[PATCH 6/7] vfs: Prevent DAX I/Os from falling back to buffered I/O

[Matthew Wilcox]

Unlike regular direct I/O, DAX will handle file holes, and there is
no desire to fall back to buffered I/O.  Buffered I/O ought to fail if
DAX I/O fails, unless we're doing a random-failure test.  So skip the
buffered I/O attempts for DAX files.
---
 mm/filemap.c | 19 ++++++++++++-------
 1 file changed, 12 insertions(+), 7 deletions(-)

diff --git a/mm/filemap.c b/mm/filemap.c
index 19bdb68..e69b586 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -1717,9 +1717,11 @@ generic_file_read_iter(struct kiocb *iocb, struct iov_iter *iter)
 		 * we've already read everything we wanted to, or if
 		 * there was a short read because we hit EOF, go ahead
 		 * and return.  Otherwise fallthrough to buffered io for
-		 * the rest of the read.
+		 * the rest of the read.  Buffered reads will not work for
+		 * DAX files, so don't bother trying.
 		 */
-		if (retval < 0 || !iov_iter_count(iter) || *ppos >= size) {
+		if (retval < 0 || !iov_iter_count(iter) || *ppos >= size ||
+		    IS_DAX(inode)) {
 			file_accessed(file);
 			goto out;
 		}
@@ -2582,13 +2584,16 @@ ssize_t __generic_file_write_iter(struct kiocb *iocb, struct iov_iter *from)
 		loff_t endbyte;
 
 		written = generic_file_direct_write(iocb, from, pos);
-		if (written < 0 || written == count)
-			goto out;
-
 		/*
-		 * direct-io write to a hole: fall through to buffered I/O
-		 * for completing the rest of the request.
+		 * If the write stopped short of completing, fall back to
+		 * buffered writes.  Some filesystems do this for writes to
+		 * holes, for example.  For DAX files, a buffered write will
+		 * not succeed (even if it did, DAX does not handle dirty
+		 * page-cache pages correctly).
 		 */
+		if (written < 0 || written == count || IS_DAX(inode))
+			goto out;
+
 		pos += written;
 		count -= written;
 
-- 
2.1.0


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