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

To: Dave Chinner <david@xxxxxxxxxxxxx>
Subject: Re: [PATCH v2] xfs: Make fiemap works with sparse file.
From: Tao Ma <tao.ma@xxxxxxxxxx>
Date: Mon, 14 Jun 2010 21:37:18 +0800
Cc: xfs@xxxxxxxxxxx, linux-kernel@xxxxxxxxxxxxxxx, sandeen@xxxxxxxxxxx, Alex Elder <aelder@xxxxxxx>, Christoph Hellwig <hch@xxxxxx>
In-reply-to: <20100614122912.GD6590@dastard>
Organization: Oracle Corporation
References: <1276308495-14267-1-git-send-email-tao.ma@xxxxxxxxxx> <20100614002705.GA6590@dastard> <20100614122912.GD6590@dastard>
User-agent: Thunderbird 2.0.0.24 (Windows/20100228)
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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