linux-kernel - Re: [PATCH] xfstests 255: add a seek_data/seek

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Date:	Wed, 29 Jun 2011 10:10:07 -0700
From:	Sunil Mushran <sunil.mushran@...cle.com>
To:	Christoph Hellwig <hch@...radead.org>
CC:	Dave Chinner <david@...morbit.com>, Josef Bacik <josef@...hat.com>,
	linux-fsdevel@...r.kernel.org, viro@...IV.linux.org.uk,
	linux-kernel@...r.kernel.org, linux-btrfs@...r.kernel.org,
	xfs@....sgi.com
Subject: Re: [PATCH] xfstests 255: add a seek_data/seek_hole tester

On 06/29/2011 12:40 AM, Christoph Hellwig wrote:
> On Wed, Jun 29, 2011 at 04:53:07PM +1000, Dave Chinner wrote:
>> On Tue, Jun 28, 2011 at 11:33:19AM -0400, Josef Bacik wrote:
>>> This is a test to make sure seek_data/seek_hole is acting like it does on
>>> Solaris.  It will check to see if the fs supports finding a hole or not and will
>>> adjust as necessary.
>> So I just looked at this with an eye to validating an XFS
>> implementation, and I came up with this list of stuff that the test
>> does not cover that I'd need to test in some way:
>>
>> 	- files with clean unwritten extents. Are they a hole or
>> 	  data? What's SEEK_DATA supposed to return on layout like
>> 	  hole-unwritten-data? i.e. needs to add fallocate to the
>> 	  picture...
>>
>> 	- files with dirty unwritten extents (i.e. dirty in memory,
>> 	  not on disk). They are most definitely data, and most
>> 	  filesystems will need a separate lookup path to detect
>> 	  dirty unwritten ranges because the state is kept
>> 	  separately (page cache vs extent cache).  Plenty of scope
>> 	  for filesystem specific bugs here so needs a roubust test.
> The discussion leading up to the resurrection of SEEK_HOLE/SEEK_DATA
> was pretty much about that point.  The conclusion based on the Sun
> documentation and common sense was that SEEK_DATA may only consider
> unwritten extents as hole if the filesystem has a way to distinguish
> plain unwritten extents and those that have been dirtied.  Else it
> should be considered data.
>
> Testing for making sure dirty preallocated areas aren't wrongly
> reported sounds relatively easy, the rest falls into implementation
> details, which imho is fine.  Not reporting preallocated extents
> as holes just is a quality of implementation issue and not a bug.

I agree. And if I might add my 2 cents that it would be much easier
if we added another test that created files with all the worrisome boundary
conditions and used SEEK_DATA/HOLE to copy the files and compared
using md5sum. This would be far easier than one that expects a certain
pos for each operation.
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