Re: [PATCH] xfs: speed up parent pointer operations

["Darrick J. Wong" <djwong@kernel.org>]

On Thu, Jan 08, 2026 at 01:30:30AM -0800, Christoph Hellwig wrote:
> On Wed, Jan 07, 2026 at 10:22:42AM -0800, Darrick J. Wong wrote:
> > > > xattr mechanism.
> > > 
> > > Or that, yes.
> > 
> > It turns out that for replace, it's more convenient to do it separately
> > in the xattr and parent pointer code because parent pointer replacements
> > require switching new_name -> name and new_value -> value after the
> > remove step; and the rename optimization is different for parent
> > pointers vs. every other xattr.
> 
> I don't really follow, but I trust you on the parent pointers.
> 
> > > for parent pointers, or due to a value change otherwise can just move
> > > things beyond the attribute and update in place trivially.  For
> > > replacing values with values of the same size things are even simpler.
> > 
> > Yes it is pretty simple:
> 
> This is the same sized value and name, and it indeed is trivial.
> 
> But even a change in size of the value (or name for that matter, except
> that outside of parent pointers that operation doesn't exist) is trivial
> in an sf attr fork with enough space, you just need to either move out
> anything beyond the attr first (larger name + value size) or down after
> (smaller large and value size).

<nod> Though if it's not an exact size match, then calling _sf_remove
and _sf_add already does that -- the _sf_remove will compact the entire
sf attr structure before _sf_add puts in the new attr.

The downsides of that sequence is that while we could probably save a
trip through xfs_idata_realloc, then I have to write and QA new code.
So for now I'd rather focus on optimizing xattr replace where the old
and new size are the same.

> > IIRC there's no rounding applied to shortform attr entries, so we have
> > to have an exact match on the value length.
> 
> Exactly.
> 
> > > > I also wonder how much benefit anyone really gets from doing this to
> > > > regular xattrs, but once I'm more convinced that it's solid w.r.t.
> > > > parent pointers it's trivial to move it to xattrs too.
> > > 
> > > Not sure what counts as regular, but I'm pretty sure it would help
> > > quite a bit for inheriting xattrs or other security attributes.
> > 
> > Here, by "regular" I meant "not parent pointers" but yeah.  It'll
> > probably help everyone to implement the shortcuts.
> 
> For user attributes it really depends on how people use it.  But for
> applications that update fixed sized attributes, which doesn't sound too
> unusual, it would be a nice improvement as well.

<nod>

--D