Re: [RESEND PATCHv3 1/2] dm-crypt: allow unaligned bio_vecs for direct io

[Keith Busch <kbusch@kernel.org>]

On Wed, Mar 18, 2026 at 05:19:39PM +0100, Mikulas Patocka wrote:
> On Mon, 16 Mar 2026, Keith Busch wrote:
> > +	if (!unaligned_allowed) {
> > +		cc->io_alignment = cc->sector_size - 1;
> > +	} else {
> > +		set_bit(CRYPT_DISCONTIGUOUS_SEGS, &cc->cipher_flags);
> > +		cc->io_alignment = 3;
> > +	}
> >  	return 0;
> >  }
> 
> Why is "3" there? Should there be the dma_alignment of the underlying 
> block device instead?

It doesn't matter what the underlying block device reports because
dm-crypt bounces the incoming plain text buffers to something aligned to
the backing device for the cypher text.

Why 3? This can theoretically do 0, but you can't express that as a
block limit; block layer will assume you didn't set anything and use a
511 default, which is exactly what I don't want. So why not 1 instead of
3? Three is the smallest non-zero cra_alignmask currently reported, and
its inefficient to do byte aligned direct io anyway.
 
> > --- a/drivers/md/dm-table.c
> > +++ b/drivers/md/dm-table.c
> > @@ -1767,6 +1767,7 @@ int dm_calculate_queue_limits(struct dm_table *t,
> >  	bool zoned = false;
> >  
> >  	dm_set_stacking_limits(limits);
> > +	limits->dma_alignment = 0;
> >  
> >  	t->integrity_supported = true;
> >  	for (unsigned int i = 0; i < t->num_targets; i++) {
> 
> dm-integrity doesn't set dma_alignment if it is using 512-byte sector size 
> (assuming that there is default 511). This should be fixed in dm-integrity 
> before making this change.
> 
> Other device mapper targets should also be reviewed to make sure that this 
> change doens't break them.

The previous call to dm_set_stacking_limits() sets it to 511, which sets
an unnecessary minimum. Setting it to 0 here means that the stacked
device will inhereit whatever the mapper overrides it to, or go back to
511 if it decides not to override it.