Re: [PATCH] smaps: count large pages smaller than PMD size to anonymous_thp

[Wenchao Hao <haowenchao22@gmail.com>]

On 2024/12/3 22:42, Ryan Roberts wrote:
> On 03/12/2024 14:17, David Hildenbrand wrote:
>> On 03.12.24 14:49, Wenchao Hao wrote:
>>> Currently, /proc/xxx/smaps reports the size of anonymous huge pages for
>>> each VMA, but it does not include large pages smaller than PMD size.
>>>
>>> This patch adds the statistics of anonymous huge pages allocated by
>>> mTHP which is smaller than PMD size to AnonHugePages field in smaps.
>>>
>>> Signed-off-by: Wenchao Hao <haowenchao22@gmail.com>
>>> ---
>>>   fs/proc/task_mmu.c | 6 ++++++
>>>   1 file changed, 6 insertions(+)
>>>
>>> diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
>>> index 38a5a3e9cba2..b655011627d8 100644
>>> --- a/fs/proc/task_mmu.c
>>> +++ b/fs/proc/task_mmu.c
>>> @@ -717,6 +717,12 @@ static void smaps_account(struct mem_size_stats *mss,
>>> struct page *page,
>>>           if (!folio_test_swapbacked(folio) && !dirty &&
>>>               !folio_test_dirty(folio))
>>>               mss->lazyfree += size;
>>> +
>>> +        /*
>>> +         * Count large pages smaller than PMD size to anonymous_thp
>>> +         */
>>> +        if (!compound && PageHead(page) && folio_order(folio))
>>> +            mss->anonymous_thp += folio_size(folio);
>>>       }
>>>         if (folio_test_ksm(folio))
>>
>>
>> I think we decided to leave this (and /proc/meminfo) be one of the last
>> interfaces where this is only concerned with PMD-sized ones:
>>
>> Documentation/admin-guide/mm/transhuge.rst:
>>
>> The number of PMD-sized anonymous transparent huge pages currently used by the
>> system is available by reading the AnonHugePages field in ``/proc/meminfo``.
>> To identify what applications are using PMD-sized anonymous transparent huge
>> pages, it is necessary to read ``/proc/PID/smaps`` and count the AnonHugePages
>> fields for each mapping. (Note that AnonHugePages only applies to traditional
>> PMD-sized THP for historical reasons and should have been called
>> AnonHugePmdMapped).
>>
> 
> Agreed. If you need per-process metrics for mTHP, we have a python script at
> tools/mm/thpmaps which does a fairly good job of parsing pagemap. --help gives
> you all the options.
> 

I tried this tool, and it is very powerful and practical IMO.
However, thereare two disadvantages:

- This tool is heavily dependent on Python and Python libraries.
  After installing several libraries with the pip command, I was able to
  get it running.
  In practice, the environment we need to analyze may be a mobile or
  embedded environment, where it is very difficult to deploy these
  libraries.
- It seems that this tool only counts file-backed large pages? During
  the actual test, I mapped a region of anonymous pages and mapped it
  as large pages, but the tool did not display those large pages.
  Below is my test file(mTHP related sysfs interface is set to "always"
  to make sure using large pages):

int main()
{
        int i;
        char *c;
        unsigned long *p;

        p = mmap(NULL, SIZE, PROT_READ | PROT_WRITE, MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
        if (!p) {
                perror("fail to get memory");
                exit(-1);
        }

        c = (unsigned char *)p;

        for (i = 0; i < SIZE / 8; i += 8)
                *(p + i) = 0xffff + i;

        while (1)
                sleep(10);

        return 0;
}

Thanks,
wenchao