linux-kernel - Re: [PATCH] smaps: count large pages smaller than PMD size to anonymous

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Message-ID: <500d1007-56f5-43cb-be9d-4a39fccc6e53@arm.com>
Date: Wed, 4 Dec 2024 17:05:45 +0000
From: Ryan Roberts <ryan.roberts@....com>
To: Wenchao Hao <haowenchao22@...il.com>, David Hildenbrand
 <david@...hat.com>, Andrew Morton <akpm@...ux-foundation.org>,
 Matthew Wilcox <willy@...radead.org>, Oscar Salvador <osalvador@...e.de>,
 Muhammad Usama Anjum <usama.anjum@...labora.com>,
 Andrii Nakryiko <andrii@...nel.org>, Peter Xu <peterx@...hat.com>,
 Barry Song <21cnbao@...il.com>, linux-kernel@...r.kernel.org,
 linux-fsdevel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH] smaps: count large pages smaller than PMD size to
 anonymous_thp

On 04/12/2024 14:40, Wenchao Hao wrote:
> 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@...il.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.

I think numpy is the only package it uses which is not in the standard library?
What other libraries did you need to install?

>   In practice, the environment we need to analyze may be a mobile or
>   embedded environment, where it is very difficult to deploy these
>   libraries.

Yes, I agree that's a problem, especially for Android. The script has proven
useful to me for debugging in a traditional Linux distro environment though.

> - It seems that this tool only counts file-backed large pages? During

No; the tool counts file-backed and anon memory. But it reports it in separate
counters. See `thpmaps --help` for full details.

>   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):

Which mTHP sizes did you enable? Depending on your value of SIZE and which mTHP
sizes are enabled, you may not have a correctly aligned region in p. So mTHP
would not be allocated. Best to over-allocate then explicitly align p to the
mTHP size, then fault it in.

> 
> int main()
> {
>         int i;
>         char *c;
>         unsigned long *p;
> 
>         p = mmap(NULL, SIZE, PROT_READ | PROT_WRITE, MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);

What is SIZE here?

>         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;

Err... what's your intent here? I think you're writting to 1 in every 8 longs?
Probably just write to the first byte of every page.

Thanks,
Ryan

> 
>         while (1)
>                 sleep(10);
> 
>         return 0;
> }
> 
> Thanks,
> wenchao
>