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
