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

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Message-ID: <CANGUGtCi85Sgrr5R0E8iuN75ubbMX9txZMwnsvp4Wv3Xh+938g@mail.gmail.com>
Date:	Mon, 22 Aug 2011 12:56:54 +0200
From:	Marco Stornelli <marco.stornelli@...il.com>
To:	Sunil Mushran <sunil.mushran@...cle.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

2011/8/22 Sunil Mushran <sunil.mushran@...cle.com>:
> On 08/20/2011 09:32 AM, Marco Stornelli wrote:
>>
>> Il 20/08/2011 17:36, Sunil Mushran ha scritto:
>>>
>>> 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.
>>>
>>
>> Thank. Yes the word "next" is not very clear. I re-read the proposal for
>> the standard, actually it's seems to me that if we are in the last hole we
>> should return the file size, if we are not in the last hole than it's ok the
>> same offset - "....except that
>> if offset falls beyond the last byte not within a hole, then the file
>> offset may be set to the file size instead".
>
> Any proposal that differentiates between holes is wrong. It should not
> matter where the hole is.
>
> Think of it from the usage-pov.
>
> doff = 0;
> while ((doff = lseek(SEEK_DATA, doff)) != -ENXIO) {
>    hoff = lseek(SEEK_HOLE, doff);
>    read_offset = doff;
>    read_len = hoff -doff;
>    process();
>    doff = hoff;
> }
>
> The goal is to make this as efficient as follows. Treating the last
> hole differently adds more code for no benefit.
>
>

Mmmm.....It seems that Josef has to be clear in this point. However I
looked for the seek hole test in xfs test suite, but I didn't find
anything. Btrfs guys, how have you got tested the implementation? What
do you think about this corner case? Al, what do you think about it?

Marco
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