Re: [PATCH 4/6] fs: Introduce FALLOC_FL_ZERO_RANGE flag for fallocate

["Lukáš Czerner" <lczerner@redhat.com>]

On Tue, 18 Feb 2014, Dave Chinner wrote:

> Date: Tue, 18 Feb 2014 13:51:12 +1100
> From: Dave Chinner <david@fromorbit.com>
> To: Lukas Czerner <lczerner@redhat.com>
> Cc: linux-ext4@vger.kernel.org, linux-fsdevel@vger.kernel.org, tytso@mit.edu,
>     xfs@oss.sgi.com
> Subject: Re: [PATCH 4/6] fs: Introduce FALLOC_FL_ZERO_RANGE flag for fallocate
> 
> On Mon, Feb 17, 2014 at 04:08:21PM +0100, Lukas Czerner wrote:
> > Introduce new FALLOC_FL_ZERO_RANGE flag for fallocate. This has the same
> > functionality as xfs ioctl XFS_IOC_ZERO_RANGE.
> > 
> > It can be used to convert a range of file to zeros preferably without
> > issuing data IO. Blocks should be preallocated for the regions that span
> > holes in the file, and the entire range is preferable converted to
> > unwritten extents - even though file system may choose to zero out the
> > extent or do whatever which will result in reading zeros from the range
> > while the range remains allocated for the file.
> > 
> > This can be also used to preallocate blocks past EOF in the same way as
> > with fallocate. Flag FALLOC_FL_KEEP_SIZE which should cause the inode
> > size to remain the same.
> > 
> > Signed-off-by: Lukas Czerner <lczerner@redhat.com>
> > ---
> >  fs/open.c                   | 7 ++++++-
> >  include/uapi/linux/falloc.h | 1 +
> >  2 files changed, 7 insertions(+), 1 deletion(-)
> > 
> > diff --git a/fs/open.c b/fs/open.c
> > index 4b3e1ed..6dc46c1 100644
> > --- a/fs/open.c
> > +++ b/fs/open.c
> > @@ -231,7 +231,12 @@ 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 & ~(FALLOC_FL_KEEP_SIZE | FALLOC_FL_PUNCH_HOLE))
> > +	if (mode & ~(FALLOC_FL_KEEP_SIZE | FALLOC_FL_PUNCH_HOLE |
> > +		     FALLOC_FL_ZERO_RANGE))
> > +		return -EOPNOTSUPP;
> > +
> > +	/* Punch hole and zero range are mutually exclusive */
> > +	if (mode & FALLOC_FL_PUNCH_HOLE && mode & FALLOC_FL_ZERO_RANGE)
> 
> I would have expected gcc to throw a warning on this. Even if it
> doesn't, it's so easy to mix up & an && and & it needs parenthesis
> around it to make it obvious what you actually meant and it doesn't
> have a && where an & should be or vice versa.  Better, IMO, is this:
> 
> 	/* Punch hole and zero range are mutually exclusive */
> 	if ((mode & (FALLOC_FL_PUNCH_HOLE | FALLOC_FL_ZERO_RANGE)) ==
> 		    (FALLOC_FL_PUNCH_HOLE | FALLOC_FL_ZERO_RANGE))
> 		return -EOPNOTSUPP;
> 
> because it's obvious what the intent is and easy to spot typos.

Fair enough, I'll change it.

Thanks!
-Lukas

> 
> Cheers,
> 
> Dave.
>