[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>

1 files changed, 29 insertions, 17 deletions

diff --git a/fs/splice.c b/fs/splice.c
index a285fd746dc0..05fd2787be98 100644
--- 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,