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

[Matthew Wilcox <willy@infradead.org>]

On Fri, Oct 07, 2022 at 10:36:56PM +0900, Hyeonggon Yoo wrote:
> > 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

That's the part I wasn't sure of.  I think we do, in
prep_compound_head().

>         - _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.

Really?  It seems to me that it returns true at the moment.  Look:

__PAGEFLAG(Slab, slab, PF_NO_TAIL)

#define PF_NO_TAIL(page, enforce) ({                                    \
                VM_BUG_ON_PGFLAGS(enforce && PageTail(page), page);     \
                PF_POISONED_CHECK(compound_head(page)); })

so AFAICS, PageSlab checks the Slab bit on the head page, not the
tail page.

> 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?

There are tools like tools/vm/page-types.c which expect PageSlab to
return true for tail pages.

> > 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.

No problem.  I suggested (in an off-list email) that you consider counting
'active' by subtraction rather than addition because I have a feeling that

int active(struct slab *slab)
{
	return ~(slab->page_type | PG_slab);
}

would be better than

int active(struct slab *slab)
{
	return slab->page_type & 0xffff;
}

at least in part because you don't have to clear the bottom 16 bits of
page_type when you clear PG_slab, and you don't have to re-set them
when you set PG_slab.