[PATCH RFC 05/30] mm: Clear vmf->pte after pte_unmap_same() returns

From: Peter Xu <peterx@redhat.com>
To: linux-kernel@vger.kernel.org, linux-mm@kvack.org
Cc: Mike Rapoport <rppt@linux.vnet.ibm.com>,
	Mike Kravetz <mike.kravetz@oracle.com>,
	peterx@redhat.com, Jerome Glisse <jglisse@redhat.com>,
	"Kirill A . Shutemov" <kirill@shutemov.name>,
	Hugh Dickins <hughd@google.com>,
	Axel Rasmussen <axelrasmussen@google.com>,
	Matthew Wilcox <willy@infradead.org>,
	Andrew Morton <akpm@linux-foundation.org>,
	Andrea Arcangeli <aarcange@redhat.com>,
	Nadav Amit <nadav.amit@gmail.com>
Subject: [PATCH RFC 05/30] mm: Clear vmf->pte after pte_unmap_same() returns
Date: Fri, 15 Jan 2021 12:08:42 -0500	[thread overview]
Message-ID: <20210115170907.24498-6-peterx@redhat.com> (raw)
In-Reply-To: <20210115170907.24498-1-peterx@redhat.com>

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.

pte_unmap_same() will be used in other places in follow up patches, so that
vmf->pte will not always be re-written.  This patch enables us to call
functions like finish_fault() because that'll conditionally unmap the pte by
checking vmf->pte first.  Or, alloc_set_pte() will make sure to allocate a new
pte even after calling pte_unmap_same().

Since we'll need to modify vmf->pte, directly pass in vmf into pte_unmap_same()
and then we can also avoid the long parameter list.

Signed-off-by: Peter Xu <peterx@redhat.com>
---
 mm/memory.c | 13 +++++++------
 1 file changed, 7 insertions(+), 6 deletions(-)

diff --git a/mm/memory.c b/mm/memory.c
index d6d2873368e1..5ab3106cdd35 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -2559,19 +2559,20 @@ 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);
+	/* After unmap of pte, the pointer is invalid now - clear it. */
+	vmf->pte = NULL;
 	return same;
 }
 
@@ -3251,7 +3252,7 @@ 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);
-- 
2.26.2

