block: use gcd() to fix chunk_sectors limit stacking

commit 22ada80 ("block: use lcm_not_zero() when stacking
chunk_sectors") broke chunk_sectors limit stacking. chunk_sectors must
reflect the most limited of all devices in the IO stack.

Otherwise malformed IO may result. E.g.: prior to this fix,
->chunk_sectors = lcm_not_zero(8, 128) would result in
blk_max_size_offset() splitting IO at 128 sectors rather than the
required more restrictive 8 sectors.

And since commit 07d098e ("block: allow 'chunk_sectors' to be
non-power-of-2") care must be taken to properly stack chunk_sectors to
be compatible with the possibility that a non-power-of-2 chunk_sectors
may be stacked. This is why gcd() is used instead of reverting back
to using min_not_zero().

Fixes: 22ada80 ("block: use lcm_not_zero() when stacking chunk_sectors")
Fixes: 07d098e ("block: allow 'chunk_sectors' to be non-power-of-2")
Reported-by: John Dorminy <jdorminy@redhat.com>
Reported-by: Bruce Johnston <bjohnsto@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>
Reviewed-by: John Dorminy <jdorminy@redhat.com>
Cc: stable@vger.kernel.org
Reviewed-by: Martin K. Petersen <martin.petersen@oracle.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

Author: snitm

block/blk-settings.c

@@ -547,7 +547,10 @@ int blk_stack_limits(struct queue_limits *t, struct queue_limits *b,
 
 	t->io_min = max(t->io_min, b->io_min);
 	t->io_opt = lcm_not_zero(t->io_opt, b->io_opt);
-	t->chunk_sectors = lcm_not_zero(t->chunk_sectors, b->chunk_sectors);
+
+	/* Set non-power-of-2 compatible chunk_sectors boundary */
+	if (b->chunk_sectors)
+		t->chunk_sectors = gcd(t->chunk_sectors, b->chunk_sectors);
 
 	/* Physical block size a multiple of the logical block size? */
 	if (t->physical_block_size & (t->logical_block_size - 1)) {