iomap_dio_actor(): fix iov_iter bugs

1) Ignoring return value from iov_iter_zero() is wrong
for iovec-backed case as well as for pipes - it can fail.

2) Failure to fault destination pages in 25Mb into a 50Mb iovec
should not act as if nothing in the area had been read, nevermind
that the first 25Mb might have *already* been read by that point.
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

fs/iomap.c

@@ -848,6 +848,7 @@ iomap_dio_actor(struct inode *inode, loff_t pos, loff_t length,
 	struct bio *bio;
 	bool need_zeroout = false;
 	int nr_pages, ret;
+	size_t copied = 0;
 
 	if ((pos | length | align) & ((1 << blkbits) - 1))
 		return -EINVAL;
@@ -859,7 +860,7 @@ iomap_dio_actor(struct inode *inode, loff_t pos, loff_t length,
 		/*FALLTHRU*/
 	case IOMAP_UNWRITTEN:
 		if (!(dio->flags & IOMAP_DIO_WRITE)) {
-			iov_iter_zero(length, dio->submit.iter);
+			length = iov_iter_zero(length, dio->submit.iter);
 			dio->size += length;
 			return length;
 		}
@@ -896,8 +897,11 @@ iomap_dio_actor(struct inode *inode, loff_t pos, loff_t length,
 	}
 
 	do {
-		if (dio->error)
+		size_t n;
+		if (dio->error) {
+			iov_iter_revert(dio->submit.iter, copied);
 			return 0;
+		}
 
 		bio = bio_alloc(GFP_KERNEL, nr_pages);
 		bio_set_dev(bio, iomap->bdev);
@@ -910,20 +914,24 @@ iomap_dio_actor(struct inode *inode, loff_t pos, loff_t length,
 		ret = bio_iov_iter_get_pages(bio, &iter);
 		if (unlikely(ret)) {
 			bio_put(bio);
-			return ret;
+			return copied ? copied : ret;
 		}
 
+		n = bio->bi_iter.bi_size;
 		if (dio->flags & IOMAP_DIO_WRITE) {
 			bio_set_op_attrs(bio, REQ_OP_WRITE, REQ_SYNC | REQ_IDLE);
-			task_io_account_write(bio->bi_iter.bi_size);
+			task_io_account_write(n);
 		} else {
 			bio_set_op_attrs(bio, REQ_OP_READ, 0);
 			if (dio->flags & IOMAP_DIO_DIRTY)
 				bio_set_pages_dirty(bio);
 		}
 
-		dio->size += bio->bi_iter.bi_size;
-		pos += bio->bi_iter.bi_size;
+		iov_iter_advance(dio->submit.iter, n);
+
+		dio->size += n;
+		pos += n;
+		copied += n;
 
 		nr_pages = iov_iter_npages(&iter, BIO_MAX_PAGES);
 
@@ -939,9 +947,7 @@ iomap_dio_actor(struct inode *inode, loff_t pos, loff_t length,
 		if (pad)
 			iomap_dio_zero(dio, iomap, pos, fs_block_size - pad);
 	}
-
-	iov_iter_advance(dio->submit.iter, length);
-	return length;
+	return copied;
 }
 
 ssize_t