[PATCH RFC 4/4] block: use chunk_sectors when evaluating stacked atomic write limits

[John Garry <john.g.garry@oracle.com>]

The atomic write unit max is limited by any stack device stripe size.

It is required that the atomic write unit is a power-of-2 factor of the
stripe size.

Currently we use io_min limit to hold the stripe size, and check for a
io_min <= SECTOR_SIZE when deciding if we have a striped stacked device.

Nilay reports that this causes a problem when the physical block size is
greater than SECTOR_SIZE [0].

Furthermore, io_min may be mutated when stacking devices, and this makes
it a poor candidate to hold the stripe size. Such an example would be
when the io_min is less than the physical block size.

Use chunk_sectors to hold the stripe size, which is more appropriate.

[0] https://lore.kernel.org/linux-block/888f3b1d-7817-4007-b3b3-1a2ea04df771@linux.ibm.com/T/#mecca17129f72811137d3c2f1e477634e77f06781

Signed-off-by: John Garry <john.g.garry@oracle.com>
---
 block/blk-settings.c | 8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

diff --git a/block/blk-settings.c b/block/blk-settings.c
index a000daafbfb4..5b0f1a854e81 100644
--- a/block/blk-settings.c
+++ b/block/blk-settings.c
@@ -594,11 +594,13 @@ static bool blk_stack_atomic_writes_boundary_head(struct queue_limits *t,
 static bool blk_stack_atomic_writes_head(struct queue_limits *t,
 				struct queue_limits *b)
 {
+	unsigned int chunk_size = t->chunk_sectors << SECTOR_SHIFT;
+
 	if (b->atomic_write_hw_boundary &&
 	    !blk_stack_atomic_writes_boundary_head(t, b))
 		return false;
 
-	if (t->io_min <= SECTOR_SIZE) {
+	if (!t->chunk_sectors) {
 		/* No chunk sectors, so use bottom device values directly */
 		t->atomic_write_hw_unit_max = b->atomic_write_hw_unit_max;
 		t->atomic_write_hw_unit_min = b->atomic_write_hw_unit_min;
@@ -617,12 +619,12 @@ static bool blk_stack_atomic_writes_head(struct queue_limits *t,
 	 * aligned with both limits, i.e. 8K in this example.
 	 */
 	t->atomic_write_hw_unit_max = b->atomic_write_hw_unit_max;
-	while (t->io_min % t->atomic_write_hw_unit_max)
+	while (chunk_size % t->atomic_write_hw_unit_max)
 		t->atomic_write_hw_unit_max /= 2;
 
 	t->atomic_write_hw_unit_min = min(b->atomic_write_hw_unit_min,
 					  t->atomic_write_hw_unit_max);
-	t->atomic_write_hw_max = min(b->atomic_write_hw_max, t->io_min);
+	t->atomic_write_hw_max = min(b->atomic_write_hw_max, chunk_size);
 
 	return true;
 }
-- 
2.31.1