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

[Baolin Wang <baolin.wang@linux.alibaba.com>]

On 2024/4/23 19:39, Ryan Roberts wrote:
> On 23/04/2024 02:46, Baolin Wang wrote:
>>
>>
>> On 2024/4/23 09:17, 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?
>>
>> Thanks for reviewing.
>>
>> IMO, I think both approaches are acceptable, which also reflects the dual nature
>> of anonymous shared pages: on the one hand they are anonymous pages, and on the
>> other hand, they are backed by a pseudo file. From the user's perspective, I
>> prefer to use the term "anonymous shmem", which can be distinguished from the
>> real file-backed THP.
>>
>> Anyway, let's see what others think.
> 
>  From a quick look at the code, it looks like the shmem alloc/fallback/charge
> events are all lumped in with FILE_THP. But the instantaneous "how many are
> allocated" and "how many are mapped" have their own NR_SHMEM_THPS and
> NR_SHMEM_PMDMAPPED counters? So its a bit inconsistent today.
> 
> My preference would be to add these to be consistent with the anon stats:
> 
> MTHP_STAT_SHMEM_FAULT_ALLOC,
> MTHP_STAT_SHMEM_FAULT_FALLBACK,
> MTHP_STAT_SHMEM_FAULT_FALLBACK_CHARGE,
> 
> But it looks like these aren't always allocated due to faults? So perhaps:
> 
> MTHP_STAT_SHMEM_ALLOC,
> MTHP_STAT_SHMEM_FALLBACK,
> MTHP_STAT_SHMEM_FALLBACK_CHARGE,

This looks good to me.

> If I've understood the code correctly (I know nothing about shmem), the
> allocation can be for both mmap(SHARED|ANON) and for tmpfs? So "SHMEM_ANON"

This is allowed, but the 'fd' for tmpfs will be ignored (see 
ksys_mmap_pgoff()), which is same with anonymous shmem.

> probably isn't quite right?
> 
> 
>>
>>>>           __MTHP_STAT_COUNT
>>>>    };
>>>>
>>>> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
>>>> index 9e52c0db7580..dc15240c1ab3 100644
>>>> --- a/mm/huge_memory.c
>>>> +++ b/mm/huge_memory.c
>>>> @@ -557,6 +557,8 @@ DEFINE_MTHP_STAT_ATTR(anon_alloc_fallback,
>>>> MTHP_STAT_ANON_ALLOC_FALLBACK);
>>>>    DEFINE_MTHP_STAT_ATTR(anon_swpout, MTHP_STAT_ANON_SWPOUT);
>>>>    DEFINE_MTHP_STAT_ATTR(anon_swpout_fallback, MTHP_STAT_ANON_SWPOUT_FALLBACK);
>>>>    DEFINE_MTHP_STAT_ATTR(anon_swpin_refault, MTHP_STAT_ANON_SWPIN_REFAULT);
>>>> +DEFINE_MTHP_STAT_ATTR(shmem_anon_alloc, MTHP_STAT_SHMEM_ANON_ALLOC);
>>>> +DEFINE_MTHP_STAT_ATTR(shmem_anon_alloc_fallback,
>>>> MTHP_STAT_SHMEM_ANON_ALLOC_FALLBACK);
>>>>
>>>>    static struct attribute *stats_attrs[] = {
>>>>           &anon_alloc_attr.attr,
>>>> @@ -564,6 +566,8 @@ static struct attribute *stats_attrs[] = {
>>>>           &anon_swpout_attr.attr,
>>>>           &anon_swpout_fallback_attr.attr,
>>>>           &anon_swpin_refault_attr.attr,
>>>> +       &shmem_anon_alloc_attr.attr,
>>>> +       &shmem_anon_alloc_fallback_attr.attr,
>>>>           NULL,
>>>>    };
>>>>
>>>> diff --git a/mm/shmem.c b/mm/shmem.c
>>>> index 8b009e7040b2..4a0aa75ab29c 100644
>>>> --- a/mm/shmem.c
>>>> +++ b/mm/shmem.c
>>>> @@ -1706,11 +1706,14 @@ static struct folio *shmem_alloc_and_add_folio(struct
>>>> vm_fault *vmf,
>>>>                           pages = 1 << order;
>>>>                           index = round_down(index, pages);
>>>>                           folio = shmem_alloc_hugefolio(gfp, info, index, order);
>>>> -                       if (folio)
>>>> +                       if (folio) {
>>>> +                               count_mthp_stat(order,
>>>> MTHP_STAT_SHMEM_ANON_ALLOC);
> 
> is there any reason why this can't go next to the existing PMD-size stat?

No, will move to the existing PMD-size stat.