memory unplug: migration by kernel

In usual, migrate_pages(page,,) is called with holding mm->sem by system call.
(mm here is a mm_struct which maps the migration target page.)
This semaphore helps avoiding some race conditions.

But, if we want to migrate a page by some kernel codes, we have to avoid
some races. This patch adds check code for following race condition.

1. A page which page->mapping==NULL can be target of migration. Then, we have
   to check page->mapping before calling try_to_unmap().

2. anon_vma can be freed while page is unmapped, but page->mapping remains as
   it was. We drop page->mapcount to be 0. Then we cannot trust page->mapping.
   So, use rcu_read_lock() to prevent anon_vma pointed by page->mapping from
   being freed during migration.

Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Acked-by: Christoph Lameter <clameter@sgi.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 19 insertions, 2 deletions

diff --git a/mm/migrate.c b/mm/migrate.c
index 34d8ada053e4..c8d87221f368 100644
--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -632,18 +632,35 @@ static int unmap_and_move(new_page_t get_new_page, unsigned long private,
                         goto unlock;
                 wait_on_page_writeback(page);
         }
-
         /*
-         * Establish migration ptes or remove ptes
+         * By try_to_unmap(), page->mapcount goes down to 0 here. In this case,
+         * we cannot notice that anon_vma is freed while we migrates a page.
+         * This rcu_read_lock() delays freeing anon_vma pointer until the end
+         * of migration. File cache pages are no problem because of page_lock()
+         */
+        rcu_read_lock();
+        /*
+         * This is a corner case handling.
+         * When a new swap-cache is read into, it is linked to LRU
+         * and treated as swapcache but has no rmap yet.
+         * Calling try_to_unmap() against a page->mapping==NULL page is
+         * BUG. So handle it here.
          */
+        if (!page->mapping)
+                goto rcu_unlock;
+        /* Establish migration ptes or remove ptes */
         try_to_unmap(page, 1);
+
         if (!page_mapped(page))
                 rc = move_to_new_page(newpage, page);
 
         if (rc)
                 remove_migration_ptes(page, page);
+rcu_unlock:
+        rcu_read_unlock();
 
 unlock:
+
         unlock_page(page);
 
         if (rc != -EAGAIN) {