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

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Message-ID: <alpine.LSU.2.00.1205151529140.1571@eggly.anvils>
Date:	Tue, 15 May 2012 15:38:33 -0700 (PDT)
From:	Hugh Dickins <hughd@...gle.com>
To:	Allison Henderson <achender@...ux.vnet.ibm.com>
cc:	Jan Kara <jack@...e.cz>, 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 Tue, 15 May 2012, Allison Henderson wrote:
> 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.
> 
> 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 

Thanks, Allison.  I just added Jan to the Cc list to make sure he sees,
since we mentioned this in the inode_dio_wait thread (which I skilfully
directed to an almost disjoint set of addressees - though I expect he
already saw via linux-ext4).

Hugh
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