Re: [PATCH 2/2] kpageflags: fix wrong KPF_THP on non-pmd-mappable compound pages

[Lance Yang <ioworker0@gmail.com>]

On Wed, Jun 26, 2024 at 10:42 PM Ryan Roberts <ryan.roberts@arm.com> wrote:
>
> On 26/06/2024 15:40, Zi Yan wrote:
> > On Wed Jun 26, 2024 at 7:07 AM EDT, Ryan Roberts wrote:
> >> On 26/06/2024 04:06, Zi Yan wrote:
> >>> On Tue Jun 25, 2024 at 10:49 PM EDT, ran xiaokai wrote:
> >>>> From: Ran Xiaokai <ran.xiaokai@zte.com.cn>
> >>>>
> >>>> KPF_COMPOUND_HEAD and KPF_COMPOUND_TAIL are set on "common" compound
> >>>> pages, which means of any order, but KPF_THP should only be set
> >>>> when the folio is a 2M pmd mappable THP.
> >>
> >> Why should KPF_THP only be set on 2M THP? What problem does it cause as it is
> >> currently configured?
> >>
> >> I would argue that mTHP is still THP so should still have the flag. And since
> >> these smaller mTHP sizes are disabled by default, only mTHP-aware user space
> >> will be enabling them, so I'll naively state that it should not cause compat
> >> issues as is.
> >>
> >> Also, the script at tools/mm/thpmaps relies on KPF_THP being set for all mTHP
> >> sizes to function correctly. So that would need to be reworked if making this
> >> change.
> >
> > + more folks working on mTHP
> >
> > I agree that mTHP is still THP, but we might want different
> > stats/counters for it, since people might want to keep the old THP counters
> > consistent. See recent commits on adding mTHP counters:
> > ec33687c6749 ("mm: add per-order mTHP anon_fault_alloc and anon_fault_fallback
> > counters"), 1f97fd042f38 ("mm: shmem: add mTHP counters for anonymous shmem")
> >
> > and changes to make THP counter to only count PMD THP:
> > 835c3a25aa37 ("mm: huge_memory: add the missing folio_test_pmd_mappable() for
> > THP split statistics")
> >
> > In this case, I wonder if we want a new KPF_MTHP bit for mTHP and some
> > adjustment on tools/mm/thpmaps.
>
> That would work for me, assuming we have KPF bits to spare?

+1

Let's check on that and see if we're good ;)

Thanks,
Lance
>