On Thu, Jun 20, 2024 at 08:56:39AM -0600, Jens Axboe wrote:
> 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".
Oh, cool! https://man7.org/linux/man-pages/man2/open.2.html
doesn't know about this yet; is anyone working on updating it?
