Re: [PATCH 4/4] xfs: rearrange xfs_da_args a bit to use less space

Darrick J. Wong
2024-04-10T20:56:54Z

On Tue, Apr 09, 2024 at 10:02:45PM -0700, Christoph Hellwig wrote:
> On Tue, Apr 09, 2024 at 05:50:23PM -0700, Darrick J. Wong wrote:
> > From: Darrick J. Wong <djwong@kernel.org>
> >
> > A few notes about struct xfs_da_args:
> >
> > The XFS_ATTR_* flags only go up as far as XFS_ATTR_INCOMPLETE, which
> > means that attr_filter could be a u8 field.
> >
> > The XATTR_* flags only have two values, which means that xattr_flags
> > could be shrunk to a u8.
> >
> > I've reduced the number of XFS_DA_OP_* flags down to the point where
> > op_flags would also fit into a u8.
> >
> > filetype has 7 bytes of slack after it, which is wasteful.
> >
> > namelen will never be greater than MAXNAMELEN, which is 256. This field
> > could be reduced to a short.
> >
> > Rearrange the fields in xfs_da_args to waste less space. This reduces
> > the structure size from 136 bytes to 128. Later when we add extra
> > fields to support parent pointer replacement, this will only bloat the
> > structure to 144 bytes, instead of 168.
>
> Looks good:
>
> Reviewed-by: Christoph Hellwig <hch@lst.de>
>
> Eventually we should probaly split this up, at lot of fields are
> used only by the attr set code, and a few less only by dir vs attr.

Agreed, though we're veering dangerously close to object inheritance.

But it would be useful for code analysis if dir operations would pass in
an xfs_dir_op structure containing a much smaller xfs_da_args and
likewise for xattrs.

--D