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.
