[PATCH 2/2] mm,migration: Prevent rmap_walk_[anon|ksm] seeing the wrong VMA information

[Mel Gorman <mel@csn.ul.ie>]

vma_adjust() is updating anon VMA information without any locks taken.
In contrast, file-backed mappings use the i_mmap_lock and this lack of
locking can result in races with page migration. During rmap_walk(),
vma_address() can return -EFAULT for an address that will soon be valid.
This leaves a dangling migration PTE behind which can later cause a BUG_ON
to trigger when the page is faulted in.

With the recent anon_vma changes, there can be more than one anon_vma->lock
that can be taken in a anon_vma_chain but a second lock cannot be spinned
upon in case of deadlock. Instead, the rmap walker tries to take locks of
different anon_vma's. If the attempt fails, the operation is restarted.

Signed-off-by: Mel Gorman <mel@csn.ul.ie>
---
 mm/ksm.c  |   13 +++++++++++++
 mm/mmap.c |    6 ++++++
 mm/rmap.c |   22 +++++++++++++++++++---
 3 files changed, 38 insertions(+), 3 deletions(-)

diff --git a/mm/ksm.c b/mm/ksm.c
index 3666d43..baa5b4d 100644
--- a/mm/ksm.c
+++ b/mm/ksm.c
@@ -1674,9 +1674,22 @@ again:
 		spin_lock(&anon_vma->lock);
 		list_for_each_entry(vmac, &anon_vma->head, same_anon_vma) {
 			vma = vmac->vma;
+
+			/* See comment in mm/rmap.c#rmap_walk_anon on locking */
+			if (anon_vma != vma->anon_vma) {
+				if (!spin_trylock(&vma->anon_vma->lock)) {
+					spin_unlock(&anon_vma->lock);
+					goto again;
+				}
+			}
+
 			if (rmap_item->address < vma->vm_start ||
 			    rmap_item->address >= vma->vm_end)
 				continue;
+
+			if (anon_vma != vma->anon_vma)
+				spin_unlock(&vma->anon_vma->lock);
+
 			/*
 			 * Initially we examine only the vma which covers this
 			 * rmap_item; but later, if there is still work to do,
diff --git a/mm/mmap.c b/mm/mmap.c
index f90ea92..61d6f1d 100644
--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -578,6 +578,9 @@ again:			remove_next = 1 + (end > next->vm_end);
 		}
 	}
 
+	if (vma->anon_vma)
+		spin_lock(&vma->anon_vma->lock);
+
 	if (root) {
 		flush_dcache_mmap_lock(mapping);
 		vma_prio_tree_remove(vma, root);
@@ -620,6 +623,9 @@ again:			remove_next = 1 + (end > next->vm_end);
 	if (mapping)
 		spin_unlock(&mapping->i_mmap_lock);
 
+	if (vma->anon_vma)
+		spin_unlock(&vma->anon_vma->lock);
+
 	if (remove_next) {
 		if (file) {
 			fput(file);
diff --git a/mm/rmap.c b/mm/rmap.c
index 85f203e..bc313a6 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -1368,15 +1368,31 @@ static int rmap_walk_anon(struct page *page, int (*rmap_one)(struct page *,
 	 * are holding mmap_sem. Users without mmap_sem are required to
 	 * take a reference count to prevent the anon_vma disappearing
 	 */
+retry:
 	anon_vma = page_anon_vma(page);
 	if (!anon_vma)
 		return ret;
 	spin_lock(&anon_vma->lock);
 	list_for_each_entry(avc, &anon_vma->head, same_anon_vma) {
 		struct vm_area_struct *vma = avc->vma;
-		unsigned long address = vma_address(page, vma);
-		if (address == -EFAULT)
-			continue;
+		unsigned long address;
+
+		/*
+		 * Guard against deadlocks by not spinning against
+		 * vma->anon_vma->lock. If contention is found, release our
+		 * lock and try again until VMA list can be traversed without
+		 * contention.
+		 */
+		if (anon_vma != vma->anon_vma) {
+			if (!spin_trylock(&vma->anon_vma->lock)) {
+				spin_unlock(&anon_vma->lock);
+				goto retry;
+			}
+		}
+		address = vma_address(page, vma);
+		if (anon_vma != vma->anon_vma)
+			spin_unlock(&vma->anon_vma->lock);
+
 		ret = rmap_one(page, vma, address, arg);
 		if (ret != SWAP_AGAIN)
 			break;
-- 
1.6.5

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