[PATCH for-4.4] block: split bios to max possible length

[Ming Lei]

On Thu, Jan 7, 2016 at 12:21 AM, Keith Busch <keith.busch at intel.com> wrote:
> On Wed, Jan 06, 2016 at 11:43:45PM +0800, Ming Lei wrote:
>> Please see the 1st line code of __blk_segment_map_sg(), in which only
>> one whole bvec is handled, and partial bvec can't be figured out there.
>>
>> Think of it further, drivers often use bv.bv_len directly in the
>> iterator, for example:
>>
>>         bio_for_each_segment(bvec, bio, iter)
>>                 memcpy(page_address(bvec.bv_page) +
>>                                             bvec.bv_offset,  addr +
>> offset, bvec.bv_len);
>>
>> So your patch will break these drivers, won't it?
>
> CC'ing Kent in hopes he will clarify what happens on a split.
>
> The bio_advance() code comments say it's handled:
>
> "
>  * This updates bi_sector, bi_size and bi_idx; if the number of bytes to
>  * complete doesn't align with a bvec boundary, then bv_len and bv_offset will
>  * be updated on the last bvec as well.
> "

One exception is that both can be reseted in bio_clone_bioset().

>
> I admit I'm having a hard time seeing where bv_len and bv_offset updated
> in this path. It was obviously handled after 054bdf646e then changed
> with 4550dd6c6b.
>
> If I follow correctly, 4550dd6c6b will implicity update the bvec's offset
> and length during the split here since bio_iter_iovec resets the bvec's
> length and offset:
> ---
> #define __bio_for_each_segment(bvl, bio, iter, start)                   \
>         for (iter = (start);                                            \
>              (iter).bi_size &&                                          \
>                 ((bvl = bio_iter_iovec((bio), (iter))), 1);             \
>              bio_advance_iter((bio), &(iter), (bvl).bv_len))
> --



-- 
Ming Lei