why do hash index builds use smgrextend() for new splitpoint pages

I'm trying to understand why hash indexes are built primarily in shared
buffers except when allocating a new splitpoint's worth of bucket pages
-- which is done with smgrextend() directly in _hash_alloc_buckets().

Is this just so that the value returned by smgrnblocks() includes the
new splitpoint's worth of bucket pages?

All writes of tuple data to pages in this new splitpoint will go
through shared buffers (via hash_getnewbuf()).

I asked this and got some thoughts from Robert in [1], but I still don't
really get it.

When a new page is needed during the hash index build, why can't
_hash_expandtable() just call ReadBufferExtended() with P_NEW instead of
_hash_getnewbuf()? Does it have to do with the BUCKET_TO_BLKNO mapping?

- Melanie

[1] https://www.postgresql.org/message-id/CA%2BTgmoa%2BQFFhkHgPxyxv6t8aVU0r7GZmu7z8io4vGG7RHPpGzA%40mail.gmail.com