scsi: sd: Do not override max_sectors_kb sysfs setting

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@microsemi.com>
Reviewed-by: Martin Wilck <mwilck@suse.com>
Tested-by: Don Brace <don.brace@microsemi.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/drivers/scsi/sd.c b/drivers/scsi/sd.c
index 8750c86f95f9534683d22c9f17e6c613e2135c2a..7e1681cf287c1e226e770035505a4b0b02958f79 100644 (file)
--- a/drivers/scsi/sd.c
+++ b/drivers/scsi/sd.c
@@ -2878,8 +2878,6 @@ static int sd_revalidate_disk(struct gendisk *disk)
                sd_read_write_same(sdkp, buffer);
        }
 
-       sdkp->first_scan = 0;
-
        /*
         * We now have all cache related info, determine how we deal
         * with flush requests.
@@ -2894,7 +2892,7 @@ 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.
         */
@@ -2908,8 +2906,19 @@ 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);