block: bio_iov_iter_get_pages: pin more pages for multi-segment IOs

bio_iov_iter_get_pages() currently only adds pages for the next non-zero
segment from the iov_iter to the bio. That's suboptimal for callers,
which typically try to pin as many pages as fit into the bio. This patch
converts the current bio_iov_iter_get_pages() into a static helper, and
introduces a new helper that allocates as many pages as

 1) fit into the bio,
 2) are present in the iov_iter,
 3) and can be pinned by MM.

Error is returned only if zero pages could be pinned. Because of 3), a
zero return value doesn't necessarily mean all pages have been pinned.
Callers that have to pin every page in the iov_iter must still call this
function in a loop (this is currently the case).

This change matters most for __blkdev_direct_IO_simple(), which calls
bio_iov_iter_get_pages() only once. If it obtains less pages than
requested, it returns a "short write" or "short read", and
__generic_file_write_iter() falls back to buffered writes, which may
lead to data corruption.

Fixes: 72ecad22d9f1 ("block: support a full bio worth of IO for simplified bdev direct-io")
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Martin Wilck <mwilck@suse.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

block/bio.c

@@ -903,14 +903,16 @@ int bio_add_page(struct bio *bio, struct page *page,
 EXPORT_SYMBOL(bio_add_page);
 
 /**
- * bio_iov_iter_get_pages - pin user or kernel pages and add them to a bio
+ * __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
+ * Pins pages from *iter and appends them to @bio's bvec array. The
  * pages will have to be released using put_page() when done.
+ * For multi-segment *iter, this function only adds pages from the
+ * the next non-empty segment of the iov iterator.
  */
-int bio_iov_iter_get_pages(struct bio *bio, struct iov_iter *iter)
+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;
 	struct bio_vec *bv = bio->bi_io_vec + bio->bi_vcnt;
@@ -947,6 +949,33 @@ int bio_iov_iter_get_pages(struct bio *bio, struct iov_iter *iter)
 	iov_iter_advance(iter, size);
 	return 0;
 }
+
+/**
+ * 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 pages from *iter and appends them to @bio's bvec array. The
+ * pages will have to be released using put_page() when done.
+ * The function tries, but does not guarantee, to pin as many pages as
+ * fit into the bio, or are requested in *iter, whatever is smaller.
+ * If MM encounters an error pinning the requested pages, it stops.
+ * Error is returned only if 0 pages could be pinned.
+ */
+int bio_iov_iter_get_pages(struct bio *bio, struct iov_iter *iter)
+{
+	unsigned short orig_vcnt = bio->bi_vcnt;
+
+	do {
+		int ret = __bio_iov_iter_get_pages(bio, iter);
+
+		if (unlikely(ret))
+			return bio->bi_vcnt > orig_vcnt ? 0 : ret;
+
+	} while (iov_iter_count(iter) && !bio_full(bio));
+
+	return 0;
+}
 EXPORT_SYMBOL_GPL(bio_iov_iter_get_pages);
 
 static void submit_bio_wait_endio(struct bio *bio)