linux-kernel - Re: [RFC PATCH 5/5] mm: shmem: add anonymous share mTHP counters

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Date: Wed, 24 Apr 2024 17:31:20 +0800
From: Baolin Wang <baolin.wang@...ux.alibaba.com>
To: Ryan Roberts <ryan.roberts@....com>, David Hildenbrand
 <david@...hat.com>, Barry Song <21cnbao@...il.com>
Cc: akpm@...ux-foundation.org, hughd@...gle.com, willy@...radead.org,
 wangkefeng.wang@...wei.com, ying.huang@...el.com, shy828301@...il.com,
 ziy@...dia.com, linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH 5/5] mm: shmem: add anonymous share mTHP counters



On 2024/4/24 16:15, Ryan Roberts wrote:
> On 24/04/2024 08:11, David Hildenbrand wrote:
>> On 24.04.24 08:10, Baolin Wang wrote:
>>>
>>>
>>> On 2024/4/23 19:37, David Hildenbrand wrote:
>>>> On 23.04.24 03:17, Barry Song wrote:
>>>>> On Mon, Apr 22, 2024 at 3:03 PM Baolin Wang
>>>>> <baolin.wang@...ux.alibaba.com> wrote:
>>>>>>
>>>>>> Signed-off-by: Baolin Wang <baolin.wang@...ux.alibaba.com>
>>>>>> ---
>>>>>>     include/linux/huge_mm.h | 2 ++
>>>>>>     mm/huge_memory.c        | 4 ++++
>>>>>>     mm/shmem.c              | 5 ++++-
>>>>>>     3 files changed, 10 insertions(+), 1 deletion(-)
>>>>>>
>>>>>> diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h
>>>>>> index 26b6fa98d8ac..67b9c1acad31 100644
>>>>>> --- a/include/linux/huge_mm.h
>>>>>> +++ b/include/linux/huge_mm.h
>>>>>> @@ -270,6 +270,8 @@ enum mthp_stat_item {
>>>>>>            MTHP_STAT_ANON_SWPOUT,
>>>>>>            MTHP_STAT_ANON_SWPOUT_FALLBACK,
>>>>>>            MTHP_STAT_ANON_SWPIN_REFAULT,
>>>>>> +       MTHP_STAT_SHMEM_ANON_ALLOC,
>>>>>> +       MTHP_STAT_SHMEM_ANON_ALLOC_FALLBACK,
>>>>>
>>>>> not quite sure about this. for 2MB pmd-mapped THP shmem, we count them
>>>>> as FILE_THP.
>>>>> here we are counting as SHMEM_ANON. To me, SHMEM_ANON is more correct but
>>>>> it doesn't align with pmd-mapped THP. David, Ryan, what do you think?
>>>>
>>>> The term "anonymous share" in the patch subject is weird to begin with
>>>> ;) Easy to confuse with anonymous cow-shared memory. Let's just call it
>>>> "anonymous shmem", which it is under the hood.
>>>
>>> Sure.
>>>
>>>> ... regarding the question: if we add FILE_ALLOC and friends, at least
>>>> initially, we wouldn't account other large pagecache folios.
>>>>
>>>> ... likely we should add that then as well so the counter matches the
>>>> actual name?
>>>>
>>>> If we later realize that we need separate FILE vs. SHMEM vs. WHATEVER
>>>> counters, we can always add more fine-grained counters later. Doing it
>>>> consistently w.r.t. traditional THPs first sounds reasonable.
>>>
>>> Um, once we expose it to userspace through the sysfs interface, the
>>> sysfs interface should be explicit as much as possible and avoid
>>> confusing users, otherwise it will be difficult to change this kind of
>>> interface in the future. Personally, I prefer to Ryan's suggestion.
>>
>> Inconsistency is confusing. As long as you avoid that, I don't particularly care.
> 
> This is a good point. We have been careful to make sure the 2M ANON mTHP stats
> match the existing PMD-size stats. So we should definitely make sure that any
> future 2M FILE mTHP stats match too, which I guess means counting both SHMEM and
> FILE events.
> 
> So perhaps it makes more sense to add FILE counters to start with. If we need
> the SHMEM-specific counters, we could add them later?
> 
> I'm happy to go with the crowd on this...

(Seems I'm the only one who prefers the term 'SHMEM_' now.) Fine, I have 
no strong preference, and let's keep consistency first. Thanks guys.