Re: [RFC][PATCH 1/4] mm, page_owner: add folio allocate post callback for struct page_owner to make the owner clearer

[Matthew Wilcox <willy@infradead.org>]

On Thu, Nov 09, 2023 at 04:04:39PM +0000, jeff.xie@linux.dev wrote:
> November 9, 2023 at 11:36 PM, "Matthew Wilcox" <willy@infradead.org> wrote:
> > But we want that anyway (or at least I do). You're right that vmalloc
> > pages are not marked as being vmalloc pages and don't contain the
> > information about which vmalloc area they belong to. I've talked about
> > ways we can add that information to folios in the past, but I have a lot
> > of other projects I'm working on. Are you interested in doing that?
> >
> 
> Certainly, I'm willing to give it a try. If a folio can include vmalloc information
> or more information, this is great. I may need to understand the background of why
> you proposed this method in the past.

I can't find the proposal now, but it's basically this:

Turn PG_slab into a PG_kernel.  If PG_kernel is set, then other flags
change their meaning.  Flags that should be reusable: writeback,
referenced, uptodate, lru, active, workingset, private, reclaim

One of those flags gets reused to be the new slab.  So, eg
folio_test_slab() becomes:

	return (folio->flags & (PG_kernel | PG_slab)) == (PG_kernel | PG_slab);

Now we have somewhere that we can use for PG_vmalloc (also PG_reserved
can become a PG_kernel sub-flag, freeing up a page flag).

We'd need to change some helpers.  eg folio_mapping() currently does:

        if (unlikely(folio_test_slab(folio)))
                return NULL;

and that should be:

	if (unlikely(folio_test_kernel(folio)))
		return NULL;

With that in place, we can reuse the folio->mapping space to point to
the struct vm_struct that allocated it.

This isn't an easy project and will require a lot of testing.  It has
some upsides, like freeing up a page flag.