bio segment constraints

[Christoph Hellwig]

On Mon, Apr 07, 2025 at 02:46:32PM +0100, Keith Busch wrote:
> > > - Is is possible to have segments not even aligned to SECTOR_SIZE?
> > 
> > No.
> 
> O_DIRECT only requires each user iovec be a multiple of the logical
> block size with the address aligned to the dma_alignment. If the
> dma_alignment is smaller than the logical block size, then this could
> create bvec segments that are smaller. For nvme where we have 4-byte
> dma alignment, you could have the first segment be the last 4 bytes of a
> page, then the remaing 508 bytes from a different page in the next
> segment.

Oh, right - with a smaller dma alignment this can actually happen.