Re: [PATCHv2 3/3] block: relax direct io memory alignment

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On Wed, May 18, 2022 at 05:14:49PM -0700, Eric Biggers wrote:
> On Wed, May 18, 2022 at 10:11:31AM -0700, Keith Busch wrote:
> > diff --git a/block/fops.c b/block/fops.c
> > index b9b83030e0df..d8537c29602f 100644
> > --- a/block/fops.c
> > +++ b/block/fops.c
> > @@ -54,8 +54,9 @@ static ssize_t __blkdev_direct_IO_simple(struct kiocb *iocb,
> >  	struct bio bio;
> >  	ssize_t ret;
> >  
> > -	if ((pos | iov_iter_alignment(iter)) &
> > -	    (bdev_logical_block_size(bdev) - 1))
> > +	if ((pos | iov_iter_count(iter)) & (bdev_logical_block_size(bdev) - 1))
> > +		return -EINVAL;
> > +	if (iov_iter_alignment(iter) & bdev_dma_alignment(bdev))
> >  		return -EINVAL;
> 
> The block layer makes a lot of assumptions that bios can be split at any bvec
> boundary.  With this patch, bios whose length isn't a multiple of the logical
> block size can be generated by splitting, which isn't valid.

How? This patch ensures every segment is block size aligned. We can always
split a bio in the middle of a bvec for any lower level hardware constraints,
and I'm not finding any splitting criteria that would try to break a bio on a
non-block aligned boundary.

> Also some devices aren't compatible with logical blocks spanning bdevs at all.
> dm-crypt errors out in this case, for example.

I'm sorry, but I am not understanding this.



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