On Thu, Jan 7, 2016 at 12:21 AM, Keith Busch <keith.busch@xxxxxxxxx> 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 -- 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
