Re: [PATCH v1 0/5] mm, kpageflags: support folio and fix output for compound pages

[David Hildenbrand]

On 12.10.23 17:02, Naoya Horiguchi wrote:
> On Thu, Oct 12, 2023 at 10:33:04AM +0200, David Hildenbrand wrote:
> > On 10.10.23 16:27, Naoya Horiguchi wrote:
> > > Hi everyone,
> > >
> > > This patchset addresses 2 issues in /proc/kpageflags.
> > >
> > >     1. We can't easily tell folio from thp, because currently both pages are
> > >        judged as thp, and
> > >     2. we see some garbage data in records of compound tail pages because
> > >        we use tail pages to store some internal data.
> > >
> > > These issues require userspace programs to do additional work to understand
> > > the page status, which makes situation more complicated.
> > >
> > > This patchset tries to solve these by defining KPF_FOLIO for issue 1., and
> > > by hiding part of page flag info on tail pages of compound pages for issue 2.
> > >
> > > I think that technically some compound pages like thp/hugetlb/slab could be
> > > considered as folio, but in this version KPF_FOLIO is set only on folios
> >
> > At least thp+hugetlb are most certainly folios. Regarding slab, I suspect we
> > no longer call them folios (cannot be mapped to user space). But Im not sure
> > about the type hierarchy.
>
> I'm not sure about the exact definition of "folio", and I think it's better
> to make KPF_FOLIO set based on the definition.

Me neither. But in any case a THP *is* a folio. So you'd have to set 
that flag in any case.

And any order-0 page (i.e., anon, pagecache) is also a folio. What you 
seem to imply with folio is "large folio". So KPF_FOLIO is really wrong 
as far as I can tell.

> "being mapped to userspace" can be one possible criteria for the definition.
> But reading source code, folio_slab() and slab_folio() convert between
> struct slab and struct folio, so I feel that someone might think a slab is
> a kind of folio.

I keep forgetting if "folio" is just the generic term for any order-0 or 
compound page, or only for some of them. I usually live in the "anon" 
world, so I don't get reminded that often :)


> > > in pagecache (so "folios in narrower meaning").  I'm not confident about
> > > this choice, so if you have any idea about this, please let me know.
> >
> > It does sound inconsistent. What exactly do you want to tell user space with
> > the new flag?
>
> The current most problematic behavior is to report folio as thp (order-2
> pagecache page is definitely a folio but not a thp), and this is what the
> new flag is intended to tell.

We are currently considering calling these sub-PMD sized THPs 
"small-sized THP". [1] Arguably, we're starting with the anon part where 
we won't get around exposing them to the user in sysfs.

So I wouldn't immediately say that these things are not THPs. They are 
not PMD-sized THP. A slab/hugetlb is certainly not a thp but a folio. 
Whereby slabs can also be order-0 folios, but hugetlb can't.


Looking at other interfaces, we do expose:

include/uapi/linux/kernel-page-flags.h:#define KPF_COMPOUND_HEAD        15
include/uapi/linux/kernel-page-flags.h:#define KPF_COMPOUND_TAIL        16

So maybe we should just continue talking about compound pages or do we 
have to use both terms here in this interface?

[1] https://lkml.kernel.org/r/20230929114421.3761121-1-ryan.roberts@xxxxxxx

--
Cheers,

David / dhildenb