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

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Message-ID: <CAK1f24=0KYAMRnamw+NTB43m1fsFqgjMewaFwtCSjY10gtkAOw@mail.gmail.com>
Date: Thu, 27 Jun 2024 09:54:14 +0800
From: Lance Yang <ioworker0@...il.com>
To: Ryan Roberts <ryan.roberts@....com>
Cc: Zi Yan <ziy@...dia.com>, ran xiaokai <ranxiaokai627@....com>, akpm@...ux-foundation.org, 
	willy@...radead.org, vbabka@...e.cz, svetly.todorov@...verge.com, 
	ran.xiaokai@....com.cn, baohua@...nel.org, peterx@...hat.com, 
	linux-kernel@...r.kernel.org, linux-mm@...ck.org, 
	linux-fsdevel@...r.kernel.org, David Hildenbrand <david@...hat.com>, 
	Baolin Wang <baolin.wang@...ux.alibaba.com>, Kefeng Wang <wangkefeng.wang@...wei.com>, 
	Barry Song <21cnbao@...il.com>
Subject: Re: [PATCH 2/2] kpageflags: fix wrong KPF_THP on non-pmd-mappable
 compound pages

On Wed, Jun 26, 2024 at 10:42 PM Ryan Roberts <ryan.roberts@....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@....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
>