Re: [PATCH RFC 12/16] xfs: Add support for fallocate2

[Dave Chinner <david@fromorbit.com>]

On Fri, May 05, 2023 at 03:23:33PM -0700, Darrick J. Wong wrote:
> On Thu, May 04, 2023 at 09:26:16AM +1000, Dave Chinner wrote:
> > On Wed, May 03, 2023 at 06:38:17PM +0000, John Garry wrote:
> > If we want fallocate() operations to apply filesystem atomic write
> > constraints to operations, then add a new modifier flag to
> > fallocate(), say FALLOC_FL_ATOMIC. The filesystem can then
> > look up it's atomic write alignment constraints and apply them to
> > the operation being performed appropriately.
> > 
> > > The alignment flag is not sticky, so further extent mutation will not
> > > obey this original alignment request.
> > 
> > IOWs, you want the specific allocation to behave exactly as if an
> > extent size hint of the given alignment had been set on that inode.
> > Which could be done with:
> > 
> > 	ioctl(FS_IOC_FSGETXATTR, &fsx)
> > 	old_extsize = fsx.fsx_extsize;
> > 	fsx.fsx_extsize = atomic_align_size;
> > 	ioctl(FS_IOC_FSSETXATTR, &fsx)
> 
> Eww, multiple threads doing fallocates can clobber each other here.

Sure, this was just an example of how the same behaviour could be be
acheived without the new ioctl. Locking and other trivialities were
left as an exercise for the reader.

> 
> > 	fallocate(....)
> > 	fsx.fsx_extsize = old_extsize;
> > 	ioctl(FS_IOC_FSSETXATTR, &fsx)
> 
> Also, you can't set extsize if the data fork has any mappings in it,
> so you can't set the old value.  But perhaps it's not so bad to expect
> that programs will set this up once and not change the underlying
> storage?
> 
> I'm not actually sure why you can't change the extent size hint.  Why is
> that?

Hysterical raisins, I think.

IIUC, it was largely to do with the fact that pre-existing
allocation could not be realigned, so once allocation has been done
the extent size hint can't guarantee extent size hint aligned/sized
extents are actually allocated for the file.

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com