On Wed, Jan 06, 2016 at 04:21:17PM +0000, Keith Busch 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.
> "
>
> 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))
> --
Yes, splitting in the middle of a bvec is perfectly fine. The reason
bio_for_each_segment takes a struct bvec and not a struct bvec * is because it's
computing what bv_len should be (taking the min of bv_len and bi_size, roughly).
See include/linux/bio.h:
bio_for_each_segment()
bio_iter_iovec()
bvec_iter_bvec()
bvec_iter_len()
which does the actual bv_len computation.
--
To unsubscribe from this list: send the line "unsubscribe linux-block" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
