On 6/20/24 8:49 AM, Matthew Wilcox wrote:

On Thu, Jun 20, 2024 at 10:16:02AM -0400, Kent Overstreet wrote:
I'm more sympathetic to "lets relax the alignment requirements", since
most IO devices actually can do IO to arbitrary boundaries (or at least
reasonable boundaries, eg cacheline alignment or 4-byte alignment).
The 512 byte alignment doesn't seem particularly rooted in any hardware
restrictions.

We already did, based on real world use cases to avoid copies just
because the memory wasn't aligned on a sector size boundary. It's
perfectly valid now to do:

struct queue_limits lim {
	.dma_alignment = 3,
};

disk = blk_mq_alloc_disk(&tag_set, &lim, NULL);

and have O_DIRECT with a 32-bit memory alignment work just fine, where
before it would EINVAL. The sector size memory alignment thing has
always been odd and never rooted in anything other than "oh let's just
require the whole combination of size/disk offset/alignment to be sector
based".

But size?  Fundamentally, we're asking the device to do IO directly to
this userspace address.  That means you get to do the entire IO, not
just the part of it that you want.  I know some devices have bitbucket
descriptors, but many don't.

We did poke at that a bit for nvme with bitbuckets, but I don't even
know how prevalent that support is in hardware. Definitely way iffier
and spotty than the alignment, where that limit was never based on
anything remotely resembling a hardware restraint.

quoted

quoted

I'm against it.  Block devices only do sector-aligned IO and we should
not pretend otherwise.

Eh?

bio isn't really specific to the block layer anyways, given that an
iov_iter can be a bio underneath. We _really_ should be trying for
better commonality of data structures.

bio is absolutely specific to the block layer.  Look at it:

It's literally "block IO", so would have to concur with that.

-- 
Jens Axboe