aio/migratepages: make aio migrate pages sane

The arbitrary restriction on page counts offered by the core
migrate_page_move_mapping() code results in rather suspicious looking
fiddling with page reference counts in the aio_migratepage() operation.
To fix this, make migrate_page_move_mapping() take an extra_count parameter
that allows aio to tell the code about its own reference count on the page
being migrated.

While cleaning up aio_migratepage(), make it validate that the old page
being passed in is actually what aio_migratepage() expects to prevent
misbehaviour in the case of races.

Signed-off-by: Benjamin LaHaise <bcrl@kvack.org>

1 files changed, 7 insertions, 6 deletions

diff --git a/mm/migrate.c b/mm/migrate.c
index e9b710201335..9194375b2307 100644
--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -317,14 +317,15 @@ static inline bool buffer_migrate_lock_buffers(struct buffer_head *head,
  */
 int migrate_page_move_mapping(struct address_space *mapping,
                 struct page *newpage, struct page *page,
-                struct buffer_head *head, enum migrate_mode mode)
+                struct buffer_head *head, enum migrate_mode mode,
+                int extra_count)
 {
-        int expected_count = 0;
+        int expected_count = 1 + extra_count;
         void **pslot;
 
         if (!mapping) {
                 /* Anonymous page without mapping */
-                if (page_count(page) != 1)
+                if (page_count(page) != expected_count)
                         return -EAGAIN;
                 return MIGRATEPAGE_SUCCESS;
         }
@@ -334,7 +335,7 @@ int migrate_page_move_mapping(struct address_space *mapping,
         pslot = radix_tree_lookup_slot(&mapping->page_tree,
                                         page_index(page));
 
-        expected_count = 2 + page_has_private(page);
+        expected_count += 1 + page_has_private(page);
         if (page_count(page) != expected_count ||
                 radix_tree_deref_slot_protected(pslot, &mapping->tree_lock) != page) {
                 spin_unlock_irq(&mapping->tree_lock);
@@ -584,7 +585,7 @@ int migrate_page(struct address_space *mapping,
 
         BUG_ON(PageWriteback(page));    /* Writeback must be complete */
 
-        rc = migrate_page_move_mapping(mapping, newpage, page, NULL, mode);
+        rc = migrate_page_move_mapping(mapping, newpage, page, NULL, mode, 0);
 
         if (rc != MIGRATEPAGE_SUCCESS)
                 return rc;
@@ -611,7 +612,7 @@ int buffer_migrate_page(struct address_space *mapping,
 
         head = page_buffers(page);
 
-        rc = migrate_page_move_mapping(mapping, newpage, page, head, mode);
+        rc = migrate_page_move_mapping(mapping, newpage, page, head, mode, 0);
 
         if (rc != MIGRATEPAGE_SUCCESS)
                 return rc;