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

[Naoya Horiguchi <naoya.horiguchi@linux.dev>]

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.
"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.

> 
> > 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.

Thanks,
Naoya Horiguchi