linux-kernel - Re: [PATCH v2] xfs: Make fiemap works with sparse file.

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Date:	Mon, 14 Jun 2010 21:37:18 +0800
From:	Tao Ma <tao.ma@...cle.com>
To:	Dave Chinner <david@...morbit.com>
CC:	xfs@....sgi.com, linux-kernel@...r.kernel.org, sandeen@...deen.net,
	Alex Elder <aelder@....com>, Christoph Hellwig <hch@....de>
Subject: Re: [PATCH v2] xfs: Make fiemap works with sparse file.

Hi Dave,

Dave Chinner wrote:
> On Mon, Jun 14, 2010 at 10:27:06AM +1000, Dave Chinner wrote:
>   
>> On Sat, Jun 12, 2010 at 10:08:15AM +0800, Tao Ma wrote:
>>     
>>> The reason is that in xfs_getbmap we will
>>> calculate holes and set it in 'out', while out is malloced by
>>> bmv_count(fi_extent_max+1) which didn't consider holes. So in the
>>> worst case, if 'out' vector looks like
>>> [hole, extent, hole, extent, hole, ... hole, extent, hole],
>>> we will only return half of fi_extent_max extents.
>>>       
>> Right, it's not broken, we simply return less than fi_extent_mex
>> extents when there are holes. I don't see that as a problem as
>> applications have to handle that case anyway, and....
>>
>>     
>>> So in xfs_vn_fiemap, we should consider this worst case. If the
>>> user wants fi_extent_max extents, we need a 'out' with size of
>>> 2 *fi_extent_max + 2(one more the header).
>>>       
>> That's rather dangerous, I think. It relies on other code to catch
>> the buffer overrun that this sets up for fragmented, non-sparse
>> files. Personally I'd much prefer to return fewer extents for sparse
>> files than to add a landmine like this into the kernel code....
>>     
>
> I just had a thought - if you want to avoid holes being reported to
> fiemap, then add a BMV_IF_NO_HOLES flag to xfs_getbmap() and skip
> holes in the mappin gloop when this flag is set. That will make
> fiemap fill in the full number of extents without hacking the
> extent count...
>   
yeah, that should work and I will try to generate a patch for it.
I am not quite familiar with xfs, so please be kind to me if I make some 
stupid mistake in the patch. ;)

Regards,
Tao
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