[PATCH v2 1/3] ext4: Add alignment check for DAX mount

[Toshi Kani <toshi.kani@hpe.com>]

When a partition is not aligned by 4KB, mount -o dax succeeds,
but any read/write access to the filesystem fails, except for
metadata update.

Call bdev_direct_access to check the alignment when -o dax is
specified.

Reported-by: Micah Parrish <micah.parrish@hpe.com>
Signed-off-by: Toshi Kani <toshi.kani@hpe.com>
Cc: "Theodore Ts'o" <tytso@mit.edu>
Cc: Andreas Dilger <adilger.kernel@dilger.ca>
Cc: Jan Kara <jack@suse.cz>
Cc: Dan Williams <dan.j.williams@intel.com>
Cc: Ross Zwisler <ross.zwisler@linux.intel.com>
Cc: Christoph Hellwig <hch@infradead.org>
Cc: Boaz Harrosh <boaz@plexistor.com>
---
 fs/ext4/super.c |   20 ++++++++++++++++++--
 1 file changed, 18 insertions(+), 2 deletions(-)

diff --git a/fs/ext4/super.c b/fs/ext4/super.c
index 304c712..51ac78e 100644
--- a/fs/ext4/super.c
+++ b/fs/ext4/super.c
@@ -3416,14 +3416,30 @@ static int ext4_fill_super(struct super_block *sb, void *data, int silent)
 	}
 
 	if (sbi->s_mount_opt & EXT4_MOUNT_DAX) {
+		struct blk_dax_ctl dax = {
+			.sector = 0,
+			.size = PAGE_SIZE,
+		};
 		if (blocksize != PAGE_SIZE) {
 			ext4_msg(sb, KERN_ERR,
 					"error: unsupported blocksize for dax");
 			goto failed_mount;
 		}
-		if (!sb->s_bdev->bd_disk->fops->direct_access) {
-			ext4_msg(sb, KERN_ERR,
+		err = bdev_direct_access(sb->s_bdev, &dax);
+		if (err < 0) {
+			switch (err) {
+			case -EOPNOTSUPP:
+				ext4_msg(sb, KERN_ERR,
 					"error: device does not support dax");
+				break;
+			case -EINVAL:
+				ext4_msg(sb, KERN_ERR,
+					"error: unaligned partition for dax");
+				break;
+			default:
+				ext4_msg(sb, KERN_ERR,
+					"error: dax access failed (%d)", err);
+			}
 			goto failed_mount;
 		}
 	}