmap_same(struct vm_fault *vmf)
 {
 	int same = 1;
 #if defined(CONFIG_SMP) || defined(CONFIG_PREEMPTION)
 	if (sizeof(pte_t) > sizeof(unsigned long)) {
-		spinlock_t *ptl = pte_lockptr(mm, pmd);
+		spinlock_t *ptl = pte_lockptr(vmf->vma->vm_mm, vmf->pmd);
 		spin_lock(ptl);
-		same = pte_same(*page_table, orig_pte);
+		same = pte_same(*vmf->pte, vmf->orig_pte);
 		spin_unlock(ptl);
 	}
 #endif
-	pte_unmap(page_table);
+	pte_unmap(vmf->pte);
+	vmf->pte = NULL;
 	return same;
 }
 
@@ -3487,7 +3487,7 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
 	vm_fault_t ret = 0;
 	void *shadow = NULL;
 
-	if (!pte_unmap_same(vma->vm_mm, vmf->pmd, vmf->pte, vmf->orig_pte))
+	if (!pte_unmap_same(vmf))
 		goto out;
 
 	entry = pte_to_swp_entry(vmf->orig_pte);
-- 
2.31.1