The code is loosely based on filemap_read() and might belong in
mm/filemap.c with that as it needs to use filemap_get_pages().

Yes, I thunk it should go into filemap.c

+	while (spliced < size &&
+	       !pipe_full(pipe->head, pipe->tail, pipe->max_usage)) {
+		struct pipe_buffer *buf = &pipe->bufs[pipe->head & (pipe->ring_size - 1)];

Can you please facto this calculation, that is also duplicated in patch
one into a helper?

static inline struct pipe_buffer *pipe_head_buf(struct pipe_inode_info *pipe)
{
	return &pipe->bufs[pipe->head & (pipe->ring_size - 1)];
}

+	struct folio_batch fbatch;
+	size_t total_spliced = 0, used, npages;
+	loff_t isize, end_offset;
+	bool writably_mapped;
+	int i, error = 0;
+
+	struct kiocb iocb = {

Why the empty line before this declaration?

+		.ki_filp	= in,
+		.ki_pos		= *ppos,
+	};

Also why doesn't this use init_sync_kiocb?

 	if (in->f_flags & O_DIRECT)
 		return generic_file_direct_splice_read(in, ppos, pipe, len, flags);
+	return generic_file_buffered_splice_read(in, ppos, pipe, len, flags);

Btw, can we drop the verbose generic_file_ prefix here?

generic_file_buffered_splice_read really should be filemap_splice_read
and be in filemap.c.  generic_file_direct_splice_read I'd just name
direct_splice_read.