[PATCH] splice: retrieve mapping after locking the page

Otherwise we could be racing with truncate/mapping removal.

Problem found/fixed by Nick Piggin <npiggin@suse.de>, logic rewritten
by me.

Signed-off-by: Jens Axboe <axboe@suse.de>

diff --git a/fs/splice.c b/fs/splice.c
index a285fd7..05fd278 100644 (file)
--- a/fs/splice.c
+++ b/fs/splice.c
@@ -55,31 +55,43 @@ static int page_cache_pipe_buf_steal(struct pipe_inode_info *pipe,
                                     struct pipe_buffer *buf)
 {
        struct page *page = buf->page;
-       struct address_space *mapping = page_mapping(page);
+       struct address_space *mapping;
 
        lock_page(page);
 
-       WARN_ON(!PageUptodate(page));
+       mapping = page_mapping(page);
+       if (mapping) {
+               WARN_ON(!PageUptodate(page));
 
-       /*
-        * At least for ext2 with nobh option, we need to wait on writeback
-        * completing on this page, since we'll remove it from the pagecache.
-        * Otherwise truncate wont wait on the page, allowing the disk
-        * blocks to be reused by someone else before we actually wrote our
-        * data to them. fs corruption ensues.
-        */
-       wait_on_page_writeback(page);
+               /*
+                * At least for ext2 with nobh option, we need to wait on
+                * writeback completing on this page, since we'll remove it
+                * from the pagecache.  Otherwise truncate wont wait on the
+                * page, allowing the disk blocks to be reused by someone else
+                * before we actually wrote our data to them. fs corruption
+                * ensues.
+                */
+               wait_on_page_writeback(page);
 
-       if (PagePrivate(page))
-               try_to_release_page(page, mapping_gfp_mask(mapping));
+               if (PagePrivate(page))
+                       try_to_release_page(page, mapping_gfp_mask(mapping));
 
-       if (!remove_mapping(mapping, page)) {
-               unlock_page(page);
-               return 1;
+               /*
+                * If we succeeded in removing the mapping, set LRU flag
+                * and return good.
+                */
+               if (remove_mapping(mapping, page)) {
+                       buf->flags |= PIPE_BUF_FLAG_LRU;
+                       return 0;
+               }
        }
 
-       buf->flags |= PIPE_BUF_FLAG_LRU;
-       return 0;
+       /*
+        * Raced with truncate or failed to remove page from current
+        * address space, unlock and return failure.
+        */
+       unlock_page(page);
+       return 1;
 }
 
 static void page_cache_pipe_buf_release(struct pipe_inode_info *pipe,