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

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Date:	Sun, 24 Apr 2011 23:11:20 -0400
From:	Valdis.Kletnieks@...edu
To:	Josef Bacik <josef@...hat.com>
Cc:	linux-kernel@...r.kernel.org, linux-btrfs@...r.kernel.org,
	linux-fsdevel@...r.kernel.org, hch@...radead.org
Subject: Re: [PATCH 1/2] fs: add SEEK_HOLE and SEEK_DATA flags

On Thu, 21 Apr 2011 15:42:33 EDT, Josef Bacik said:

> -SEEK_HOLE: this moves the file pos to the nearest hole in the file from the
> given position. 

Do we want the semantic to be "the nearest" hole?  Or did we really want "the
next" hole?  Loops like a bullet loaded in the chamber and pointed at the
programmer's foot if they aren't allowing for the fact that this can go
*backwards* in the file if the closest hole is towards the beginning.  Good way
to end up in an infinite loop or other messy...

Consider the obvious implementation of "skip over a hole" - lseek(SEEK_HOLE),
lseek(SEEK_DATA).  and start reading data because we've skipped over the hole.
Wrong - the second seek may have gone backwards if the data was only 4K away
but the hole was 64K in size...

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