[PATCH v3] vfs: fix readahead(2) on block devices

[Reuben Hawkins <reubenhwk@gmail.com>]

Readahead was factored to call generic_fadvise.  That refactor added an S_ISREG
restriction which broke readahead on block devices.

This change swaps out the existing restrictions with an FMODE_LSEEK check to
fix block device readahead.

The readahead01.c and readahead02.c tests pass in ltp/testcases/...

Fixes: 3d8f7615319b ("vfs: implement readahead(2) using POSIX_FADV_WILLNEED")
Reviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: Reuben Hawkins <reubenhwk@gmail.com>
---
 mm/readahead.c | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/mm/readahead.c b/mm/readahead.c
index e815c114de21..0ff6fffe3c84 100644
--- a/mm/readahead.c
+++ b/mm/readahead.c
@@ -734,8 +734,7 @@ ssize_t ksys_readahead(int fd, loff_t offset, size_t count)
 	 * on this file, then we must return -EINVAL.
 	 */
 	ret = -EINVAL;
-	if (!f.file->f_mapping || !f.file->f_mapping->a_ops ||
-	    !S_ISREG(file_inode(f.file)->i_mode))
+	if (!(f.file->f_mode & FMODE_LSEEK))
 		goto out;
 
 	ret = vfs_fadvise(f.file, offset, count, POSIX_FADV_WILLNEED);
-- 
2.34.1