[PATCH] scsi: sd: Do not override max_sectors_kb sysfs setting

Martin K. Petersen
2017-09-28T01:39:06Z

A user may lower the max_sectors_kb setting in sysfs to accommodate
certain workloads. Previously we would always set the max I/O size to
either the block layer default or the optional preferred I/O size
reported by the device.

Keep the current heuristics for the initial setting of max_sectors_kb.
For subsequent invocations, only update the current queue limit if it
exceeds the capabilities of the hardware.

Reported-by: Don Brace <don.brace@microsemi.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>
---
drivers/scsi/sd.c | 19 ++++++++++++++-----
1 file changed, 14 insertions(+), 5 deletions(-)

diff --git a/drivers/scsi/sd.c b/drivers/scsi/sd.c
index 6549e5ce09ca..b18ba3235900 100644
--- a/drivers/scsi/sd.c
+++ b/drivers/scsi/sd.c
@@ -3098,8 +3098,6 @@ static int sd_revalidate_disk(struct gendisk *disk)
sd_read_security(sdkp, buffer);
}

- sdkp->first_scan = 0;
-
/*
* We now have all cache related info, determine how we deal
* with flush requests.
@@ -3114,7 +3112,7 @@ static int sd_revalidate_disk(struct gendisk *disk)
q->limits.max_dev_sectors = logical_to_sectors(sdp, dev_max);

/*
- * Use the device's preferred I/O size for reads and writes
+ * Determine the device's preferred I/O size for reads and writes
* unless the reported value is unreasonably small, large, or
* garbage.
*/
@@ -3128,8 +3126,19 @@ static int sd_revalidate_disk(struct gendisk *disk)
rw_max = min_not_zero(logical_to_sectors(sdp, dev_max),
(sector_t)BLK_DEF_MAX_SECTORS);

- /* Combine with controller limits */
- q->limits.max_sectors = min(rw_max, queue_max_hw_sectors(q));
+ /* Do not exceed controller limit */
+ rw_max = min(rw_max, queue_max_hw_sectors(q));
+
+ /*
+ * Only update max_sectors if previously unset or if the current value
+ * exceeds the capabilities of the hardware.
+ */
+ if (sdkp->first_scan ||
+ q->limits.max_sectors > q->limits.max_dev_sectors ||
+ q->limits.max_sectors > q->limits.max_hw_sectors)
+ q->limits.max_sectors = rw_max;
+
+ sdkp->first_scan = 0;

set_capacity(disk, logical_to_sectors(sdp, sdkp->capacity));
sd_config_write_same(sdkp);
--
2.14.1