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

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Date: Thu, 27 Jun 2024 21:16:46 +1200
From: Barry Song <21cnbao@...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, 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>, Lance Yang <ioworker0@...il.com>
Subject: Re: [PATCH 2/2] kpageflags: fix wrong KPF_THP on non-pmd-mappable
 compound pages

On Thu, Jun 27, 2024 at 8:39 PM Ryan Roberts <ryan.roberts@....com> wrote:
>
> On 27/06/2024 05:10, Barry Song wrote:
> > On Thu, Jun 27, 2024 at 2:40 AM Zi Yan <ziy@...dia.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@....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:-)
>
> I don't quite follow your logic for that last part; If there are 2 separate
> bits; KPF_THP and KPF_MTHP, and KPF_THP is only set for PMD-sized THP, that
> would be a safe/compatible approach, right? Where as your suggestion requires
> changes to existing tools to work.

Right, my point is that mTHP and THP are both types of THP. The only difference
is whether they are PMD-mapped or PTE-mapped. Adding a bit to describe how
the page is mapped would more accurately reflect reality. However, this change
would disrupt tools that assume KPF_THP always means PMD-mapped THP.
Therefore, we would still need separate bits for THP and mTHP in this case.

I saw Willy complain about mTHP being called "mTHP," but in this case, calling
it "mTHP" or just "THP" doesn't change anything if old tools continue to assume
that KPF_THP means PMD-mapped THP.

>
> Thinking about this a bit more, I wonder if PKF_MTHP is the right name for a new
> flag; We don't currently expose the term "mTHP" to user space. I can't think of
> a better name though.

Yes.  If "compatibility" is a requirement, we cannot disregard it.

> I'd still like to understand what is actually broken that this change is fixing.
> Is the concern that a user could see KPF_THP and advance forward by
> "/sys/kernel/mm/transparent_hugepage/hpage_pmd_size / getpagesize()" entries?
>

Maybe we need an example which is thinking that KPF_THP is PMD-mapped.

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

Thanks
Barry