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Date: Wed, 26 Feb 2014 17:30:35 -0800 (PST)
From: Hugh Dickins <hughd@google.com>
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To: Dave Chinner <david@fromorbit.com>
cc: Hugh Dickins <hughd@google.com>, Namjae Jeon <linkinjeon@gmail.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Matthew Wilcox <matthew@wil.cx>, "Theodore Ts'o" <tytso@mit.edu>,
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        linux-ext4@vger.kernel.org, linux-kernel@vger.kernel.org,
        linux-mm@kvack.org, Namjae Jeon <namjae.jeon@samsung.com>
Subject: Re: [PATCH v5 0/10] fs: Introduce new flag(FALLOC_FL_COLLAPSE_RANGE)
 for fallocate
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On Thu, 27 Feb 2014, Dave Chinner wrote:
> On Wed, Feb 26, 2014 at 03:08:58PM -0800, Hugh Dickins wrote:
> > 
> > Thanks for explaining more, I was just about to acknowledge what a good
> > example that is.  Indeed, it seems not unreasonable to be editing the
> > earlier part of a file while the later part of it is still streaming in.
> > 
> > But damn, it now occurs to me that there's still a problem at the
> > streaming end: its file write offset won't be updated to reflect
> > the collapse, so there would be a sparse hole at that end.  And
> > collapse returns -EPERM if IS_APPEND(inode).
> 
> Well, we figure that most applications won't be using append only
> inode flags for files that they know they want to edit at random
> offsets later on. ;)
> 
> However, I can see how DVR apps would use open(O_APPEND) to obtain
> the fd they write to because that sets the write position to the EOF
> on every write() call (i.e. in generic_write_checks()). And collapse
> range should behave sanely with this sort of usage.
> 
> e.g. XFS calls generic_write_checks() after it has taken the IO lock
> to set the current write position to EOF. Hence it will be correctly
> serialised against collapse range calls and so O_APPEND writes will
> not leave sparse holes if collapse range calls are interleaved with
> the write stream....

Right, I was getting confused between O_APPEND and APPEND_Only!
Thanks, I'm back to being convinced by your example.

Hugh