linux-kernel - Re: [PATCH v2 18/20] fs/proc/task_mmu: remove per-page mapcount dependency for "mapmax" (CONFIG_NO_PAGE

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Message-ID: <a84a78bc-2446-4678-b8fb-fa9b37695355@redhat.com>
Date: Mon, 24 Feb 2025 22:04:53 +0100
From: David Hildenbrand <david@...hat.com>
To: Zi Yan <ziy@...dia.com>, linux-kernel@...r.kernel.org
Cc: linux-doc@...r.kernel.org, cgroups@...r.kernel.org, linux-mm@...ck.org,
 linux-fsdevel@...r.kernel.org, linux-api@...r.kernel.org,
 Andrew Morton <akpm@...ux-foundation.org>,
 "Matthew Wilcox (Oracle)" <willy@...radead.org>, Tejun Heo <tj@...nel.org>,
 Zefan Li <lizefan.x@...edance.com>, Johannes Weiner <hannes@...xchg.org>,
 Michal Koutný <mkoutny@...e.com>,
 Jonathan Corbet <corbet@....net>, Andy Lutomirski <luto@...nel.org>,
 Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>,
 Borislav Petkov <bp@...en8.de>, Dave Hansen <dave.hansen@...ux.intel.com>,
 Muchun Song <muchun.song@...ux.dev>,
 "Liam R. Howlett" <Liam.Howlett@...cle.com>,
 Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
 Vlastimil Babka <vbabka@...e.cz>, Jann Horn <jannh@...gle.com>,
 owner-linux-mm@...ck.org
Subject: Re: [PATCH v2 18/20] fs/proc/task_mmu: remove per-page mapcount
 dependency for "mapmax" (CONFIG_NO_PAGE_MAPCOUNT)

On 24.02.25 21:45, Zi Yan wrote:
> On Mon Feb 24, 2025 at 11:56 AM EST, David Hildenbrand wrote:
>> Let's implement an alternative when per-page mapcounts in large folios are
>> no longer maintained -- soon with CONFIG_NO_PAGE_MAPCOUNT.
>>
>> For calculating "mapmax", we now use the average per-page mapcount in
>> a large folio instead of the per-page mapcount.
>>
>> For hugetlb folios and folios that are not partially mapped into MMs,
>> there is no change.
>>
>> Likely, this change will not matter much in practice, and an alternative
>> might be to simple remove this stat with CONFIG_NO_PAGE_MAPCOUNT.
>> However, there might be value to it, so let's keep it like that and
>> document the behavior.
>>
>> Signed-off-by: David Hildenbrand <david@...hat.com>
>> ---
>>   Documentation/filesystems/proc.rst | 5 +++++
>>   fs/proc/task_mmu.c                 | 7 ++++++-
>>   2 files changed, 11 insertions(+), 1 deletion(-)
>>
>> diff --git a/Documentation/filesystems/proc.rst b/Documentation/filesystems/proc.rst
>> index 09f0aed5a08ba..1aa190017f796 100644
>> --- a/Documentation/filesystems/proc.rst
>> +++ b/Documentation/filesystems/proc.rst
>> @@ -686,6 +686,11 @@ Where:
>>   node locality page counters (N0 == node0, N1 == node1, ...) and the kernel page
>>   size, in KB, that is backing the mapping up.
>>   
>> +Note that some kernel configurations do not track the precise number of times
>> +a page part of a larger allocation (e.g., THP) is mapped. In these
>> +configurations, "mapmax" might corresponds to the average number of mappings
>> +per page in such a larger allocation instead.
>> +
>>   1.2 Kernel data
>>   ---------------
>>   
>> diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
>> index 80839bbf9657f..d7ee842367f0f 100644
>> --- a/fs/proc/task_mmu.c
>> +++ b/fs/proc/task_mmu.c
>> @@ -2862,7 +2862,12 @@ static void gather_stats(struct page *page, struct numa_maps *md, int pte_dirty,
>>   			unsigned long nr_pages)
>>   {
>>   	struct folio *folio = page_folio(page);
>> -	int count = folio_precise_page_mapcount(folio, page);
>> +	int count;
>> +
>> +	if (IS_ENABLED(CONFIG_PAGE_MAPCOUNT))
>> +		count = folio_precise_page_mapcount(folio, page);
>> +	else
>> +		count = min_t(int, folio_average_page_mapcount(folio), 1);
> 
> s/min/max ?

Indeed, thanks!

> 
> Otherwise, count is at most 1. Anyway, if you change
> folio_average_page_mapcount() as I indicated in patch 16, this
> will become count = folio_average_page_mapcount(folio).

No, the average should not be 1 just because a single subpage is mapped.

-- 
Cheers,

David / dhildenb