[PATCH V2] mkfs: avoid divide-by-zero when hardware reports optimal i/o size as 0

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From: Jeff Mahoney <jeffm@xxxxxxxx>

Commit 051b4e37f5e (mkfs: factor AG alignment) factored out the
AG alignment code into a separate function.  It got rid of
redundant checks for dswidth != 0 since calc_stripe_factors was
supposed to guarantee that if dsunit is non-zero dswidth will be
as well.  Unfortunately, there's hardware out there that reports its
optimal i/o size as larger than the maximum i/o size, which the kernel
treats as broken and zeros out the optimal i/o size.

To resolve this we can check the topology before consuming it, and
ignore the bad stripe geometry.

[sandeen: remove guessing heuristic, just warn and ignore bad data.]

Fixes: 051b4e37f5e (mkfs: factor AG alignment)
Signed-off-by: Jeff Mahoney <jeffm@xxxxxxxx>
Reviewed-by: Eric Sandeen <sandeen@xxxxxxxxxx>
Signed-off-by: Eric Sandeen <sandeen@xxxxxxxxxxx>
---

so, I rewrote this a bit.  I'm not a fan of guessing what the kernel
really must have meant, becaue next time the root cause may be differnt.
In other cases we ignore bad geometry, I think we should in this case as
well.  This will also let me go forward with a factored-out geometry checker,
and for user-specified badness we'll warn and exit, for kernel-provided
badness we'll warn and ignore.

diff --git a/mkfs/xfs_mkfs.c b/mkfs/xfs_mkfs.c
index 1074886..2e53c1e 100644
--- a/mkfs/xfs_mkfs.c
+++ b/mkfs/xfs_mkfs.c
@@ -2281,11 +2281,20 @@ _("data stripe width (%d) must be a multiple of the data stripe unit (%d)\n"),
 
 	/* if no stripe config set, use the device default */
 	if (!dsunit) {
-		dsunit = ft->dsunit;
-		dswidth = ft->dswidth;
-		use_dev = true;
+		/* Ignore nonsense from device.  XXX add more validation */
+		if (ft->dsunit && ft->dswidth == 0) {
+			fprintf(stderr,
+_("%s: Volume reports stripe unit of %d bytes and stripe width of 0, ignoring.\n"),
+				progname, BBTOB(ft->dsunit));
+			ft->dsunit = 0;
+			ft->dswidth = 0;
+		} else {
+			dsunit = ft->dsunit;
+			dswidth = ft->dswidth;
+			use_dev = true;
+		}
 	} else {
-		/* check and warn is alignment is sub-optimal */
+		/* check and warn if user-specified alignment is sub-optimal */
 		if (ft->dsunit && ft->dsunit != dsunit) {
 			fprintf(stderr,
 _("%s: Specified data stripe unit %d is not the same as the volume stripe unit %d\n"),

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