Re: Folios for 5.15 request - Was: re: Folio discussion recap -

[Kent Overstreet]

On Wed, Sep 22, 2021 at 11:08:58AM -0400, Johannes Weiner wrote:
> On Tue, Sep 21, 2021 at 05:22:54PM -0400, Kent Overstreet wrote:
> >  - it's become apparent that there haven't been any real objections to the code
> >    that was queued up for 5.15. There _are_ very real discussions and points of
> >    contention still to be decided and resolved for the work beyond file backed
> >    pages, but those discussions were what derailed the more modest, and more
> >    badly needed, work that affects everyone in filesystem land
> 
> Unfortunately, I think this is a result of me wanting to discuss a way
> forward rather than a way back.
> 
> To clarify: I do very much object to the code as currently queued up,
> and not just to a vague future direction.
> 
> The patches add and convert a lot of complicated code to provision for
> a future we do not agree on. The indirections it adds, and the hybrid
> state it leaves the tree in, make it directly more difficult to work
> with and understand the MM code base. Stuff that isn't needed for
> exposing folios to the filesystems.
> 
> As Willy has repeatedly expressed a take-it-or-leave-it attitude in
> response to my feedback, I'm not excited about merging this now and
> potentially leaving quite a bit of cleanup work to others if the
> downstream discussion don't go to his liking.
> 
> Here is the roughly annotated pull request:

Thanks for breaking this out, Johannes.

So: mm/filemap.c and mm/page-writeback.c - I disagree about folios not really
being needed there. Those files really belong more in fs/ than mm/, and the code
in those files needs folios the most - especially filemap.c, a lot of those
algorithms have to change from block based to extent based, making the analogy
with filesystems.

I think it makes sense to drop the mm/lru stuff, as well as the mm/memcg,
mm/migrate and mm/workingset and mm/swap stuff that you object to - that is, the
code paths that are for both file + anonymous pages, unless Matthew has
technical reasons why that would break the rest of the patch set.

And then, we really should have a pow wow and figure out what our options are
going forward. I think we have some agreement now that not everything is going
to be a folio going forwards (Matthew already split out his slab conversion to a
new type) - so if anonymous pages aren't becoming folios, we should prototype
some stuff and see where that helps and hurts us.

> As per the other email I still think it would have been good to have a
> high-level discussion about the *legitimate* entry points and data
> structures that will continue to deal with tail pages down the
> line. To scope the actual problem that is being addressed by this
> inverted/whitelist approach - so we don't annotate the entire world
> just to box in a handful of page table walkers...

That discussion can still happen... and there's still the potential to get a lot
more done if we're breaking open struct page and coming up with new types. I got
Matthew on board with what you wanted, re: using the slab allocator for larger
allocations