linux-kernel - Re: [PATCH] tmpfs: support SEEK_DATA and SEEK

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Date:	Wed, 28 Nov 2012 18:02:21 -0800 (PST)
From:	Hugh Dickins <hughd@...gle.com>
To:	Jaegeuk Hanse <jaegeuk.hanse@...il.com>
cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Theodore Ts'o <tytso@....edu>,
	Zheng Liu <wenqing.lz@...bao.com>,
	Jeff liu <jeff.liu@...cle.com>,
	Jim Meyering <jim@...ering.net>,
	Paul Eggert <eggert@...ucla.edu>,
	Christoph Hellwig <hch@...radead.org>,
	Josef Bacik <josef@...hat.com>,
	Andi Kleen <andi@...stfloor.org>,
	Andreas Dilger <adilger@...ger.ca>,
	Dave Chinner <david@...morbit.com>,
	Marco Stornelli <marco.stornelli@...il.com>,
	Chris Mason <chris.mason@...ionio.com>,
	Sunil Mushran <sunil.mushran@...cle.com>,
	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	linux-mm@...ck.org
Subject: Re: [PATCH] tmpfs: support SEEK_DATA and SEEK_HOLE (reprise)

On Thu, 29 Nov 2012, Jaegeuk Hanse wrote:
> On Wed, Nov 28, 2012 at 05:22:03PM -0800, Hugh Dickins wrote:
> >Revert 3.5's f21f8062201f ("tmpfs: revert SEEK_DATA and SEEK_HOLE")
> >to reinstate 4fb5ef089b28 ("tmpfs: support SEEK_DATA and SEEK_HOLE"),
> >with the intervening additional arg to generic_file_llseek_size().
> >
> >In 3.8, ext4 is expected to join btrfs, ocfs2 and xfs with proper
> >SEEK_DATA and SEEK_HOLE support; and a good case has now been made
> >for it on tmpfs, so let's join the party.
> >
> 
> Hi Hugh,
> 
> IIUC, several months ago you revert the patch. You said, 
> 
> "I don't know who actually uses SEEK_DATA or SEEK_HOLE, and whether it
> would be of any use to them on tmpfs.  This code adds 92 lines and 752
> bytes on x86_64 - is that bloat or worthwhile?"

YUC.

> 
> But this time in which scenario will use it?

I was not very convinced by the grep argument from Jim and Paul:
that seemed to be grep holding on to a no-arbitrary-limits dogma,
at the expense of its users, causing an absurd line-length issue,
which use of SEEK_DATA happens to avoid in some cases.

The cp of sparse files from Jeff and Dave was more convincing;
but I still didn't see why little old tmpfs needed to be ahead
of the pack.

But at LinuxCon/Plumbers in San Diego in August, a more convincing
case was made: I was hoping you would not ask, because I did not take
notes, and cannot pass on the details - was it rpm building on tmpfs?
I was convinced enough to promise support on tmpfs when support on
ext4 goes in.

Hugh
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