Re: [PATCH 1/4] iomap: Lift blocksize restriction on atomic writes

[John Garry]

On 17/01/2025 18:29, Darrick J. Wong wrote:
> > > Ok, let's do that then.  Just to be clear -- for any RWF_ATOMIC direct
> > > write that's correctly aligned and targets a single mapping in the
> > > correct state, we can build the untorn bio and submit it.  For
> > > everything else, prealloc some post EOF blocks, write them there, and
> > > exchange-range them.
> > I have some doubt about this, but I may be misunderstanding the concept:
> >
> > So is there any guarantee that what we write into is aligned (after the
> > exchange-range routine)? If not, surely every subsequent write with
> > RWF_ATOMIC to that logical range will require this exchange-range routine
> > until we get something aligned (and correct granularity) - correct?
> Correct, you'd still need forcealign to make sure that the new
> allocations for exchange-range are aligned to awumin.

Sure, but I think that if we align sb_agblocks to the bdev atomic write 
limits at mkfs time and also set extszhint appropriately, then 
probability of desired alignment and granularity is appreciably higher.

Thanks,
John