Re: [PATCH v5 0/10] fs: Introduce new flag(FALLOC_FL_COLLAPSE_RANGE) for fallocate

Hugh Dickins
2014-02-27T01:31:36Z

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