Re: [PATCH] mm: move PG_slab flag to page_type

[Hyeonggon Yoo]

On Sun, Sep 25, 2022 at 12:04:40AM +0100, Matthew Wilcox wrote:
> On Mon, Sep 19, 2022 at 09:57:08PM +0900, Hyeonggon Yoo wrote:
> > For now, only SLAB uses _mapcount field as a number of active objects in
> > a slab, and other slab allocators do not use it. As 16 bits are enough
> > for that, use remaining 16 bits of _mapcount as page_type even when
> > SLAB is used. And then move PG_slab flag to page_type!
> > 
> > Note that page_type is always placed in upper 16 bits of _mapcount to
> > avoid confusing normal _mapcount as page_type. As underflow (actually
> > I mean, yeah, overflow) is not a concern anymore, use more lower bits
> > except bit zero.
> > 
> > Add more folio helpers for PAGE_TYPE_OPS() not to break existing
> > slab implementations.
> > 
> > Remove PG_slab check from PAGE_FLAGS_CHECK_AT_FREE. buddy will still
> > check if _mapcount is properly set at free.
> > 
> > Exclude PG_slab from hwpoison and show_page_flags() for now.
> > 
> > Note that with this patch, page_mapped() and folio_mapped() always return
> > false for slab page.
> 
> This is an interesting approach.  It raises some questions.

Hello Matthew, sorry for late reply and I didn't mean to ignore your
feedback. I realized compound pages and folio stuffs are my weak side and
needed some time to learn :)

> First, you say that folio_mapped() returns false for slab pages.  That's
> only true for order-0 slab pages.  For larger pages,
> 
>         if (!folio_test_large(folio))
>                 return atomic_read(&folio->_mapcount) >= 0;
>         if (atomic_read(folio_mapcount_ptr(folio)) >= 0)
>                 return true;
> 
> so that's going to depend what folio_mapcount_ptr() aliases with.

IIUC it's true for order > 0 slab too.

As slab pages are not mapped to userspace at all,
entire compound page nor base pages are not mapped to userspace.

AFAIK followings are true for order > 0 slab:
        - (first tail page)->compound_mapcount is -1
        - _mapcount of base pages are -1

So:
        folio_mapped() and page_mapped() (if applied to head page)
        returns false for larger pages with this patch.

I wrote simple testcase and did check that folio_mapped() and page_mapped()
returns false for both order-0 page and larger pages. (and SLAB
returned true for them before)

> Second, this patch changes the behaviour of PageSlab() when applied to
> tail pages.

Altough it changes the way it checks the flag,

it does not change behavior when applied to tail pages - PageSlab() on tail
page returns false with or without this patch.

If PageSlab() need to return true for tail pages too,
we may make it check page_type at head page.

But I'm not sure when it the behavior is needed.
Can you please share your insight on this?

> Which raises the further question of what PageBuddy(),
> PageTable(), PageGuard() and PageIsolated() should do for multi-page
> folios, if that is even possible.

For users that uses real compound page like slab, we can make it check
page_type of head page. (if needed)

But for cases David described, there isn't much thing we can do
except making them to use real compound pages.

> Third, can we do this without that awkward __u16 thing?  Perhaps
> 
> -#define PG_buddy        0x00000080
> -#define PG_offline      0x00000100
> -#define PG_table        0x00000200
> -#define PG_guard        0x00000400
> +#define PG_buddy        0x00010000
> +#define PG_offline      0x00020000
> +#define PG_table        0x00040000
> +#define PG_guard        0x00080000
> +#define PG_slab         0x00100000
> 
> ... and then use wrappers in slab.c to access the bottom 16 bits?

Definitely! I prefer that way and will adjust in RFC v2.

Thank you for precious feedback.

-- 
Hyeonggon