On Fri, Aug 07, 2020 at 03:11:48PM +0100, Matthew Wilcox wrote: > On Fri, Aug 07, 2020 at 09:41:14PM +0800, Ming Lei wrote: > > On Fri, Aug 07, 2020 at 01:38:54PM +0100, Al Viro wrote: > > > FWIW, my preference would be to have for_each_bvec() advance past zero-length > > > segments; I'll need to go through its uses elsewhere in the tree first, though > > > (after I grab some sleep), > > > > Usually block layer doesn't allow/support zero bvec, however we can make > > for_each_bvec() to support it only. > > > > Tetsuo, can you try the following patch? > > > > diff --git a/include/linux/bvec.h b/include/linux/bvec.h > > index ac0c7299d5b8..b03c793dd28d 100644 > > --- a/include/linux/bvec.h > > +++ b/include/linux/bvec.h > > @@ -117,11 +117,19 @@ static inline bool bvec_iter_advance(const struct bio_vec *bv, > > return true; > > } > > > > +static inline void bvec_iter_skip_zero_vec(const struct bio_vec *bv, > > + struct bvec_iter *iter) > > +{ > > + iter->bi_idx++; > > + iter->bi_bvec_done = 0; > > +} > > + > > #define for_each_bvec(bvl, bio_vec, iter, start) \ > > for (iter = (start); \ > > (iter).bi_size && \ > > - ((bvl = bvec_iter_bvec((bio_vec), (iter))), 1); \ > > - bvec_iter_advance((bio_vec), &(iter), (bvl).bv_len)) > > + ((bvl = bvec_iter_bvec((bio_vec), (iter))), 1); \ > > + (bvl).bv_len ? bvec_iter_advance((bio_vec), &(iter), (bvl).bv_len) : \ > > + bvec_iter_skip_zero_vec((bio_vec), &(iter))) > > Uhm, bvec_iter_advance() already skips over zero length bio_vecs. > > while (bytes && bytes >= bv[idx].bv_len) { > bytes -= bv[idx].bv_len; > idx++; > } The issue is that zero (bvl).bv_len passed to bvec_iter_advance(), so the iterator can't move on. And I tried to avoid change to bvec_iter_advance() since this exact issue only exists on for_each_bvec, and block layer won't support/allow zero-length bvec. > > The problem is when the _first_ bio_vec is zero length. It can be any zero-length bvec during the iterating. > Maybe something more > like this (which doesn't even compile, but hopefully makes my point): > > @@ -86,12 +86,24 @@ struct bvec_iter_all { > (mp_bvec_iter_page((bvec), (iter)) + \ > mp_bvec_iter_page_idx((bvec), (iter))) > > -#define bvec_iter_bvec(bvec, iter) \ > -((struct bio_vec) { \ > - .bv_page = bvec_iter_page((bvec), (iter)), \ > - .bv_len = bvec_iter_len((bvec), (iter)), \ > - .bv_offset = bvec_iter_offset((bvec), (iter)), \ > -}) > +static inline bool bvec_iter_bvec(struct bio_vec *bv, struct bio_vec *bvec, > + struct bvec_iter *iter) > +{ > + unsigned int idx = iter->bi_idx; > + > + if (!iter->bi_size) > + return false; > + > + while (!bv[idx].bv_len) > + idx++; > + iter->bi_idx = idx; > + > + bv->bv_page = bvec_iter_page(bvec, *iter); > + bv->bv_len = bvec_iter_len(bvec, *iter); > + bv->bv_offset = bvec_iter_offset(bvec, *iter); > + > + return true; > +} > > static inline bool bvec_iter_advance(const struct bio_vec *bv, > struct bvec_iter *iter, unsigned bytes) > @@ -119,8 +131,7 @@ static inline bool bvec_iter_advance(const struct bio_vec *bv, > > #define for_each_bvec(bvl, bio_vec, iter, start) \ > for (iter = (start); \ > - (iter).bi_size && \ > - ((bvl = bvec_iter_bvec((bio_vec), (iter))), 1); \ > + bvec_iter_bvec(&(bvl), (bio_vec), &(iter)); \ > bvec_iter_advance((bio_vec), &(iter), (bvl).bv_len)) > > /* for iterating one bio from start to end */ > > (I find the whole bvec handling a mess of confusing macros and would > welcome more of it being inline functions, in general). The above change may bring more code duplication. Meantime, it can't work because (bvl).bv_len isn't taken into account into bvec_iter_bvec(), then how can the iterator advance? Thanks, Ming