[PATCHv3 2/2] block: save user max_sectors limit

[Keith Busch <kbusch@meta.com>]

From: Keith Busch <kbusch@kernel.org>

The user can set the max_sectors limit to any valid value via sysfs
/sys/block/<dev>/queue/max_sectors_kb attribute. If the device limits
are ever rescanned, though, the limit reverts back to the potentially
artificially low BLK_DEF_MAX_SECTORS value.

Preserve the user's setting as the max_sectors limit as long as it's
valid. The user can reset back to defaults by writing 0 to the sysfs
file.

Signed-off-by: Keith Busch <kbusch@kernel.org>
---
v2->v3:

  Added documentation update (Damien)

  Using the unsigned BLK_DEF_MAX_SECTORS (Christoph)

 Documentation/ABI/stable/sysfs-block |  3 ++-
 block/blk-settings.c                 |  9 +++++++--
 block/blk-sysfs.c                    | 13 ++++++++++---
 include/linux/blkdev.h               |  1 +
 4 files changed, 20 insertions(+), 6 deletions(-)

diff --git a/Documentation/ABI/stable/sysfs-block b/Documentation/ABI/stable/sysfs-block
index cd14ecb3c9a5a..ac1e519272aa2 100644
--- a/Documentation/ABI/stable/sysfs-block
+++ b/Documentation/ABI/stable/sysfs-block
@@ -432,7 +432,8 @@ Contact:	linux-block@vger.kernel.org
 Description:
 		[RW] This is the maximum number of kilobytes that the block
 		layer will allow for a filesystem request. Must be smaller than
-		or equal to the maximum size allowed by the hardware.
+		or equal to the maximum size allowed by the hardware. Write 0
+		to use default kernel settings.
 
 
 What:		/sys/block/<disk>/queue/max_segment_size
diff --git a/block/blk-settings.c b/block/blk-settings.c
index 9875ca131eb0c..9c9713c9269cc 100644
--- a/block/blk-settings.c
+++ b/block/blk-settings.c
@@ -40,7 +40,7 @@ void blk_set_default_limits(struct queue_limits *lim)
 	lim->virt_boundary_mask = 0;
 	lim->max_segment_size = BLK_MAX_SEGMENT_SIZE;
 	lim->max_sectors = lim->max_hw_sectors = BLK_SAFE_MAX_SECTORS;
-	lim->max_dev_sectors = 0;
+	lim->max_user_sectors = lim->max_dev_sectors = 0;
 	lim->chunk_sectors = 0;
 	lim->max_write_zeroes_sectors = 0;
 	lim->max_zone_append_sectors = 0;
@@ -135,7 +135,12 @@ void blk_queue_max_hw_sectors(struct request_queue *q, unsigned int max_hw_secto
 	limits->max_hw_sectors = max_hw_sectors;
 
 	max_sectors = min_not_zero(max_hw_sectors, limits->max_dev_sectors);
-	max_sectors = min(max_sectors, BLK_DEF_MAX_SECTORS);
+
+	if (limits->max_user_sectors)
+		max_sectors = min(max_sectors, limits->max_user_sectors);
+	else
+		max_sectors = min(max_sectors, BLK_DEF_MAX_SECTORS);
+
 	max_sectors = round_down(max_sectors,
 				 limits->logical_block_size >> SECTOR_SHIFT);
 	limits->max_sectors = max_sectors;
diff --git a/block/blk-sysfs.c b/block/blk-sysfs.c
index 93d9e9c9a6ea8..e67acd859d072 100644
--- a/block/blk-sysfs.c
+++ b/block/blk-sysfs.c
@@ -249,9 +249,16 @@ queue_max_sectors_store(struct request_queue *q, const char *page, size_t count)
 
 	max_hw_sectors_kb = min_not_zero(max_hw_sectors_kb, (unsigned long)
 					 q->limits.max_dev_sectors >> 1);
-
-	if (max_sectors_kb > max_hw_sectors_kb || max_sectors_kb < page_kb)
-		return -EINVAL;
+	if (max_sectors_kb == 0) {
+		q->limits.max_user_sectors = 0;
+		max_sectors_kb = min(max_hw_sectors_kb,
+				     BLK_DEF_MAX_SECTORS >> 1);
+	} else {
+		if (max_sectors_kb > max_hw_sectors_kb ||
+		    max_sectors_kb < page_kb)
+			return -EINVAL;
+		q->limits.max_user_sectors = max_sectors_kb << 1;
+	}
 
 	spin_lock_irq(&q->queue_lock);
 	q->limits.max_sectors = max_sectors_kb << 1;
diff --git a/include/linux/blkdev.h b/include/linux/blkdev.h
index 69f7199d38da8..8f5bb00d12ece 100644
--- a/include/linux/blkdev.h
+++ b/include/linux/blkdev.h
@@ -288,6 +288,7 @@ struct queue_limits {
 	unsigned int		max_dev_sectors;
 	unsigned int		chunk_sectors;
 	unsigned int		max_sectors;
+	unsigned int		max_user_sectors;
 	unsigned int		max_segment_size;
 	unsigned int		physical_block_size;
 	unsigned int		logical_block_size;
-- 
2.30.2