linux-kernel - Re: [PATCH 1/2] fs: add SEEK_HOLE and SEEK

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Date:	Fri, 22 Apr 2011 09:28:13 -0700
From:	Sunil Mushran <sunil.mushran@...cle.com>
To:	Eric Blake <eblake@...hat.com>
CC:	Markus Trippelsdorf <markus@...ppelsdorf.de>,
	Christoph Hellwig <hch@...radead.org>,
	Josef Bacik <josef@...icpanda.com>,
	linux-kernel@...r.kernel.org, linux-btrfs@...r.kernel.org,
	linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH 1/2] fs: add SEEK_HOLE and SEEK_DATA flags

On 04/22/2011 04:50 AM, Eric Blake wrote:
> That blog also mentioned the useful idea of adding FIND_HOLE and
> FIND_DATA, not implemented in Solaris, but which could easily be
> provided as additional lseek constants in Linux to locate the start of
> the next chunk without repositioning and which could ease application
> programmer's life a bit.  After all, cp wants to know where data ends
> without repositioning (FIND_HOLE), read() that much data which
> repositions in the process, then skip to the next chunk of data
> (SEEK_DATA) - two lseek() calls per iteration if we have 4 constants,
> but 3 per iteration if we only have SEEK_HOLE and have to manually rewind.

while(1) {
     read(block);
     if (block_all_zeroes)
         lseek(SEEK_DATA);
}

What's wrong with the above? If this is the case, even SEEK_HOLE
is not needed but should be added as it is already in Solaris.

My problem with FIND_* is that we are messing with the well understood
semantics of lseek().

And if generic_file_llseek_unlocked() treats SEEK_DATA as SEEK_CUR and
SEEK_HOLE as SEEK_END (both with zero offset) then we don't even
have to bother with the finding the "correct" error code.

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