Re: [PATCH v22 1/4] mm: add MAP_DROPPABLE for designating always lazily freeable mappings

[Linus Torvalds]

On Wed, 10 Jul 2024 at 21:46, David Hildenbrand <david@xxxxxxxxxx> wrote:
>
> Maybe we can find ways of simply never marking these pages dirty, so we
> don't have to special-case that code where we don't really have a VMA at
> hand?

That's one option. Jason's patch basically goes "ignore folio dirty
bit for these pages".

Your suggestion basically says "don't turn folios dirty in the first place".

It's mainly the pte_dirty games in mm/vmscan.c that does it
(walk_pte_range), but also the tear-down in mm/memory.c
(zap_present_folio_ptes). Possibly others that I didn't think of.

Both do have access to the vma, although in the case of
walk_pte_range() we don't actually pass it down because we haven't
needed it).

There's also page_vma_mkclean_one(), try_to_unmap_one() and
try_to_migrate_one().  And possibly many others I haven't even thought
about.

So quite a few places that do that "transfer dirty bit from pte to folio".

The other approach might be to just let all the dirty handling happen
- make droppable pages have a "page->mapping" (and not be anonymous),
and have the mapping->a_ops->writepage() just always return success
immediately.

That might actually be a conceptually simpler model. MAP_DROPPABLE
becomes a shared mapping that just has a really cheap writeback that
throws the data away. No need to worry about swap cache or anything
like that, because that's just for anonymous pages.

I say "conceptually simpler", because right now the patch does depend
on just using the regular anon page faulting etc code.

                 Linus