linux-ext4 - Re: punch-hole should go beyond i

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Message-ID: <4FB2CC79.4020200@linux.vnet.ibm.com>
Date:	Tue, 15 May 2012 14:36:57 -0700
From:	Allison Henderson <achender@...ux.vnet.ibm.com>
To:	Hugh Dickins <hughd@...gle.com>
CC:	Dave Chinner <david@...morbit.com>,
	"Theodore Ts'o" <tytso@....edu>, linux-ext4@...r.kernel.org,
	Lukas Czerner <lczerner@...hat.com>
Subject: Re: punch-hole should go beyond i_size

On 05/13/2012 02:13 PM, Hugh Dickins wrote:
> On Thu, 12 Jan 2012, Allison Henderson wrote:
>> On 01/11/2012 07:55 PM, Dave Chinner wrote:
>>> On Wed, Jan 11, 2012 at 05:02:12PM -0800, Hugh Dickins wrote:
>>>> Hi Allison,
>>>>
>>>> In thinking about fallocate() on tmpfs, I cross-check with ext4
>>>> and find this bug in its implementation of FALLOC_FL_PUNCH_HOLE:
>>>>
>>>> rm -f temp
>>>> fallocate    -l 4096 temp
>>>> du temp				# shows 4, right
>>>> fallocate -p -l 4096 temp
>>>> du temp				# shows 0, right
>>>> rm -f temp
>>>> fallocate -n -l 4096 temp
>>>> du temp				# shows 4, right
>>>> fallocate -p -l 4096 temp
>>>> du temp				# shows 4, wrong
>>>> rm temp
>>>>
>>>> ext4_ext_punch_hole() contains /* No need to punch hole beyond i_size */
>>>> early return, and trimming to i_size below, but forgets that the other
>>>> variety of fallocate(), with FALLOC_FL_KEEP_SIZE set, may have allocated
>>>> blocks beyond i_size.  They can be removed with ftruncate(), but it is
>>>> unexpected for fallocate() not to undo its own work, and xfs does so.
>>>
>>> I'm pretty sure that's a bug as XFS allows punching holes in extents
>>> beyond EOF.
>>>
>>> Cheers,
>>>
>>> Dave.
>>
>> Oh I see, I'll take a look at it, I think it will be ok to just take out the
>> early return.  Thx!
> 
> I see the -EOPNOTSUPPs have gone into 3.4's ext4_punch_hole() - thanks -
> but the i_size issue remains unfixed.  I wouldn't be surprised if it were
> more complicated than you had hoped - I had no intention of trying a patch
> myself!  It's not an actual problem for me, but I thought I'd just send a
> reminder, before I move out of the hole-punching business.
> 
> Hugh
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Hi all,

I had a fix for this a while ago and I believe Lukas had rebased it when he was working on some punch hole optimizations, but Im not sure what happened to it after that.  I think Lukas might still be working on that set?  If not, I can take a peek at it again and see if I can get it updated and resent.  Thx!

Allison Henderson 

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