[PATCH 2/3] sd: Reject optimal transfer length smaller than page size

["Martin K. Petersen" <martin.petersen@oracle.com>]

Eryu Guan reported that loading scsi_debug would fail. This turned out
to be caused by scsi_debug reporting an optimal I/O size of 32KB which
is smaller than the 64KB page size on the PowerPC system in question.

Add a check to ensure that we only use the device-reported OPTIMAL
TRANSFER LENGTH if it is bigger than or equal to the page cache size.

Reported-by: Eryu Guan <guaneryu@gmail.com>
Reported-by: Ming Lei <tom.leiming@gmail.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>
---
 drivers/scsi/sd.c | 9 ++++++---
 1 file changed, 6 insertions(+), 3 deletions(-)

diff --git a/drivers/scsi/sd.c b/drivers/scsi/sd.c
index 3d22fc3e3c1a..4e08d1cd704d 100644
--- a/drivers/scsi/sd.c
+++ b/drivers/scsi/sd.c
@@ -2885,10 +2885,13 @@ static int sd_revalidate_disk(struct gendisk *disk)
 
 	/*
 	 * Use the device's preferred I/O size for reads and writes
-	 * unless the reported value is unreasonably large (or garbage).
+	 * unless the reported value is unreasonably small, large, or
+	 * garbage.
 	 */
-	if (sdkp->opt_xfer_blocks && sdkp->opt_xfer_blocks <= dev_max &&
-	    sdkp->opt_xfer_blocks <= SD_DEF_XFER_BLOCKS)
+	if (sdkp->opt_xfer_blocks &&
+	    sdkp->opt_xfer_blocks <= dev_max &&
+	    sdkp->opt_xfer_blocks <= SD_DEF_XFER_BLOCKS &&
+	    sdkp->opt_xfer_blocks * sdp->sector_size >= PAGE_CACHE_SIZE)
 		rw_max = q->limits.io_opt =
 			logical_to_sectors(sdp, sdkp->opt_xfer_blocks);
 	else
-- 
2.5.0