[PATCH V6 03/30] block: use bio_add_page in bio_iov_iter_get_pages

Ming Lei
2018-06-09T12:31:21Z

From: Christoph Hellwig <hch@lst.de>

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.

Reviewed-by: Ming Lei <ming.lei@redhat.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
---
block/bio.c | 45 +++++++++++++++++++++------------------------
1 file changed, 21 insertions(+), 24 deletions(-)

diff --git a/block/bio.c b/block/bio.c
index ebd3ca62e037..cb0f46e2752b 100644
--- a/block/bio.c
+++ b/block/bio.c
@@ -902,6 +902,8 @@ 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
@@ -913,38 +915,33 @@ 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;
+ }

iov_iter_advance(iter, size);
return 0;
--
2.9.5