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

[Barry Song <21cnbao@gmail.com>]

On Thu, Jun 27, 2024 at 2:40 AM Zi Yan <ziy@nvidia.com> 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.

It seems we have to do this though I think keeping KPF_THP and adding a
separate bit like KPF_PMD_MAPPED makes more sense. but those tools
relying on KPF_THP need to realize this and check the new bit , which is
not done now.
whether the mTHP's name is mTHP or THP will make no difference for
this case:-)

>
>
> --
> Best Regards,
> Yan, Zi
>

Thanks
Barry