For the incoming usage of IOMAP_DIO_BOUNCE in btrfs, btrfs has set
iov_iter::nofault to prevent deadlock when a page fault is needed to
read out the buffer.

However bio_iov_iter_bounce_write() doesn't respect iov_iter::nofault
flag, and just call a plain copy_from_iter() so it can still trigger
page fault and cause deadlock in btrfs.

Fix it by utilizing copy_folio_from_iter_atomic() if nofault flag is
set, otherwise use copy_folio_from_iter().

Signed-off-by: Qu Wenruo <wqu@suse.com>
---
 block/bio.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/block/bio.c b/block/bio.c
index b33ff69bb722..01bb76d9717c 100644
--- a/block/bio.c
+++ b/block/bio.c
@@ -1335,7 +1335,10 @@ static int bio_iov_iter_bounce_write(struct bio *bio, struct iov_iter *iter,
 			break;
 		bio_add_folio_nofail(bio, folio, this_len, 0);
 
-		copied = copy_from_iter(folio_address(folio), this_len, iter);
+		if (iter->nofault)
+			copied = copy_folio_from_iter_atomic(folio, 0, this_len, iter);
+		else
+			copied = copy_folio_from_iter(folio, 0, this_len, iter);
 		if (copied < this_len) {
 			iov_iter_revert(iter, bio->bi_iter.bi_size - this_len + copied);
 			bio_free_folios(bio);
-- 
2.54.0