splice: fix generic_file_splice_read() race with page invalidation

If a page was invalidated during splicing from file to a pipe, then
generic_file_splice_read() could return a short or zero count.

This manifested itself in rare I/O errors seen on nfs exported fuse
filesystems.  This is because nfsd uses splice_direct_to_actor() to read
files, and fuse uses invalidate_inode_pages2() to invalidate stale data on
open.

Fix by redoing the page find/create if it was found to be truncated
(invalidated).
Signed-off-by: Miklos Szeredi <mszeredi@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>

fs/splice.c

@@ -379,13 +379,22 @@ __generic_file_splice_read(struct file *in, loff_t *ppos,
 				lock_page(page);
 
 			/*
-			 * page was truncated, stop here. if this isn't the
-			 * first page, we'll just complete what we already
-			 * added
+			 * Page was truncated, or invalidated by the
+			 * filesystem.  Redo the find/create, but this time the
+			 * page is kept locked, so there's no chance of another
+			 * race with truncate/invalidate.
 			 */
 			if (!page->mapping) {
 				unlock_page(page);
-				break;
+				page = find_or_create_page(mapping, index,
+						mapping_gfp_mask(mapping));
+
+				if (!page) {
+					error = -ENOMEM;
+					break;
+				}
+				page_cache_release(pages[page_nr]);
+				pages[page_nr] = page;
 			}
 			/*
 			 * page was already under io and is now done, great