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

[Ryan Roberts <ryan.roberts@arm.com>]

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@linux.alibaba.com> wrote:
>>>>>
>>>>> Signed-off-by: Baolin Wang <baolin.wang@linux.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...