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 09:39:52 +0100
From: Ryan Roberts <ryan.roberts@....com>
To: Barry Song <21cnbao@...il.com>, Zi Yan <ziy@...dia.com>
Cc: 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 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.

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.

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?

> 
>>
>>
>> --
>> Best Regards,
>> Yan, Zi
>>
> 
> Thanks
> Barry