[PATCH 2/2] mkfs: validate rt extent size hint when rtinherit is set

["Darrick J. Wong" <djwong@kernel.org>]

From: Darrick J. Wong <djwong@kernel.org>

Extent size hints exist to nudge the behavior of the file data block
allocator towards trying to make aligned allocations.  Therefore, it
doesn't make sense to allow a hint that isn't a multiple of the
fundamental allocation unit for a given file.

This means that if the sysadmin is formatting with rtinherit set on the
root dir, validate_extsize_hint needs to check the hint value on a
simulated realtime file to make sure that it's correct.  Unfortunately,
the gate check here was for a nonzero rt extent size, which is wrong
since we never format with rtextsize==0.  This leads to absurd failures
such as:

# mkfs.xfs -f /dev/sdf -r extsize=7b -d rtinherit=0,extszinherit=13
illegal extent size hint 13, must be less than 649088 and a multiple of 7.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
---
 mkfs/xfs_mkfs.c |    7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)


diff --git a/mkfs/xfs_mkfs.c b/mkfs/xfs_mkfs.c
index f84a42f9..9c14c04e 100644
--- a/mkfs/xfs_mkfs.c
+++ b/mkfs/xfs_mkfs.c
@@ -2384,10 +2384,11 @@ _("illegal extent size hint %lld, must be less than %u.\n"),
 	}
 
 	/*
-	 * Now we do it again with a realtime file so that we know the hint and
-	 * flag that get passed on to realtime files will be correct.
+	 * If the value is to be passed on to realtime files, revalidate with
+	 * a realtime file so that we know the hint and flag that get passed on
+	 * to realtime files will be correct.
 	 */
-	if (mp->m_sb.sb_rextsize == 0)
+	if (!(cli->fsx.fsx_xflags & FS_XFLAG_RTINHERIT))
 		return;
 
 	flags = XFS_DIFLAG_REALTIME;