[PATCH] mkfs.xfs: fix ASSERT on too-small device with stripe geometry

[Eric Sandeen <sandeen@redhat.com>]

When a too-small device is created with stripe geometry, we hit an
assert in align_ag_geometry():

# truncate --size=10444800 testfile
# mkfs.xfs -dsu=65536,sw=1 testfile 
mkfs.xfs: xfs_mkfs.c:2834: align_ag_geometry: Assertion `cfg->agcount != 0' failed.

This is because align_ag_geometry() finds that the size of the last
(only) AG is too small, and attempts to trim it off.  Obviously 0
AGs is invalid, and we hit the ASSERT.

Fix this by skipping the last-ag-trim if there is only one AG, and
add a new test to validate_ag_geometry() which offers a very specific,
clear warning if the device (in dblocks) is smaller than the minimum
allowed AG size.

Reported-by: Zdenek Kabelac <zkabelac@redhat.com>
Signed-off-by: Eric Sandeen <sandeen@redhat.com>
---

diff --git a/mkfs/xfs_mkfs.c b/mkfs/xfs_mkfs.c
index a687f385..da8c5986 100644
--- a/mkfs/xfs_mkfs.c
+++ b/mkfs/xfs_mkfs.c
@@ -1038,6 +1038,15 @@ validate_ag_geometry(
 	uint64_t	agsize,
 	uint64_t	agcount)
 {
+	/* Is this device simply too small? */
+	if (dblocks < XFS_AG_MIN_BLOCKS(blocklog)) {
+		fprintf(stderr,
+	_("device (%lld blocks) too small, need at least %lld blocks\n"),
+			(long long)dblocks,
+			(long long)XFS_AG_MIN_BLOCKS(blocklog));
+		usage();
+	}
+
 	if (agsize < XFS_AG_MIN_BLOCKS(blocklog)) {
 		fprintf(stderr,
 	_("agsize (%lld blocks) too small, need at least %lld blocks\n"),
@@ -2827,11 +2836,12 @@ validate:
 	 * and drop the blocks.
 	 */
 	if (cfg->dblocks % cfg->agsize != 0 &&
+	     cfg->agcount > 1 &&
 	     (cfg->dblocks % cfg->agsize < XFS_AG_MIN_BLOCKS(cfg->blocklog))) {
+printf("%d %d %d\n", cfg->dblocks, cfg->agsize, cfg->dblocks % cfg->agsize);
 		ASSERT(!cli_opt_set(&dopts, D_AGCOUNT));
 		cfg->dblocks = (xfs_rfsblock_t)((cfg->agcount - 1) * cfg->agsize);
 		cfg->agcount--;
-		ASSERT(cfg->agcount != 0);
 	}
 
 	validate_ag_geometry(cfg->blocklog, cfg->dblocks,