mm: pin address_space before dereferencing it while isolating an LRU …

…page

commit 69d763f upstream.

Minchan Kim asked the following question -- what locks protects
address_space destroying when race happens between inode trauncation and
__isolate_lru_page? Jan Kara clarified by describing the race as follows

CPU1                                            CPU2

truncate(inode)                                 __isolate_lru_page()
  ...
  truncate_inode_page(mapping, page);
    delete_from_page_cache(page)
      spin_lock_irqsave(&mapping->tree_lock, flags);
        __delete_from_page_cache(page, NULL)
          page_cache_tree_delete(..)
            ...                                   mapping = page_mapping(page);
            page->mapping = NULL;
            ...
      spin_unlock_irqrestore(&mapping->tree_lock, flags);
      page_cache_free_page(mapping, page)
        put_page(page)
          if (put_page_testzero(page)) -> false
- inode now has no pages and can be freed including embedded address_space

                                                  if (mapping && !mapping->a_ops->migratepage)
- we've dereferenced mapping which is potentially already free.

The race is theoretically possible but unlikely.  Before the
delete_from_page_cache, truncate_cleanup_page is called so the page is
likely to be !PageDirty or PageWriteback which gets skipped by the only
caller that checks the mappping in __isolate_lru_page.  Even if the race
occurs, a substantial amount of work has to happen during a tiny window
with no preemption but it could potentially be done using a virtual
machine to artifically slow one CPU or halt it during the critical
window.

This patch should eliminate the race with truncation by try-locking the
page before derefencing mapping and aborting if the lock was not
acquired.  There was a suggestion from Huang Ying to use RCU as a
side-effect to prevent mapping being freed.  However, I do not like the
solution as it's an unconventional means of preserving a mapping and
it's not a context where rcu_read_lock is obviously protecting rcu data.

Link: http://lkml.kernel.org/r/[email protected]
Fixes: c824493 ("mm: compaction: make isolate_lru_page() filter-aware again")
Signed-off-by: Mel Gorman <[email protected]>
Acked-by: Minchan Kim <[email protected]>
Cc: "Huang, Ying" <[email protected]>
Cc: Jan Kara <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>
[bwh: Backported to 3.2: adjust context]
Signed-off-by: Ben Hutchings <[email protected]>

mm/vmscan.c

@@ -1085,6 +1085,7 @@ int __isolate_lru_page(struct page *page, isolate_mode_t mode, int file)
 
 		if (PageDirty(page)) {
 			struct address_space *mapping;
+			bool migrate_dirty;
 
 			/* ISOLATE_CLEAN means only clean pages */
 			if (mode & ISOLATE_CLEAN)
@@ -1093,10 +1094,19 @@ int __isolate_lru_page(struct page *page, isolate_mode_t mode, int file)
 			/*
 			 * Only pages without mappings or that have a
 			 * ->migratepage callback are possible to migrate
-			 * without blocking
+			 * without blocking. However, we can be racing with
+			 * truncation so it's necessary to lock the page
+			 * to stabilise the mapping as truncation holds
+			 * the page lock until after the page is removed
+			 * from the page cache.
 			 */
+			if (!trylock_page(page))
+				return ret;
+
 			mapping = page_mapping(page);
-			if (mapping && !mapping->a_ops->migratepage)
+			migrate_dirty = mapping && mapping->a_ops->migratepage;
+			unlock_page(page);
+			if (!migrate_dirty)
 				return ret;
 		}
 	}