[PATCH v2] block: Increase BLK_DEF_MAX_SECTORS_CAP

[Damien Le Moal <dlemoal@kernel.org>]

Back in 2015, commit d2be537c3ba3 ("block: bump BLK_DEF_MAX_SECTORS to
2560") increased the default maximum size of a block device I/O to 2560
sectors (1280 KiB) to "accommodate a 10-data-disk stripe write with
chunk size 128k". This choice is rather arbitrary and since then,
improvements to the block layer have software RAID drivers correctly
advertize their stripe width through chunk_sectors and abuses of
BLK_DEF_MAX_SECTORS_CAP by drivers (to set the HW limit rather than the
default user controlled maximum I/O size) have been fixed.

Since many block devices can benefit from a larger value of
BLK_DEF_MAX_SECTORS_CAP, and in particular HDDs, increase this value to
be 4MiB, or 8192 sectors.

And given that BLK_DEF_MAX_SECTORS_CAP is only used in the block layer
and should not be used by drivers directly, move this macro definition
to the block layer internal header file block/blk.h.

Suggested-by: Martin K . Petersen <martin.petersen@oracle.com>
Signed-off-by: Damien Le Moal <dlemoal@kernel.org>
Reviewed-by: Martin K. Petersen <martin.petersen@oracle.com>
---
Changes from v1:
 - Move BLK_DEF_MAX_SECTORS_CAP definition to block/blk.h
 - Define the macro value using SZ_4M to make it more readable
 - Added review tag

 block/blk.h            | 9 +++++++++
 include/linux/blkdev.h | 9 ---------
 2 files changed, 9 insertions(+), 9 deletions(-)

diff --git a/block/blk.h b/block/blk.h
index 37ec459fe656..1141b343d0b5 100644
--- a/block/blk.h
+++ b/block/blk.h
@@ -13,6 +13,15 @@
 
 struct elevator_type;
 
+/*
+ * Default upper limit for the software max_sectors limit used for regular I/Os.
+ * This can be increased through sysfs.
+ *
+ * This should not be confused with the max_hw_sector limit that is entirely
+ * controlled by the block device driver, usually based on hardware limits.
+ */
+#define BLK_DEF_MAX_SECTORS_CAP	(SZ_4M >> SECTOR_SHIFT)
+
 #define	BLK_DEV_MAX_SECTORS	(LLONG_MAX >> 9)
 #define	BLK_MIN_SEGMENT_SIZE	4096
 
diff --git a/include/linux/blkdev.h b/include/linux/blkdev.h
index 85aab8bc96e7..c2b3ddea8b6d 100644
--- a/include/linux/blkdev.h
+++ b/include/linux/blkdev.h
@@ -1231,15 +1231,6 @@ enum blk_default_limits {
 	BLK_SEG_BOUNDARY_MASK	= 0xFFFFFFFFUL,
 };
 
-/*
- * Default upper limit for the software max_sectors limit used for
- * regular file system I/O.  This can be increased through sysfs.
- *
- * Not to be confused with the max_hw_sector limit that is entirely
- * controlled by the driver, usually based on hardware limits.
- */
-#define BLK_DEF_MAX_SECTORS_CAP	2560u
-
 static inline struct queue_limits *bdev_limits(struct block_device *bdev)
 {
 	return &bdev_get_queue(bdev)->limits;
-- 
2.49.0