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Date:	Fri, 15 Apr 2011 16:28:37 -0600
From:	Andreas Dilger <>
To:	Christoph Hellwig <>
Cc:	Eric Blake <>, Jim Meyering <>,
	Eric Sandeen <>,
	Dave Chinner <>, xfs-oss <>,,,
	Markus Trippelsdorf <>
Subject: Re: Files full of zeros with coreutils-8.11 and xfs (FIEMAP related?)

On 2011-04-15, at 11:26 AM, Christoph Hellwig wrote:
> On Fri, Apr 15, 2011 at 11:24:19AM -0600, Eric Blake wrote:
>> Would it be worth borrowing from Solaris' semantics and adding SEEK_HOLE
>> and SEEK_DATA to lseek(2), as a higher level (less-detailed, but easier
>> to define and easier to use) interface for discovering the regions of a
>> file that only contain NUL bytes?
> Yes, I've already suggested that both in this thread and on IRC.
> For efficient copies it's the only usable interface.

I suspect that these bugs would have still existed whether the interface is SEEK_HOLE/SEEK_DATA, or FIEMAP.  The main problem is that the delalloc pages were not accounted for correctly during layout traversal.

For ext4 I think it is sufficient to add another case to ext4_ext_fiemap_cb() to check the pagecache for unmapped pages when it finds an uninitialized extent on disk.  This will be very similar to the fix for finding holes in the on-disk mapping.

Cheers, Andreas

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