block: support zone append bvecs

Christoph reported that we'll likely trigger the WARN_ON_ONCE() checking
that we're not submitting a bvec with REQ_OP_ZONE_APPEND in
bio_iov_iter_get_pages() some time ago using zoned btrfs, but I couldn't
reproduce it back then.

Now Naohiro was able to trigger the bug as well with xfstests generic/095
on a zoned btrfs.

There is nothing that prevents bvec submissions via REQ_OP_ZONE_APPEND if
the hardware's zone append limit is met.

Reported-by: Naohiro Aota <naohiro.aota@wdc.com>
Reported-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/10bd414d9326c90cd69029077db63b363854eee5.1616600835.git.johannes.thumshirn@wdc.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>

block/bio.c

@@ -949,7 +949,7 @@ void bio_release_pages(struct bio *bio, bool mark_dirty)
 }
 EXPORT_SYMBOL_GPL(bio_release_pages);
 
-static int bio_iov_bvec_set(struct bio *bio, struct iov_iter *iter)
+static void __bio_iov_bvec_set(struct bio *bio, struct iov_iter *iter)
 {
 	WARN_ON_ONCE(bio->bi_max_vecs);
 
@@ -959,11 +959,26 @@ static int bio_iov_bvec_set(struct bio *bio, struct iov_iter *iter)
 	bio->bi_iter.bi_size = iter->count;
 	bio_set_flag(bio, BIO_NO_PAGE_REF);
 	bio_set_flag(bio, BIO_CLONED);
+}
 
+static int bio_iov_bvec_set(struct bio *bio, struct iov_iter *iter)
+{
+	__bio_iov_bvec_set(bio, iter);
 	iov_iter_advance(iter, iter->count);
 	return 0;
 }
 
+static int bio_iov_bvec_set_append(struct bio *bio, struct iov_iter *iter)
+{
+	struct request_queue *q = bio->bi_bdev->bd_disk->queue;
+	struct iov_iter i = *iter;
+
+	iov_iter_truncate(&i, queue_max_zone_append_sectors(q) << 9);
+	__bio_iov_bvec_set(bio, &i);
+	iov_iter_advance(iter, i.count);
+	return 0;
+}
+
 #define PAGE_PTRS_PER_BVEC     (sizeof(struct bio_vec) / sizeof(struct page *))
 
 /**
@@ -1094,8 +1109,8 @@ int bio_iov_iter_get_pages(struct bio *bio, struct iov_iter *iter)
 	int ret = 0;
 
 	if (iov_iter_is_bvec(iter)) {
-		if (WARN_ON_ONCE(bio_op(bio) == REQ_OP_ZONE_APPEND))
-			return -EINVAL;
+		if (bio_op(bio) == REQ_OP_ZONE_APPEND)
+			return bio_iov_bvec_set_append(bio, iter);
 		return bio_iov_bvec_set(bio, iter);
 	}