Re: [PATCH 4/7] xfs: Implement fallocate query support mode

[Lukas Czerner <lczerner@redhat.com>]

On Mon, Sep 18, 2017 at 01:48:38PM -0700, Darrick J. Wong wrote:
> On Mon, Sep 18, 2017 at 05:52:24PM +0200, Lukas Czerner wrote:
> > Return all fallcoate modes supported by xfs file system.
> > 
> > Cc: linux-xfs@vger.kernel.org
> > Signed-off-by: Lukas Czerner <lczerner@redhat.com>
> > ---
> >  fs/xfs/xfs_file.c | 6 +++++-
> >  1 file changed, 5 insertions(+), 1 deletion(-)
> > 
> > diff --git a/fs/xfs/xfs_file.c b/fs/xfs/xfs_file.c
> > index ebdd0bd..85e06c6 100644
> > --- a/fs/xfs/xfs_file.c
> > +++ b/fs/xfs/xfs_file.c
> > @@ -746,7 +746,8 @@ xfs_file_write_iter(
> >  #define	XFS_FALLOC_FL_SUPPORTED						\
> >  		(FALLOC_FL_KEEP_SIZE | FALLOC_FL_PUNCH_HOLE |		\
> >  		 FALLOC_FL_COLLAPSE_RANGE | FALLOC_FL_ZERO_RANGE |	\
> > -		 FALLOC_FL_INSERT_RANGE | FALLOC_FL_UNSHARE_RANGE)
> > +		 FALLOC_FL_INSERT_RANGE | FALLOC_FL_UNSHARE_RANGE |	\
> > +		 FALLOC_FL_QUERY_SUPPORT | FALLOC_FL_PREALLOC_RANGE)
> >  
> >  STATIC long
> >  xfs_file_fallocate(
> > @@ -768,6 +769,9 @@ xfs_file_fallocate(
> >  	if (mode & ~XFS_FALLOC_FL_SUPPORTED)
> >  		return -EOPNOTSUPP;
> >  
> > +	if (mode & FALLOC_FL_QUERY_SUPPORT)
> > +		return XFS_FALLOC_FL_SUPPORTED;
> 
> Urk, manpage update describing the goals of the query interface and how
> this is supposed to work is needed.

See linux-fsdevel for entire patch set. It's RFC, manpage update will
come later.
https://www.spinics.net/lists/linux-fsdevel/msg115740.html

> 
> Are we required to return only the mode flags that would reasonably be
> expected to work on this file, or the fs in general?  Do we return zero

Yes, as mentioned in cover letter it's supposed to return flags that
would be expected to work on this file.

> if the file is immutable (I guess the fd has to be O_RDONLY?) or if the
> fs is readonly?

It's going through the same path as any fallocate call.

> 
> And like hch said, why not {f,}pathconf?

I do not think fpathconf is suitable for this use case, see my reply to hch
and reply form Andreas.

-Lukas

> 
> --D
> 
> > +
> >  	xfs_ilock(ip, iolock);
> >  	error = xfs_break_layouts(inode, &iolock);
> >  	if (error)
> > -- 
> > 2.7.5
> > 
> > --
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