Re: [PATCH v7 11/14] xfs: add xfs_file_dio_write_atomic()

[Luis Chamberlain <mcgrof@kernel.org>]

On Tue, Apr 22, 2025 at 07:08:32AM +0100, John Garry wrote:
> On 21/04/2025 22:18, Luis Chamberlain wrote:
> > > /*
> > > +	 * The retry mechanism is based on the ->iomap_begin method returning
> > > +	 * -ENOPROTOOPT, which would be when the REQ_ATOMIC-based write is not
> > > +	 * possible. The REQ_ATOMIC-based method typically not be possible if
> > > +	 * the write spans multiple extents or the disk blocks are misaligned.
> > > +	 */
> > > +	if (ret == -ENOPROTOOPT && dops == &xfs_direct_write_iomap_ops) {
> > Based on feedback from LSFMM, due to the performance variaibility this
> > can introduce, it sounded like some folks would like to opt-in to not
> > have a software fallback and just require an error out.
> > > Could an option be added to not allow the software fallback?
> 
> I still don't see the use in this.

Its not the use, its the concern for underdeterminism in performance.

> So consider userspace wants to write something atomically and we fail as a
> HW-based atomic write is not possible.

Sounds like a terrible predicant for those that want hw atomics and
reliability for it.

> What is userspace going to do next?

It would seem that would depend on their analysis on the number of
software fallbacks where a software atomic based solution is used and
the impact on performance.

> I heard something like "if HW-based atomics are not possible, then something
> has not been configured properly for the FS" - that something would be
> extent granularity and alignment, but we don't have a method to ensure this.
> That is the whole point of having a FS fallback.

We do with LBS. Its perfectly deterministic to be aligned with a sector
size matching the block size, even for metadata writes.

> > If so, then I think the next patch would also need updating.
> > 
> > Or are you suggesting that without the software fallback atomic writes
> > greater than fs block size are not possible?
> 
> Yes, as XFS has no method to guarantee extent granularity and alignment.

Ah, I think the documentation for this featuer should make this clear,
it was not clear up to this point in patch review.

  Luis