linux-kernel - Re: regression introduced by "block: Add support for DAX reads/writes to block devices"

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Date:	Tue, 11 Aug 2015 07:27:28 +1000
From:	Dave Chinner <david@...morbit.com>
To:	Linda Knippers <linda.knippers@...com>
Cc:	Boaz Harrosh <boaz@...xistor.com>, Jeff Moyer <jmoyer@...hat.com>,
	"matthew r. wilcox" <matthew.r.wilcox@...el.com>,
	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	Vishal Verma <vishal.l.verma@...el.com>
Subject: Re: regression introduced by "block: Add support for DAX
 reads/writes to block devices"

On Mon, Aug 10, 2015 at 12:32:08PM -0400, Linda Knippers wrote:
> On 8/9/2015 4:52 AM, Boaz Harrosh wrote:
> > On 08/06/2015 11:34 PM, Dave Chinner wrote:
> >> On Thu, Aug 06, 2015 at 10:52:47AM +0300, Boaz Harrosh wrote:
> >>> On 08/06/2015 06:24 AM, Dave Chinner wrote:
> >>>> On Wed, Aug 05, 2015 at 09:42:54PM -0400, Linda Knippers wrote:
> >>>>> On 08/05/2015 06:01 PM, Dave Chinner wrote:
> >>>>>> On Wed, Aug 05, 2015 at 04:19:08PM -0400, Jeff Moyer wrote:
> >>> <>
> >>>>>>>
> >>>>>>> I sat down with Linda to look into it, and the problem is that mkfs.xfs
> >>>>>>> sets the blocksize of the device to 512 (via BLKBSZSET), and then reads
> >>>>>>> from the last sector of the device.  This results in dax_io trying to do
> >>>>>>> a page-sized I/O at 512 bytes from the end of the device.
> >>>>>>
> >>>
> >>> This part I do not understand. how is mkfs.xfs reading the sector?
> >>> Is it through open(/dev/pmem0,...) ? O_DIRECT?
> >>
> >> mkfs.xfs uses O_DIRECT. Only if open(O_DIRECT) fails or mkfs.xfs is
> >> told that it is working on an image file does it fall back to
> >> buffered IO. All of the XFS userspace tools work this way to prevent
> >> page cache pollution issues with read-once or write-once data during
> >> operation.
....
> That patch does cause 'mkfs -t xfs' to work.
> 
> Before:
> $ sudo mkfs -t xfs -f /dev/pmem3
> meta-data=/dev/pmem3             isize=256    agcount=4, agsize=524288 blks
>          =                       sectsz=512   attr=2, projid32bit=1
                                   ^^^^^^^^^^
....

> $ sudo mkfs -t xfs -f /dev/pmem3
> meta-data=/dev/pmem3             isize=256    agcount=4, agsize=524288 blks
>          =                       sectsz=4096  attr=2, projid32bit=1
                                   ^^^^^^^^^^^

So in the after case, mkfs.xfs is behaving differently and not
exercising the bug. It's seen the:

> $ cat /sys/block/pmem3/queue/logical_block_size
> 512
> $ cat /sys/block/pmem3/queue/physical_block_size
> 4096
  ^^^^

4k physical block size, and hence configured the filesystem with a
4k sector size so all IO it issues is physicallly aligned. IOWs,
mkfs.xfs's last sector read is 4k aligned and sized, and therefore
the test has not confirmed that the patch fixes the 512 byte last
sector read is fixed at all.

Isn't there a regression test suite that covers basic block device
functionality that you can use to test these simple corner cases?

Cheers,

Dave.
-- 
Dave Chinner
david@...morbit.com
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