[PATCH RFC 06/16] block: Limit atomic writes according to bio and queue limits

[John Garry]

We rely the block layer always being able to send a bio of size
atomic_write_unit_max without being required to split it due to request
queue or other bio limits. We already know at any bio should have an
alignment of atomic_write_unit_max or lower.

A bio may contain min(BIO_MAX_VECS, limits->max_segments) vectors,
and each vector of at least PAGE_SIZE, except for if start address may
not be PAGE aligned; for this case, subtract 1 to give the max guaranteed
count of pages which we may store in a bio, and limit both
atomic_write_unit_min and atomic_write_unit_max to this value.

Signed-off-by: John Garry <john.g.garry@xxxxxxxxxx>
---
 block/blk-settings.c | 23 ++++++++++++++++++++---
 1 file changed, 20 insertions(+), 3 deletions(-)

diff --git a/block/blk-settings.c b/block/blk-settings.c
index e21731715a12..f64a2f736cb8 100644
--- a/block/blk-settings.c
+++ b/block/blk-settings.c
@@ -212,6 +212,18 @@ void blk_queue_atomic_write_boundary(struct request_queue *q,
 }
 EXPORT_SYMBOL(blk_queue_atomic_write_boundary);
 
+static unsigned int blk_queue_max_guaranteed_bio_size(struct queue_limits *limits)
+{
+	unsigned int max_segments = limits->max_segments;
+	unsigned int atomic_write_max_segments =
+				min(BIO_MAX_VECS, max_segments);
+	/* subtract 1 to assume PAGE-misaligned IOV start address */
+	unsigned int size = (atomic_write_max_segments - 1) *
+				(PAGE_SIZE / SECTOR_SIZE);
+
+	return rounddown_pow_of_two(size);
+}
+
 /**
  * blk_queue_atomic_write_unit_min - smallest unit that can be written
  *				     atomically to the device.
@@ -221,7 +233,10 @@ EXPORT_SYMBOL(blk_queue_atomic_write_boundary);
 void blk_queue_atomic_write_unit_min(struct request_queue *q,
 				     unsigned int sectors)
 {
-	q->limits.atomic_write_unit_min = sectors;
+	struct queue_limits *limits= &q->limits;
+	unsigned int guaranteed = blk_queue_max_guaranteed_bio_size(limits);
+
+	limits->atomic_write_unit_min = min(guaranteed, sectors);
 }
 EXPORT_SYMBOL(blk_queue_atomic_write_unit_min);
 
@@ -234,8 +249,10 @@ EXPORT_SYMBOL(blk_queue_atomic_write_unit_min);
 void blk_queue_atomic_write_unit_max(struct request_queue *q,
 				     unsigned int sectors)
 {
-	struct queue_limits *limits = &q->limits;
-	limits->atomic_write_unit_max = sectors;
+	struct queue_limits *limits= &q->limits;
+	unsigned int guaranteed = blk_queue_max_guaranteed_bio_size(limits);
+
+	limits->atomic_write_unit_max = min(guaranteed, sectors);
 }
 EXPORT_SYMBOL(blk_queue_atomic_write_unit_max);
 
-- 
2.31.1