block: use bio_add_page in bio_iov_iter_get_pages

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: NChristoph Hellwig <hch@lst.de>
Signed-off-by: NJens Axboe <axboe@kernel.dk>

block/bio.c

@@ -827,6 +827,8 @@ 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
@@ -839,38 +841,35 @@ EXPORT_SYMBOL(bio_add_page);
  */
 static int __bio_iov_iter_get_pages(struct bio *bio, struct iov_iter *iter)
 {
-	unsigned short nr_pages = bio->bi_max_vecs - bio->bi_vcnt, idx;
+	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;
+	ssize_t size, left;
+	unsigned len, i;
 	size_t offset;
-	ssize_t size;
+
+	/*
+	 * 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;
-	idx = 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;
+	for (left = size, i = 0; left > 0; left -= len, i++) {
+		struct page *page = pages[i];
 
-	while (idx--) {
-		bv[idx].bv_page = pages[idx];
-		bv[idx].bv_len = PAGE_SIZE;
-		bv[idx].bv_offset = 0;
+		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;
 	}
 
-	bv[0].bv_offset += offset;
-	bv[0].bv_len -= offset;
-	bv[nr_pages - 1].bv_len -= nr_pages * PAGE_SIZE - offset - size;
-
 	iov_iter_advance(iter, size);
 	return 0;
 }