[dm-devel] Max length limitation with BLKZEROOUT on dm device ?

[Mike Snitzer]

On Mon, Jul 01 2013 at  5:04am -0400,
Bharata B Rao <bharata.rao at gmail.com> wrote:

> Hi,
> 
> While experimenting on zeroing out a dm device from a storage that
> supports WRITE SAME, I see the below warnings from kernel (3.10.0):
> 
> ******************
> INFO: task a.out:21365 blocked for more than 120 seconds.
> "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
> a.out           D ffffffff8182d620     0 21365  19945 0x00000080
> ffff882079321bf8 0000000000000086 ffff882079321fd8 0000000000013fc0
> ffff882079320010 0000000000013fc0 0000000000013fc0 0000000000013fc0
> ffff882079321fd8 0000000000013fc0 ffff882032a6eb10 ffff8810795ea0c0
> Call Trace:
> [<ffffffff816426f9>] schedule+0x29/0x70
> [<ffffffff81640945>] schedule_timeout+0x195/0x220
> [<ffffffff812ad777>] ? __blk_run_queue+0x37/0x50
> [<ffffffff812af53f>] ? blk_queue_bio+0x2ff/0x3b0
> [<ffffffff81064859>] ? read_tsc+0x9/0x20
> [<ffffffff810f5ebc>] ? ktime_get_ts+0x4c/0xf0
> [<ffffffff81641c4f>] io_schedule_timeout+0x9f/0x100
> [<ffffffff812ae44a>] ? generic_make_request+0xca/0x100
> [<ffffffff81642aec>] wait_for_completion_io+0xdc/0x110
> [<ffffffff810d8fb0>] ? try_to_wake_up+0x2b0/0x2b0
> [<ffffffff812b5c3c>] blkdev_issue_write_same+0x1bc/0x1d0
> [<ffffffff812b5cfd>] blkdev_issue_zeroout+0xad/0x120
> [<ffffffff812b6c68>] blkdev_ioctl+0x728/0x7b0
> [<ffffffff8120296c>] block_ioctl+0x3c/0x40
> [<ffffffff811ddb89>] do_vfs_ioctl+0x89/0x350
> [<ffffffff811ddef1>] SyS_ioctl+0xa1/0xb0
> [<ffffffff8164ca99>] system_call_fastpath+0x16/0x1b
> ******************
> 
> My test program looks like this:
> 
> {
> unsigned long arg[2] = {0, 21474836480}; /* len = 20G */
> fd = open("/dev/mapper/ws_vg-lvol0", O_RDWR);
> ioctl(fd, BLKZEROOUT, &arg);
> }
> 
> # ls -l /dev/mapper/ws_vg-lvol0
> lrwxrwxrwx. 1 root root 8 Jul  1 12:25 /dev/mapper/ws_vg-lvol0 -> ../dm-17
> 
> # cat /sys/block/dm-17/queue/write_same_max_bytes
> 268435456
> 
> I don't see such messages when len is 512M, 1G, 10G or 15G, but with
> 20G, such messages are seen.
> 
> Though I don't see any limitation on the number of bytes that could be
> zeroed out from a single BLKZEROOUT ioctl call, could this be the
> result of using a len as large as 20G in a single ioctl ?

It is quite likely a function of the storage.  The 20G BLKZEROOUT
eventually completes right?

It could be that 15G just happens to complete before the 120s threshold,
but 20G causes it to exceed the 120s hung task timeout?

Mike