diff --git a/fs/iomap/bio.c b/fs/iomap/bio.c
index 153b779949e9..256564ebe238 100644
--- a/fs/iomap/bio.c
+++ b/fs/iomap/bio.c
@@ -109,7 +109,7 @@ int iomap_bio_read_folio_range(const struct iomap_iter *iter,
 
 	if (!bio ||
 	    bio_end_sector(bio) != iomap_sector(&iter->iomap, iter->pos) ||
-	    bio->bi_iter.bi_size > iomap_max_bio_size(&iter->iomap) - plen ||
+	    !iomap_bio_has_room(&iter->iomap, bio, plen) ||
 	    !bio_add_folio(bio, folio, plen, offset_in_folio(folio, iter->pos)))
 		iomap_read_alloc_bio(iter, ctx, plen);
 	return 0;
diff --git a/fs/iomap/buffered-io.c b/fs/iomap/buffered-io.c
index 678f4c5329e7..a6ab6f31cbbf 100644
--- a/fs/iomap/buffered-io.c
+++ b/fs/iomap/buffered-io.c
@@ -920,6 +920,7 @@ static int __iomap_write_begin(const struct iomap_iter *iter,
 		if (iomap_block_needs_zeroing(iter, block_start)) {
 			if (WARN_ON_ONCE(iter->flags & IOMAP_UNSHARE))
 				return -EIO;
+			/* uptodate once copied, see __iomap_write_end() */
 			folio_zero_segments(folio, poff, from, to, poff + plen);
 		} else {
 			const struct iomap *iomap = iomap_iter_srcmap(iter);
@@ -943,8 +944,8 @@ static int __iomap_write_begin(const struct iomap_iter *iter,
 
 			if (iomap->flags & IOMAP_F_ZERO_TAIL)
 				folio_zero_segment(folio, to, poff + plen);
+			iomap_set_range_uptodate(folio, poff, plen);
 		}
-		iomap_set_range_uptodate(folio, poff, plen);
 	} while ((block_start += plen) < block_end);
 
 	return 0;
diff --git a/fs/iomap/internal.h b/fs/iomap/internal.h
index 74e898b196dc..e69b3ac02cc2 100644
--- a/fs/iomap/internal.h
+++ b/fs/iomap/internal.h
@@ -17,6 +17,15 @@ static inline size_t iomap_max_bio_size(const struct iomap *iomap)
 	return BIO_MAX_SIZE;
 }
 
+/* Can @len more bytes go into @bio? */
+static inline bool iomap_bio_has_room(const struct iomap *iomap,
+		const struct bio *bio, size_t len)
+{
+	size_t max_size = iomap_max_bio_size(iomap);
+
+	return len <= max_size && bio->bi_iter.bi_size <= max_size - len;
+}
+
 u32 iomap_finish_ioend_buffered_read(struct iomap_ioend *ioend);
 u32 iomap_finish_ioend_direct(struct iomap_ioend *ioend);
 
diff --git a/fs/iomap/ioend.c b/fs/iomap/ioend.c
index 92b89fbf95b9..4da248ff99f8 100644
--- a/fs/iomap/ioend.c
+++ b/fs/iomap/ioend.c
@@ -133,8 +133,7 @@ static bool iomap_can_add_to_ioend(struct iomap_writepage_ctx *wpc, loff_t pos,
 {
 	struct iomap_ioend *ioend = wpc->wb_ctx;
 
-	if (ioend->io_bio.bi_iter.bi_size >
-	    iomap_max_bio_size(&wpc->iomap) - map_len)
+	if (!iomap_bio_has_room(&wpc->iomap, &ioend->io_bio, map_len))
 		return false;
 	if (ioend_flags & IOMAP_IOEND_BOUNDARY)
 		return false;