linux-ext4 - Re: [PATCH v5 1/10] fs: Add new flag(FALLOC_FL_COLLAPSE

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Date:	Tue, 25 Feb 2014 15:41:20 -0800 (PST)
From:	Hugh Dickins <hughd@...gle.com>
To:	Dave Chinner <david@...morbit.com>
cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Theodore Ts'o <tytso@....edu>,
	Namjae Jeon <linkinjeon@...il.com>, viro@...iv.linux.org.uk,
	bpm@....com, adilger.kernel@...ger.ca, jack@...e.cz,
	mtk.manpages@...il.com, lczerner@...hat.com,
	linux-fsdevel@...r.kernel.org, xfs@....sgi.com,
	linux-ext4@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org, Namjae Jeon <namjae.jeon@...sung.com>,
	Ashish Sangwan <a.sangwan@...sung.com>
Subject: Re: [PATCH v5 1/10] fs: Add new flag(FALLOC_FL_COLLAPSE_RANGE) for
 fallocate

On Mon, 24 Feb 2014, Dave Chinner wrote:
> On Sat, Feb 22, 2014 at 09:06:25AM -0500, Theodore Ts'o wrote:
> > On Wed, Feb 19, 2014 at 01:37:43AM +0900, Namjae Jeon wrote:
> > > +	/*
> > > +	 * There is no need to overlap collapse range with EOF, in which case
> > > +	 * it is effectively a truncate operation
> > > +	 */
> > > +	if ((mode & FALLOC_FL_COLLAPSE_RANGE) &&
> > > +	    (offset + len >= i_size_read(inode)))
> > > +		return -EINVAL;
> > > +
> > 
> > I wonder if we should just translate a collapse range that is
> > equivalent to a truncate operation to, in fact, be a truncate
> > operation?
> 
> Trying to collapse a range that extends beyond EOF, IMO, is likely
> to only happen if the DVR/NLE application is buggy. Hence I think
> that telling the application it is doing something that is likely to
> be wrong is better than silently truncating the file....

I do agree with Ted on this point.  This is not an xfs ioctl added
for one DVR/NLE application, it's a mode of a Linux system call.

We do not usually reject with an error when one system call happens
to ask for something which can already be accomplished another way;
nor nanny our callers.

It seems natural to me that COLLAPSE_RANGE should support beyond EOF;
unless that adds significantly to implementation difficulties?

Actually, is it even correct to fail at EOF?  What if fallocation
with FALLOC_FL_KEEP_SIZE was used earlier, to allocate beyond EOF:
shouldn't it be possible to shift that allocation down, along with
the EOF, rather than leave it behind as a stranded island?

Hugh
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