Re: [PATCH V11 15/19] block: enable multipage bvecs

Ming Lei
2018-11-21T15:49:17Z

On Wed, Nov 21, 2018 at 03:55:02PM +0100, Christoph Hellwig wrote:
> On Wed, Nov 21, 2018 at 11:23:23AM +0800, Ming Lei wrote:
> > if (bio->bi_vcnt > 0) {
> > - struct bio_vec *bv = &bio->bi_io_vec[bio->bi_vcnt - 1];
> > + struct bio_vec bv;
> > + struct bio_vec *seg = &bio->bi_io_vec[bio->bi_vcnt - 1];
> >
> > - if (page == bv->bv_page && off == bv->bv_offset + bv->bv_len) {
> > - bv->bv_len += len;
> > + bvec_last_segment(seg, &bv);
> > +
> > + if (page == bv.bv_page && off == bv.bv_offset + bv.bv_len) {
>
> I think this we can simplify the try to merge into bio case a bit,
> and also document it better with something like this:
>
> diff --git a/block/bio.c b/block/bio.c
> index 854676edc438..cc913281a723 100644
> --- a/block/bio.c
> +++ b/block/bio.c
> @@ -822,54 +822,40 @@ EXPORT_SYMBOL(bio_add_pc_page);
> * @page: page to add
> * @len: length of the data to add
> * @off: offset of the data in @page
> + * @same_page: if %true only merge if the new data is in the same physical
> + * page as the last segment of the bio.
> *
> - * Try to add the data at @page + @off to the last page of @bio. This is a
> + * Try to add the data at @page + @off to the last bvec of @bio. This is a
> * a useful optimisation for file systems with a block size smaller than the
> * page size.
> *
> * Return %true on success or %false on failure.
> */
> bool __bio_try_merge_page(struct bio *bio, struct page *page,
> - unsigned int len, unsigned int off)
> + unsigned int len, unsigned int off, bool same_page)
> {
> if (WARN_ON_ONCE(bio_flagged(bio, BIO_CLONED)))
> return false;
>
> if (bio->bi_vcnt > 0) {
> - struct bio_vec bv;
> - struct bio_vec *seg = &bio->bi_io_vec[bio->bi_vcnt - 1];
> -
> - bvec_last_segment(seg, &bv);
> -
> - if (page == bv.bv_page && off == bv.bv_offset + bv.bv_len) {
> - seg->bv_len += len;
> - bio->bi_iter.bi_size += len;
> - return true;
> - }
> + struct bio_vec *bv = &bio->bi_io_vec[bio->bi_vcnt - 1];
> + phys_addr_t vec_addr = page_to_phys(bv->bv_page);
> + phys_addr_t page_addr = page_to_phys(page);
> +
> + if (vec_addr + bv->bv_offset + bv->bv_len != page_addr + off)
> + return false;
> + if (same_page &&
> + (vec_addr & PAGE_SIZE) != (page_addr & PAGE_SIZE))
> + return false;

I guess the correct check should be:

end_addr = vec_addr + bv->bv_offset + bv->bv_len;
if (same_page &&
(end_addr & PAGE_MASK) != (page_addr & PAGE_MASK))
return false;

And this approach is good, will take it in V12.

Thanks,
Ming