On Tue, Jun 05, 2018 at 11:41:19AM +0800, Ming Lei wrote:
> On Mon, Jun 04, 2018 at 03:58:53PM +0200, Christoph Hellwig wrote:
> > Replace a nasty hack with a different nasty hack to prepare for multipage
> > bio_vecs. By moving the temporary page array as far up as possible in
> > the space allocated for the bio_vec array we can iterate forward over it
> > and thus use bio_add_page. Using bio_add_page means we'll be able to
> > merge physically contiguous pages once support for multipath bio_vecs is
> > merged.
> >
> > Signed-off-by: Christoph Hellwig <hch@xxxxxx>
> > ---
> > block/bio.c | 45 +++++++++++++++++++++------------------------
> > 1 file changed, 21 insertions(+), 24 deletions(-)
> >
> > diff --git a/block/bio.c b/block/bio.c
> > index d95fab72acb5..b09c133a1e24 100644
> > --- a/block/bio.c
> > +++ b/block/bio.c
> > @@ -871,6 +871,8 @@ int bio_add_page(struct bio *bio, struct page *page,
> > }
> > EXPORT_SYMBOL(bio_add_page);
> >
> > +#define PAGE_PTRS_PER_BVEC (sizeof(struct bio_vec) / sizeof(struct page *))
> > +
> > /**
> > * bio_iov_iter_get_pages - pin user or kernel pages and add them to a bio
> > * @bio: bio to add pages to
> > @@ -882,38 +884,33 @@ EXPORT_SYMBOL(bio_add_page);
> > int bio_iov_iter_get_pages(struct bio *bio, struct iov_iter *iter)
> > {
> > unsigned short nr_pages = bio->bi_max_vecs - bio->bi_vcnt;
> > + unsigned short entries_left = bio->bi_max_vecs - bio->bi_vcnt;
> > struct bio_vec *bv = bio->bi_io_vec + bio->bi_vcnt;
> > struct page **pages = (struct page **)bv;
> > - size_t offset, diff;
> > - ssize_t size;
> > + ssize_t size, left;
> > + unsigned len, i;
> > + size_t offset;
> > +
> > + /*
> > + * Move page array up in the allocated memory for the bio vecs as
> > + * far as possible so that we can start filling biovecs from the
> > + * beginning without overwriting the temporary page array.
> > + */
> > + BUILD_BUG_ON(PAGE_PTRS_PER_BVEC < 2);
> > + pages += entries_left * (PAGE_PTRS_PER_BVEC - 1);
> >
> > size = iov_iter_get_pages(iter, pages, LONG_MAX, nr_pages, &offset);
> > if (unlikely(size <= 0))
> > return size ? size : -EFAULT;
> > - nr_pages = (size + offset + PAGE_SIZE - 1) / PAGE_SIZE;
> >
> > - /*
> > - * Deep magic below: We need to walk the pinned pages backwards
> > - * because we are abusing the space allocated for the bio_vecs
> > - * for the page array. Because the bio_vecs are larger than the
> > - * page pointers by definition this will always work. But it also
> > - * means we can't use bio_add_page, so any changes to it's semantics
> > - * need to be reflected here as well.
> > - */
> > - bio->bi_iter.bi_size += size;
> > - bio->bi_vcnt += nr_pages;
> > -
> > - diff = (nr_pages * PAGE_SIZE - offset) - size;
> > - while (nr_pages--) {
> > - bv[nr_pages].bv_page = pages[nr_pages];
> > - bv[nr_pages].bv_len = PAGE_SIZE;
> > - bv[nr_pages].bv_offset = 0;
> > - }
> > + for (left = size, i = 0; left > 0; left -= len, i++) {
> > + struct page *page = pages[i];
> >
> > - bv[0].bv_offset += offset;
> > - bv[0].bv_len -= offset;
> > - if (diff)
> > - bv[bio->bi_vcnt - 1].bv_len -= diff;
> > + len = min_t(size_t, PAGE_SIZE - offset, left);
> > + if (WARN_ON_ONCE(bio_add_page(bio, page, len, offset) != len))
> > + return -EINVAL;
> > + offset = 0;
> > + }
>
> One invariant is that the page index 'i' is always <= bvec index
oops, the above "<=" should have been ">=".
Thanks,
Ming
