diff --git a/fs/bcachefs/fs-io.c b/fs/bcachefs/fs-io.c
index 944fffd9f7b5..77037574cb0d 100644
--- a/fs/bcachefs/fs-io.c
+++ b/fs/bcachefs/fs-io.c
@@ -3074,9 +3074,7 @@ static int __bchfs_fallocate(struct bch_inode_info *inode, int mode,
 
 	while (!ret && bkey_lt(iter.pos, end_pos)) {
 		s64 i_sectors_delta = 0;
-		struct disk_reservation disk_res = { 0 };
 		struct quota_res quota_res = { 0 };
-		struct bkey_i_reservation reservation;
 		struct bkey_s_c k;
 		unsigned sectors;
 		u32 snapshot;
@@ -3107,16 +3105,7 @@ static int __bchfs_fallocate(struct bch_inode_info *inode, int mode,
 			continue;
 		}
 
-		bkey_reservation_init(&reservation.k_i);
-		reservation.k.type	= KEY_TYPE_reservation;
-		reservation.k.p		= k.k->p;
-		reservation.k.size	= k.k->size;
-
-		bch2_cut_front(iter.pos,	&reservation.k_i);
-		bch2_cut_back(end_pos,		&reservation.k_i);
-
-		sectors = reservation.k.size;
-		reservation.v.nr_replicas = bch2_bkey_nr_ptrs_allocated(k);
+		sectors = bpos_min(k.k->p, end_pos).offset - iter.pos.offset;
 
 		if (!bkey_extent_is_allocation(k.k)) {
 			ret = bch2_quota_reservation_add(c, inode,
@@ -3126,25 +3115,15 @@ static int __bchfs_fallocate(struct bch_inode_info *inode, int mode,
 				goto bkey_err;
 		}
 
-		if (reservation.v.nr_replicas < opts.data_replicas ||
-		    bch2_bkey_sectors_compressed(k)) {
-			ret = bch2_disk_reservation_get(c, &disk_res, sectors,
-							opts.data_replicas, 0);
-			if (unlikely(ret))
-				goto bkey_err;
-
-			reservation.v.nr_replicas = disk_res.nr_replicas;
-		}
-
-		ret = bch2_extent_update(&trans, inode_inum(inode), &iter,
-				&reservation.k_i, &disk_res,
-				0, &i_sectors_delta, true);
+		ret = bch2_extent_fallocate(&trans, inode_inum(inode), &iter,
+					    sectors, opts, &i_sectors_delta,
+					    writepoint_hashed((unsigned long) current));
 		if (ret)
 			goto bkey_err;
+
 		i_sectors_acct(c, inode, &quota_res, i_sectors_delta);
 bkey_err:
 		bch2_quota_reservation_put(c, inode, &quota_res);
-		bch2_disk_reservation_put(c, &disk_res);
 		if (bch2_err_matches(ret, BCH_ERR_transaction_restart))
 			ret = 0;
 	}
diff --git a/fs/bcachefs/io.c b/fs/bcachefs/io.c
index af6b9a7456f5..bde3a4c42189 100644
--- a/fs/bcachefs/io.c
+++ b/fs/bcachefs/io.c
@@ -360,6 +360,36 @@ err:
 	return ret;
 }
 
+/* Overwrites whatever was present with zeroes: */
+int bch2_extent_fallocate(struct btree_trans *trans,
+			  subvol_inum inum,
+			  struct btree_iter *iter,
+			  unsigned sectors,
+			  struct bch_io_opts opts,
+			  s64 *i_sectors_delta,
+			  struct write_point_specifier write_point)
+{
+	int ret;
+	struct bch_fs *c = trans->c;
+	struct disk_reservation disk_res = { 0 };
+	struct bkey_i_reservation *reservation =
+		bch2_trans_kmalloc(trans, sizeof(*reservation));
+
+	ret = PTR_ERR_OR_ZERO(reservation);
+	if (ret)
+		return ret;
+
+	bkey_reservation_init(&reservation->k_i);
+	reservation->k.p = iter->pos;
+	bch2_key_resize(&reservation->k, sectors);
+	reservation->v.nr_replicas = opts.data_replicas;
+
+	ret = bch2_extent_update(trans, inum, iter, &reservation->k_i, &disk_res,
+				 0, i_sectors_delta, true);
+	bch2_disk_reservation_put(c, &disk_res);
+	return ret;
+}
+
 /*
  * Returns -BCH_ERR_transacton_restart if we had to drop locks:
  */
diff --git a/fs/bcachefs/io.h b/fs/bcachefs/io.h
index 39f2aabf4f59..aafe1bf993bb 100644
--- a/fs/bcachefs/io.h
+++ b/fs/bcachefs/io.h
@@ -74,6 +74,9 @@ int bch2_sum_sector_overwrites(struct btree_trans *, struct btree_iter *,
 int bch2_extent_update(struct btree_trans *, subvol_inum,
 		       struct btree_iter *, struct bkey_i *,
 		       struct disk_reservation *, u64, s64 *, bool);
+int bch2_extent_fallocate(struct btree_trans *, subvol_inum, struct btree_iter *,
+			  unsigned, struct bch_io_opts, s64 *,
+			  struct write_point_specifier);
 
 int bch2_fpunch_at(struct btree_trans *, struct btree_iter *,
 		   subvol_inum, u64, s64 *);