Re: [PATCH v5 07/10] xfs: Commit CoW-based atomic writes atomically

[John Garry]

On 12/03/2025 07:39, Christoph Hellwig wrote:
> On Mon, Mar 10, 2025 at 06:39:43PM +0000, John Garry wrote:
> > When completing a CoW-based write, each extent range mapping update is
> > covered by a separate transaction.
> >
> > For a CoW-based atomic write, all mappings must be changed at once, so
> > change to use a single transaction.
>
> As already mentioned in a previous reply:  "all" might be to much.
> The code can only support a (relatively low) number of extents
> in a single transaction safely.

Then we would need to limit the awu max to whatever can be guaranteed
(to fit).


> > +int
> > +xfs_reflink_end_atomic_cow(
> > +	struct xfs_inode		*ip,
> > +	xfs_off_t			offset,
> > +	xfs_off_t			count)
>
> Assuming we could actually to the multi extent per transaction
> commit safely, what would be the reason to not always do it?

Yes, I suppose that it could always be used. I would suggest that as a 
later improvement, if you agree.

Thanks,
John