block: add bio_iov_iter_get_pages()

This is a helper that pins down a range from an iov_iter and adds it to
a bio without requiring a separate memory allocation for the page array.
It will be used for upcoming direct I/O implementations for block devices
and iomap based file systems.

Signed-off-by: Kent Overstreet <kent.overstreet@gmail.com>
[hch: ported to the iov_iter interface, renamed and added comments.
      All blame should be directed to me and all fame should go to Kent
      after this!]
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@fb.com>

2 files changed, 50 insertions, 0 deletions

diff --git a/block/bio.c b/block/bio.c
index db85c5753a76..2cf6ebabc68c 100644
--- a/block/bio.c
+++ b/block/bio.c
@@ -847,6 +847,55 @@ done:
 }
 EXPORT_SYMBOL(bio_add_page);
 
+/**
+ * bio_iov_iter_get_pages - pin user or kernel pages and add them to a bio
+ * @bio: bio to add pages to
+ * @iter: iov iterator describing the region to be mapped
+ *
+ * Pins as many pages from *iter and appends them to @bio's bvec array. The
+ * pages will have to be released using put_page() when done.
+ */
+int bio_iov_iter_get_pages(struct bio *bio, struct iov_iter *iter)
+{
+        unsigned short nr_pages = 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;
+
+        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;
+        }
+
+        bv[0].bv_offset += offset;
+        bv[0].bv_len -= offset;
+        if (diff)
+                bv[bio->bi_vcnt - 1].bv_len -= diff;
+
+        iov_iter_advance(iter, size);
+        return 0;
+}
+EXPORT_SYMBOL_GPL(bio_iov_iter_get_pages);
+
 struct submit_bio_ret {
         struct completion event;
         int error;
diff --git a/include/linux/bio.h b/include/linux/bio.h
index 5c604b4914bf..d367cd37a7f7 100644
--- a/include/linux/bio.h
+++ b/include/linux/bio.h
@@ -427,6 +427,7 @@ void bio_chain(struct bio *, struct bio *);
 extern int bio_add_page(struct bio *, struct page *, unsigned int,unsigned int);
 extern int bio_add_pc_page(struct request_queue *, struct bio *, struct page *,
                            unsigned int, unsigned int);
+int bio_iov_iter_get_pages(struct bio *bio, struct iov_iter *iter);
 struct rq_map_data;
 extern struct bio *bio_map_user_iov(struct request_queue *,
                                     const struct iov_iter *, gfp_t);