linux-kernel - Re: [PATCH 1/4] fs: add SEEK_HOLE and SEEK

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Date:	Sat, 20 Aug 2011 08:36:43 -0700
From:	Sunil Mushran <sunil.mushran@...cle.com>
To:	Marco Stornelli <marco.stornelli@...il.com>
CC:	Josef Bacik <josef@...hat.com>, linux-fsdevel@...r.kernel.org,
	linux-kernel@...r.kernel.org, linux-btrfs@...r.kernel.org,
	xfs@....sgi.com, viro@...IV.linux.org.uk
Subject: Re: [PATCH 1/4] fs: add SEEK_HOLE and SEEK_DATA flags

On 08/20/2011 03:03 AM, Marco Stornelli wrote:
> Il 20/08/2011 11:41, Marco Stornelli ha scritto:
>> Hi,
>>
>> Il 28/06/2011 17:33, Josef Bacik ha scritto:
>>> This just gets us ready to support the SEEK_HOLE and SEEK_DATA flags.
>>> Turns out
>>> using fiemap in things like cp cause more problems than it solves, so
>>> lets try
>>> and give userspace an interface that doesn't suck. We need to match
>>> solaris
>>> here, and the definitions are
>>>
>>> *o* If /whence/ is SEEK_HOLE, the offset of the start of the
>>> next hole greater than or equal to the supplied offset
>>> is returned. The definition of a hole is provided near
>>> the end of the DESCRIPTION.
>>>
>>> *o* If /whence/ is SEEK_DATA, the file pointer is set to the
>>> start of the next non-hole file region greater than or
>>> equal to the supplied offset.
>>>
>>
>> I'm implementing the SEEK_DATA/SEEK_HOLE management for pramfs and I've
>> got some doubts about the right behavior:
>>
>> 1) when we use SEEK_DATA/SEEK_HOLE, the offset used in lseek means
>> always the offset from the start of the file, right?
>>
>> 2) in case of a file with hole at the beginning and data at the end, if
>> I do lseek(fd, 0, SEEK_HOLE) I should receive the end of the file
>> because the idea is to search the *next* hole and we have always a
>> virtual hole at the end of the file, right?
>
> Just to be precise about this question: the alternative here, it's to 
> return the same position because we are already in a hole.

Yes, the offset is from the start of the file.

And yes, same offset is ok. I think the word next should be
dropped from the definition. It is misleading.
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