On 2022/05/19 6:56, Keith Busch wrote:
> On Wed, May 18, 2022 at 08:27:31PM -0700, Eric Biggers wrote:
>>
>> So the bio ends up with a total length that is a multiple of the logical block
>> size, but the lengths of the individual bvecs in the bio are *not* necessarily
>> multiples of the logical block size. That's the problem.
>
> I'm surely missing something here. I know the bvecs are not necessarily lbs
> aligned, but why does that matter? Is there some driver that can only take
> exactly 1 bvec, but allows it to be unaligned? If so, we could take the segment
> queue limit into account, but I am not sure that we need to.
For direct IO, the first bvec will always be aligned to a logical block size.
See __blkdev_direct_IO() and __blkdev_direct_IO_simple():
if ((pos | iov_iter_alignment(iter)) &
(bdev_logical_block_size(bdev) - 1))
return -EINVAL;
And given that, all bvecs should also be LBA aligned since the LBA size is
always a divisor of the page size. Since splitting is always done on an LBA
boundary, I do not see how we can ever get bvecs that are not LBA aligned.
Or I am missing something too...
>
>> Note, there's also lots of code that assumes that bio_vec::bv_len is a multiple
>> of 512.
>
> Could you point me to some examples?
>
>> That was implied by it being a multiple of the logical block size. But
>> the DMA alignment can be much lower, like 8 bytes (see nvme_set_queue_limits()).
>
> That's the driver this was tested on, though I just changed it to 4 bytes for
> 5.19.
--
Damien Le Moal
Western Digital Research
