block: Fix incorrect alignment offset reporting and update documentation

queue_sector_alignment_offset returned the wrong value which caused
partitions to report an incorrect alignment_offset.  Since offset
alignment calculation is needed several places it has been split into a
separate helper function.  The topology stacking function has been
updated accordingly.

Furthermore, comments have been added to clarify how the stacking
function works.

Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>
Tested-by: Mike Snitzer <snitzer@redhat.com>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>

2 files changed, 42 insertions, 13 deletions

diff --git a/block/blk-settings.c b/block/blk-settings.c
index e14fcbcedbf..d52d4adc440 100644
--- a/block/blk-settings.c
+++ b/block/blk-settings.c
@@ -505,20 +505,30 @@ static unsigned int lcm(unsigned int a, unsigned int b)
 
 /**
  * blk_stack_limits - adjust queue_limits for stacked devices
- * @t:  the stacking driver limits (top)
- * @b:  the underlying queue limits (bottom)
+ * @t:  the stacking driver limits (top device)
+ * @b:  the underlying queue limits (bottom, component device)
  * @offset:  offset to beginning of data within component device
  *
  * Description:
- *    Merges two queue_limit structs.  Returns 0 if alignment didn't
- *    change.  Returns -1 if adding the bottom device caused
- *    misalignment.
+ *    This function is used by stacking drivers like MD and DM to ensure
+ *    that all component devices have compatible block sizes and
+ *    alignments.  The stacking driver must provide a queue_limits
+ *    struct (top) and then iteratively call the stacking function for
+ *    all component (bottom) devices.  The stacking function will
+ *    attempt to combine the values and ensure proper alignment.
+ *
+ *    Returns 0 if the top and bottom queue_limits are compatible.  The
+ *    top device's block sizes and alignment offsets may be adjusted to
+ *    ensure alignment with the bottom device. If no compatible sizes
+ *    and alignments exist, -1 is returned and the resulting top
+ *    queue_limits will have the misaligned flag set to indicate that
+ *    the alignment_offset is undefined.
  */
 int blk_stack_limits(struct queue_limits *t, struct queue_limits *b,
                      sector_t offset)
 {
         sector_t alignment;
-        unsigned int top, bottom, granularity;
+        unsigned int top, bottom;
 
         t->max_sectors = min_not_zero(t->max_sectors, b->max_sectors);
         t->max_hw_sectors = min_not_zero(t->max_hw_sectors, b->max_hw_sectors);
@@ -536,15 +546,18 @@ int blk_stack_limits(struct queue_limits *t, struct queue_limits *b,
         t->max_segment_size = min_not_zero(t->max_segment_size,
                                            b->max_segment_size);
 
-        granularity = max(b->physical_block_size, b->io_min);
-        alignment = b->alignment_offset - (offset & (granularity - 1));
+        alignment = queue_limit_alignment_offset(b, offset);
 
+        /* Bottom device has different alignment.  Check that it is
+         * compatible with the current top alignment.
+         */
         if (t->alignment_offset != alignment) {
 
                 top = max(t->physical_block_size, t->io_min)
                         + t->alignment_offset;
-                bottom = granularity + alignment;
+                bottom = max(b->physical_block_size, b->io_min) + alignment;
 
+                /* Verify that top and bottom intervals line up */
                 if (max(top, bottom) & (min(top, bottom) - 1))
                         t->misaligned = 1;
         }
@@ -561,32 +574,39 @@ int blk_stack_limits(struct queue_limits *t, struct queue_limits *b,
         t->no_cluster |= b->no_cluster;
         t->discard_zeroes_data &= b->discard_zeroes_data;
 
+        /* Physical block size a multiple of the logical block size? */
         if (t->physical_block_size & (t->logical_block_size - 1)) {
                 t->physical_block_size = t->logical_block_size;
                 t->misaligned = 1;
         }
 
+        /* Minimum I/O a multiple of the physical block size? */
         if (t->io_min & (t->physical_block_size - 1)) {
                 t->io_min = t->physical_block_size;
                 t->misaligned = 1;
         }
 
+        /* Optimal I/O a multiple of the physical block size? */
         if (t->io_opt & (t->physical_block_size - 1)) {
                 t->io_opt = 0;
                 t->misaligned = 1;
         }
 
+        /* Find lowest common alignment_offset */
         t->alignment_offset = lcm(t->alignment_offset, alignment)
                 & (max(t->physical_block_size, t->io_min) - 1);
 
+        /* Verify that new alignment_offset is on a logical block boundary */
         if (t->alignment_offset & (t->logical_block_size - 1))
                 t->misaligned = 1;
 
         /* Discard alignment and granularity */
         if (b->discard_granularity) {
+                unsigned int granularity = b->discard_granularity;
+                offset &= granularity - 1;
 
-                alignment = b->discard_alignment -
-                        (offset & (b->discard_granularity - 1));
+                alignment = (granularity + b->discard_alignment - offset)
+                        & (granularity - 1);
 
                 if (t->discard_granularity != 0 &&
                     t->discard_alignment != alignment) {
@@ -598,6 +618,8 @@ int blk_stack_limits(struct queue_limits *t, struct queue_limits *b,
                                 t->discard_misaligned = 1;
                 }
 
+                t->max_discard_sectors = min_not_zero(t->max_discard_sectors,
+                                                      b->max_discard_sectors);
                 t->discard_granularity = max(t->discard_granularity,
                                              b->discard_granularity);
                 t->discard_alignment = lcm(t->discard_alignment, alignment) &
diff --git a/include/linux/blkdev.h b/include/linux/blkdev.h
index 784a919aa0d..59b832be304 100644
--- a/include/linux/blkdev.h
+++ b/include/linux/blkdev.h
@@ -1116,11 +1116,18 @@ static inline int queue_alignment_offset(struct request_queue *q)
         return q->limits.alignment_offset;
 }
 
+static inline int queue_limit_alignment_offset(struct queue_limits *lim, sector_t offset)
+{
+        unsigned int granularity = max(lim->physical_block_size, lim->io_min);
+
+        offset &= granularity - 1;
+        return (granularity + lim->alignment_offset - offset) & (granularity - 1);
+}
+
 static inline int queue_sector_alignment_offset(struct request_queue *q,
                                                 sector_t sector)
 {
-        return ((sector << 9) - q->limits.alignment_offset)
-                & (q->limits.io_min - 1);
+        return queue_limit_alignment_offset(&q->limits, sector << 9);
 }
 
 static inline int bdev_alignment_offset(struct block_device *bdev)