On 08/30/2016 10:54 AM, Andy Grover wrote:
Hi Martin, (you're the last person to touch this? :)
If I create a local SCSI device using LIO's loopback fabric, and backed
by a file, /sys/block/sda/queue/max_hw_sectors_kb reports 32767. This
reflects the loopback scsi_host_template reporting 65536 max_sectors,
but that's the "controller" -- the actual device reports via the Block
Limits VPD of supporting a max size of 16384 512-byte blocks.
Therefore, using max_hw_sectors_kb will give a wrong impression for what
the max data size that can be issued to the device is -- 32767KiB
instead of 8192KiB. Should max_hw_sectors_kb be reported as the lesser
of max_hw_sectors and max_dev_sectors? Something like:
--- a/include/linux/blkdev.h
+++ b/include/linux/blkdev.h
@@ -1196,7 +1196,7 @@ static inline unsigned int
queue_max_sectors(struct request_queue *q)
static inline unsigned int queue_max_hw_sectors(struct request_queue *q)
{
- return q->limits.max_hw_sectors;
+ return min_not_zero(q->limits.max_hw_sectors,
q->limits.max_dev_sectors);
}
...because limits.max_hw_sectors internally seems to be just the
controller limit, but what users of queue_max_hw_sectors (including
sysfs) really want is the lesser of limits.max_hw_sectors and
limits.max_dev_sectors.
It should be the maximum supported to the device, which is the minimum
of any limitations in that path. So I'd say your change is correct.
--
Jens Axboe
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