block: fix blk_stack_limits() regression due to lcm() change

Linux 3.19 commit 69c953c ("lib/lcm.c: lcm(n,0)=lcm(0,n) is 0, not n")
caused blk_stack_limits() to not properly stack queue_limits for stacked
devices (e.g. DM).

Fix this regression by establishing lcm_not_zero() and switching
blk_stack_limits() over to using it.

DM uses blk_set_stacking_limits() to establish the initial top-level
queue_limits that are then built up based on underlying devices' limits
using blk_stack_limits().  In the case of optimal_io_size (io_opt)
blk_set_stacking_limits() establishes a default value of 0.  With commit
69c953c, lcm(0, n) is no longer n, which compromises proper stacking of
the underlying devices' io_opt.

Test:
$ modprobe scsi_debug dev_size_mb=10 num_tgts=1 opt_blks=1536
$ cat /sys/block/sde/queue/optimal_io_size
786432
$ dmsetup create node --table "0 100 linear /dev/sde 0"

Before this fix:
$ cat /sys/block/dm-5/queue/optimal_io_size
0

After this fix:
$ cat /sys/block/dm-5/queue/optimal_io_size
786432

Signed-off-by: Mike Snitzer <snitzer@redhat.com>
Cc: stable@vger.kernel.org # 3.19+
Acked-by: Martin K. Petersen <martin.petersen@oracle.com>
Signed-off-by: Jens Axboe <axboe@fb.com>

3 files changed, 15 insertions, 3 deletions

diff --git a/block/blk-settings.c b/block/blk-settings.c
index 6ed2cbe5e8c9..12600bfffca9 100644
--- a/block/blk-settings.c
+++ b/block/blk-settings.c
@@ -585,7 +585,7 @@ int blk_stack_limits(struct queue_limits *t, struct queue_limits *b,
                                      b->physical_block_size);
 
         t->io_min = max(t->io_min, b->io_min);
-        t->io_opt = lcm(t->io_opt, b->io_opt);
+        t->io_opt = lcm_not_zero(t->io_opt, b->io_opt);
 
         t->cluster &= b->cluster;
         t->discard_zeroes_data &= b->discard_zeroes_data;
@@ -616,7 +616,7 @@ int blk_stack_limits(struct queue_limits *t, struct queue_limits *b,
                     b->raid_partial_stripes_expensive);
 
         /* Find lowest common alignment_offset */
-        t->alignment_offset = lcm(t->alignment_offset, alignment)
+        t->alignment_offset = lcm_not_zero(t->alignment_offset, alignment)
                 % max(t->physical_block_size, t->io_min);
 
         /* Verify that new alignment_offset is on a logical block boundary */
@@ -643,7 +643,7 @@ int blk_stack_limits(struct queue_limits *t, struct queue_limits *b,
                                                       b->max_discard_sectors);
                 t->discard_granularity = max(t->discard_granularity,
                                              b->discard_granularity);
-                t->discard_alignment = lcm(t->discard_alignment, alignment) %
+                t->discard_alignment = lcm_not_zero(t->discard_alignment, alignment) %
                         t->discard_granularity;
         }
 
diff --git a/include/linux/lcm.h b/include/linux/lcm.h
index 7bf01d779b45..1ce79a7f1daa 100644
--- a/include/linux/lcm.h
+++ b/include/linux/lcm.h
@@ -4,5 +4,6 @@
 #include <linux/compiler.h>
 
 unsigned long lcm(unsigned long a, unsigned long b) __attribute_const__;
+unsigned long lcm_not_zero(unsigned long a, unsigned long b) __attribute_const__;
 
 #endif /* _LCM_H */
diff --git a/lib/lcm.c b/lib/lcm.c
index e97dbd51e756..03d7fcb420b5 100644
--- a/lib/lcm.c
+++ b/lib/lcm.c
@@ -12,3 +12,14 @@ unsigned long lcm(unsigned long a, unsigned long b)
                 return 0;
 }
 EXPORT_SYMBOL_GPL(lcm);
+
+unsigned long lcm_not_zero(unsigned long a, unsigned long b)
+{
+        unsigned long l = lcm(a, b);
+
+        if (l)
+                return l;
+
+        return (b ? : a);
+}
+EXPORT_SYMBOL_GPL(lcm_not_zero);