scsi: sd: Do not override max_sectors_kb sysfs setting

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.

Cc: <stable@vger.kernel.org>
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>

1 files changed, 14 insertions, 5 deletions

diff --git a/drivers/scsi/sd.c b/drivers/scsi/sd.c
index 3d26a729825c..d175c5c5ccf8 100644
--- a/drivers/scsi/sd.c
+++ b/drivers/scsi/sd.c
@@ -3107,8 +3107,6 @@ 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.
@@ -3123,7 +3121,7 @@ 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.
          */
@@ -3137,8 +3135,19 @@ 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);