Re: [PATCH 1/6] fs: add hole punching to fallocate

[Josef Bacik <josef@redhat.com>]

On Tue, Nov 09, 2010 at 12:12:22PM +1100, Dave Chinner wrote:
> On Mon, Nov 08, 2010 at 03:32:02PM -0500, Josef Bacik wrote:
> > Hole punching has already been implemented by XFS and OCFS2, and has the
> > potential to be implemented on both BTRFS and EXT4 so we need a generic way to
> > get to this feature.  The simplest way in my mind is to add FALLOC_FL_PUNCH_HOLE
> > to fallocate() since it already looks like the normal fallocate() operation.
> > I've tested this patch with XFS and BTRFS to make sure XFS did what it's
> > supposed to do and that BTRFS failed like it was supposed to.  Thank you,
> > 
> > Signed-off-by: Josef Bacik <josef@redhat.com>
> > ---
> >  fs/open.c              |    2 +-
> >  include/linux/falloc.h |    1 +
> >  2 files changed, 2 insertions(+), 1 deletions(-)
> > 
> > diff --git a/fs/open.c b/fs/open.c
> > index 4197b9e..ab8dedf 100644
> > --- a/fs/open.c
> > +++ b/fs/open.c
> > @@ -223,7 +223,7 @@ int do_fallocate(struct file *file, int mode, loff_t offset, loff_t len)
> >  		return -EINVAL;
> >  
> >  	/* Return error if mode is not supported */
> > -	if (mode && !(mode & FALLOC_FL_KEEP_SIZE))
> > +	if (mode && (mode & ~(FALLOC_FL_KEEP_SIZE | FALLOC_FL_PUNCH_HOLE)))
> >  		return -EOPNOTSUPP;
> >  
> >  	if (!(file->f_mode & FMODE_WRITE))
> > diff --git a/include/linux/falloc.h b/include/linux/falloc.h
> > index 3c15510..851cba2 100644
> > --- a/include/linux/falloc.h
> > +++ b/include/linux/falloc.h
> > @@ -2,6 +2,7 @@
> >  #define _FALLOC_H_
> >  
> >  #define FALLOC_FL_KEEP_SIZE	0x01 /* default is extend size */
> > +#define FALLOC_FL_PUNCH_HOLE	0X02 /* de-allocates range */
> 
> Hole punching was not included originally in fallocate() for a
> variety of reasons. IIRC, they were along the lines of:
> 
> 	1 de-allocating of blocks in an allocation syscall is wrong.
> 	  People wanted a new syscall for this functionality.
> 	2 no glibc interface needs it
> 	3 at the time, only XFS supported punching holes, so there
> 	  is not need to support it in a generic interface
> 	4 the use cases presented were not considered compelling
> 	  enough to justify the additional complexity (!)
> 
> In the end, I gave up arguing for it to be included because just
> getting the FALLOC_FL_KEEP_SIZE functionality was a hard enough
> battle.
> 
> Anyway, #3 isn't the case any more, #4 was just an excuse not to
> support anything ext4 couldn't do and lots of apps are calling
> fallocate directly (because glibc can't use FALLOC_FL_KEEP_SIZE) so
> #2 isn't an issue, either. I guess that leaves #1 to be debated;
> I don't think there is any problem with doing what you propose.
> 
> What I will suggest is that this requires a generic xfstest to be
> written and support added to xfs_io to enable that test (and others)
> to issue hole punches. Something along the lines of test 242 which I
> wrote for testing all the edge case of XFS_IOC_ZERO_RANGE (*) would be
> good.
> 

So this was relatively simple, adding a flag to falloc for xfs_io and such.  Got
a test going and it worked great on XFS.  Then I went to make sure it worked on
non-XFS, and thats where I've run into pain.  Turns out xfs_io -c "bmap" only
works on XFS.  So I thought to myself "well how hard could it be to make this
thing use fiemap?", hahaha I'm an idiot.  So I've been adding a xfs_io -c
"fiemap" that spits things out similar to bmap, and it will probably be tomorrow
when I finish it.

So good news is my simple patches seem to work just fine for hole-punch, bad
news is its going to take me another day to have all the infrastructure to test
it on non-XFS filesystems.

Also did you want me to rebase my patches on your fallocate() version of
ZERO_RANGE?  Thanks,

Josef