block: make /sys/block/<dev>/queue/discard_max_bytes writeable

Lots of devices support huge discard sizes these days. Depending
on how the device handles them internally, huge discards can
introduce massive latencies (hundreds of msec) on the device side.

We have a sysfs file, discard_max_bytes, that advertises the max
hardware supported discard size. Make this writeable, and split
the settings into a soft and hard limit. This can be set from
'discard_granularity' and up to the hardware limit.

Add a new sysfs file, 'discard_max_hw_bytes', that shows the hw
set limit.

Reviewed-by: Jeff Moyer <[e-mail]>
Signed-off-by: Jens Axboe <[e-mail]>

Documentation/block/queue-sysfs.txt

···
 reported by the device. A value of '0' means device does not support
 the discard functionality.
 
-discard_max_bytes (RO)
+discard_max_hw_bytes (RO)
 ----------------------
 Devices that support discard functionality may have internal limits on
 the number of bytes that can be trimmed or unmapped in a single operation.
···
 number of bytes that can be discarded in a single operation. Discard
 requests issued to the device must not exceed this limit. A discard_max_bytes
 value of 0 means that the device does not support discard functionality.
+
+discard_max_bytes (RW)
+----------------------
+While discard_max_hw_bytes is the hardware limit for the device, this
+setting is the software limit. Some devices exhibit large latencies when
+large discards are issued, setting this value lower will make Linux issue
+smaller discards and potentially help reduce latencies induced by large
+discard operations.
 
 discard_zeroes_data (RO)
 ------------------------

block/blk-settings.c

···
 	lim->chunk_sectors = 0;
 	lim->max_write_same_sectors = 0;
 	lim->max_discard_sectors = 0;
+	lim->max_hw_discard_sectors = 0;
 	lim->discard_granularity = 0;
 	lim->discard_alignment = 0;
 	lim->discard_misaligned = 0;
···
 void blk_queue_max_discard_sectors(struct request_queue *q,
 		unsigned int max_discard_sectors)
 {
+	q->limits.max_hw_discard_sectors = max_discard_sectors;
 	q->limits.max_discard_sectors = max_discard_sectors;
 }
 EXPORT_SYMBOL(blk_queue_max_discard_sectors);
···
 
 		t->max_discard_sectors = min_not_zero(t->max_discard_sectors,
 						      b->max_discard_sectors);
+		t->max_hw_discard_sectors = min_not_zero(t->max_hw_discard_sectors,
+							 b->max_hw_discard_sectors);
 		t->discard_granularity = max(t->discard_granularity,
 					     b->discard_granularity);
 		t->discard_alignment = lcm_not_zero(t->discard_alignment, alignment) %

block/blk-sysfs.c

···
 	return queue_var_show(q->limits.discard_granularity, page);
 }
 
+static ssize_t queue_discard_max_hw_show(struct request_queue *q, char *page)
+{
+	unsigned long long val;
+
+	val = q->limits.max_hw_discard_sectors << 9;
+	return sprintf(page, "%llu\n", val);
+}
+
 static ssize_t queue_discard_max_show(struct request_queue *q, char *page)
 {
 	return sprintf(page, "%llu\n",
 		       (unsigned long long)q->limits.max_discard_sectors << 9);
+}
+
+static ssize_t queue_discard_max_store(struct request_queue *q,
+				       const char *page, size_t count)
+{
+	unsigned long max_discard;
+	ssize_t ret = queue_var_store(&max_discard, page, count);
+
+	if (ret < 0)
+		return ret;
+
+	if (max_discard & (q->limits.discard_granularity - 1))
+		return -EINVAL;
+
+	max_discard >>= 9;
+	if (max_discard > UINT_MAX)
+		return -EINVAL;
+
+	if (max_discard > q->limits.max_hw_discard_sectors)
+		max_discard = q->limits.max_hw_discard_sectors;
+
+	q->limits.max_discard_sectors = max_discard;
+	return ret;
 }
 
 static ssize_t queue_discard_zeroes_data_show(struct request_queue *q, char *page)
···
 	.show = queue_discard_granularity_show,
 };
 
+static struct queue_sysfs_entry queue_discard_max_hw_entry = {
+	.attr = {.name = "discard_max_hw_bytes", .mode = S_IRUGO },
+	.show = queue_discard_max_hw_show,
+};
+
 static struct queue_sysfs_entry queue_discard_max_entry = {
-	.attr = {.name = "discard_max_bytes", .mode = S_IRUGO },
+	.attr = {.name = "discard_max_bytes", .mode = S_IRUGO | S_IWUSR },
 	.show = queue_discard_max_show,
+	.store = queue_discard_max_store,
 };
 
 static struct queue_sysfs_entry queue_discard_zeroes_data_entry = {
···
 	&queue_io_opt_entry.attr,
 	&queue_discard_granularity_entry.attr,
 	&queue_discard_max_entry.attr,
+	&queue_discard_max_hw_entry.attr,
 	&queue_discard_zeroes_data_entry.attr,
 	&queue_write_same_max_entry.attr,
 	&queue_nonrot_entry.attr,

include/linux/blkdev.h

···
 	unsigned int		io_min;
 	unsigned int		io_opt;
 	unsigned int		max_discard_sectors;
+	unsigned int		max_hw_discard_sectors;
 	unsigned int		max_write_same_sectors;
 	unsigned int		discard_granularity;
 	unsigned int		discard_alignment;