Re: [RFC PATCH v4 3/4] mm: add do_set_pte_range()

[Matthew Wilcox <willy@infradead.org>]

On Mon, Feb 06, 2023 at 04:13:44PM +0100, David Hildenbrand wrote:
> > > The handling of cow pages is still very clunky.
> > > folio_add_new_anon_rmap() handles anonymous large folios just fine.  I
> > > think David was looking at current code, not the code in mm-next.
> > OK. Let's wait for further comment from David.
> 
> As I raised, page_add_new_anon_rmap() -> folio_add_new_anon_rmap() can be
> used to add a fresh (a) PMD-mapped THP or (b) order-0 folio.
>
> folio_add_new_anon_rmap() is not suitable for PTE-mapping a large folio.
> Which is what we are intending to do here unless I am completely off.

I think you are.  While the infrastructure here handles large folios
which are not PMDs, there's nobody who will allocate such a thing, so
there is no problem.  Right>

> PTE-mapping a large folio requires different accounting, different mapcount
> handling and different PG_anon_exclusive handling.
> 
> Which is all not there yet.

Assuming you mean "a large folio which is not PMD sized", I agree.  But we
haven't even started discussing how we should decide to allocate folios
which are not 0 or PMD order yet, so this worry seems premature.  When we
have figured that out, it'll be time to look at folio_add_new_anon_rmap()
to decide how to support it, but hopefully before then we'll have a
better way of handling mapcount.