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

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Date:	Mon, 22 Aug 2011 19:56:31 +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

Il 22/08/2011 17:57, Sunil Mushran ha scritto:
>>> 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?
>
>
> The following test was used to test the early implementations.
> http://oss.oracle.com/~smushran/seek_data/
>

Thank you very much!! I found another point. Your test fails with my 
implementation because here 
(http://www.austingroupbugs.net/view.php?id=415) says: "If whence is 
SEEK_DATA, the file offset shall be set to the smallest location of a 
byte not within a hole and not less than offset. It shall be an error if 
no such byte exists." So in this case I return ENXIO but the test 
expects another value. I have to say that there is a bit of confusion 
about the real behavior of this new feature :)

Marco
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