On 6/20/24 9:15 AM, Matthew Wilcox wrote:
> 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?
Probably not... At least we do have STATX_DIOALIGN which can be used to
figure out what the alignment is, but I don't recall if any man date
updates got done. Keith may remember, CC'ed.
--
Jens Axboe
