[PATCH 4.9 093/105] scsi: sd: Do not override max_sectors_kb sysfs setting

[Greg Kroah-Hartman]

4.9-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Martin K. Petersen <martin.petersen@xxxxxxxxxx>

commit 77082ca503bed061f7fbda7cfd7c93beda967a41 upstream.

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@xxxxxxxxxxxxx>
Reviewed-by: Martin Wilck <mwilck@xxxxxxxx>
Tested-by: Don Brace <don.brace@xxxxxxxxxxxxx>
Signed-off-by: Martin K. Petersen <martin.petersen@xxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>

---
 drivers/scsi/sd.c |   19 ++++++++++++++-----
 1 file changed, 14 insertions(+), 5 deletions(-)

--- a/drivers/scsi/sd.c
+++ b/drivers/scsi/sd.c
@@ -2867,8 +2867,6 @@ static int sd_revalidate_disk(struct gen
 		sd_read_write_same(sdkp, buffer);
 	}
 
-	sdkp->first_scan = 0;
-
 	/*
 	 * We now have all cache related info, determine how we deal
 	 * with flush requests.
@@ -2883,7 +2881,7 @@ static int sd_revalidate_disk(struct gen
 	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.
 	 */
@@ -2897,8 +2895,19 @@ static int sd_revalidate_disk(struct gen
 		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);