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From: Dave Chinner <david@fromorbit.com>
To: Tao Ma <tao.ma@oracle.com>
Cc: Christoph Hellwig <hch@lst.de>, Alex Elder <aelder@sgi.com>,
	sandeen@sandeen.net, linux-kernel@vger.kernel.org,
	xfs@oss.sgi.com
Subject: Re: [PATCH v2] xfs: Make fiemap works with sparse file.
Date: Mon, 14 Jun 2010 22:29:12 +1000	[thread overview]
Message-ID: <20100614122912.GD6590@dastard> (raw)
In-Reply-To: <20100614002705.GA6590@dastard>

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...

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com

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WARNING: multiple messages have this Message-ID (diff)
From: Dave Chinner <david@fromorbit.com>
To: Tao Ma <tao.ma@oracle.com>
Cc: xfs@oss.sgi.com, linux-kernel@vger.kernel.org,
	sandeen@sandeen.net, Alex Elder <aelder@sgi.com>,
	Christoph Hellwig <hch@lst.de>
Subject: Re: [PATCH v2] xfs: Make fiemap works with sparse file.
Date: Mon, 14 Jun 2010 22:29:12 +1000	[thread overview]
Message-ID: <20100614122912.GD6590@dastard> (raw)
In-Reply-To: <20100614002705.GA6590@dastard>

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...

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com

  parent reply	other threads:[~2010-06-14 12:27 UTC|newest]

Thread overview: 20+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2010-06-12  2:08 [PATCH v2] xfs: Make fiemap works with sparse file Tao Ma
2010-06-12  2:08 ` Tao Ma
2010-06-14  0:27 ` Dave Chinner
2010-06-14  0:27   ` Dave Chinner
2010-06-14  5:53   ` Tao Ma
2010-06-14  5:53     ` Tao Ma
2010-06-14 12:29   ` Dave Chinner [this message]
2010-06-14 12:29     ` Dave Chinner
2010-06-14 13:37     ` Tao Ma
2010-06-14 13:37       ` Tao Ma
2010-06-17  8:53     ` Tao Ma
2010-06-17  8:53       ` Tao Ma
2010-06-18  0:47       ` Dave Chinner
2010-06-18  0:47         ` Dave Chinner
2010-06-18  2:27         ` Tao Ma
2010-06-18  2:27           ` Tao Ma
2010-06-18  6:22           ` Dave Chinner
2010-06-18  6:22             ` Dave Chinner
2010-08-27 19:46       ` Alex Elder
2010-08-30  2:44         ` [PATCH v4] " Tao Ma

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