Re: [PATCH v2 19/20] fs/proc/task_mmu: remove per-page mapcount dependency for smaps/smaps_rollup (CONFIG_NO_PAGE_MAPCOUNT)

[David Hildenbrand <david@redhat.com>]

On 24.02.25 21:53, 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 no longer maintained -- soon with CONFIG_NO_PAGE_MAPCOUNT.
>>
>> When computing the output for smaps / smaps_rollups, in particular when
>> calculating the USS (Unique Set Size) and the PSS (Proportional Set Size),
>> we still rely on per-page mapcounts.
>>
>> To determine private vs. shared, we'll use folio_likely_mapped_shared(),
>> similar to how we handle PM_MMAP_EXCLUSIVE. Similarly, we might now
>> under-estimate the USS and count pages towards "shared" that are
>> actually "private" ("exclusively mapped").
>>
>> When calculating the PSS, we'll now also use the average per-page
>> mapcount for large folios: this can result in both, an over-estimation
>> and an under-estimation of the PSS. The difference is not expected to
>> matter much in practice, but we'll have to learn as we go.
>>
>> We can now provide folio_precise_page_mapcount() only with
>> CONFIG_PAGE_MAPCOUNT, and remove one of the last users of per-page
>> mapcounts when CONFIG_NO_PAGE_MAPCOUNT is enabled.
>>
>> Document the new behavior.
>>
>> Signed-off-by: David Hildenbrand <david@redhat.com>
>> ---
>>   Documentation/filesystems/proc.rst | 13 +++++++++++++
>>   fs/proc/internal.h                 |  8 ++++++++
>>   fs/proc/task_mmu.c                 | 17 +++++++++++++++--
>>   3 files changed, 36 insertions(+), 2 deletions(-)
>>
>> diff --git a/Documentation/filesystems/proc.rst b/Documentation/filesystems/proc.rst
>> index 1aa190017f796..57d55274a1f42 100644
>> --- a/Documentation/filesystems/proc.rst
>> +++ b/Documentation/filesystems/proc.rst
>> @@ -506,6 +506,19 @@ Note that even a page which is part of a MAP_SHARED mapping, but has only
>>   a single pte mapped, i.e.  is currently used by only one process, is accounted
>>   as private and not as shared.
>>   
>> +Note that in some kernel configurations, all pages part of a larger allocation
>> +(e.g., THP) might be considered "shared" if the large allocation is
>> +considered "shared": if not all pages are exclusive to the same process.
>> +Further, some kernel configurations might consider larger allocations "shared",
>> +if they were at one point considered "shared", even if they would now be
>> +considered "exclusive".
>> +
>> +Some kernel configurations do not track the precise number of times a page part
>> +of a larger allocation is mapped. In this case, when calculating the PSS, the
>> +average number of mappings per page in this larger allocation might be used
>> +as an approximation for the number of mappings of a page. The PSS calculation
>> +will be imprecise in this case.
>> +
>>   "Referenced" indicates the amount of memory currently marked as referenced or
>>   accessed.
>>   
>> diff --git a/fs/proc/internal.h b/fs/proc/internal.h
>> index 16aa1fd260771..70205425a2daa 100644
>> --- a/fs/proc/internal.h
>> +++ b/fs/proc/internal.h
>> @@ -143,6 +143,7 @@ unsigned name_to_int(const struct qstr *qstr);
>>   /* Worst case buffer size needed for holding an integer. */
>>   #define PROC_NUMBUF 13
>>   
>> +#ifdef CONFIG_PAGE_MAPCOUNT
>>   /**
>>    * folio_precise_page_mapcount() - Number of mappings of this folio page.
>>    * @folio: The folio.
>> @@ -173,6 +174,13 @@ static inline int folio_precise_page_mapcount(struct folio *folio,
>>   
>>   	return mapcount;
>>   }
>> +#else /* !CONFIG_PAGE_MAPCOUNT */
>> +static inline int folio_precise_page_mapcount(struct folio *folio,
>> +		struct page *page)
>> +{
>> +	BUILD_BUG();
>> +}
>> +#endif /* CONFIG_PAGE_MAPCOUNT */
>>   
>>   /**
>>    * folio_average_page_mapcount() - Average number of mappings per page in this
>> diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
>> index d7ee842367f0f..7ca0bc3bf417d 100644
>> --- a/fs/proc/task_mmu.c
>> +++ b/fs/proc/task_mmu.c
>> @@ -707,6 +707,8 @@ static void smaps_account(struct mem_size_stats *mss, struct page *page,
>>   	struct folio *folio = page_folio(page);
>>   	int i, nr = compound ? compound_nr(page) : 1;
>>   	unsigned long size = nr * PAGE_SIZE;
>> +	bool exclusive;
>> +	int mapcount;
>>   
>>   	/*
>>   	 * First accumulate quantities that depend only on |size| and the type
>> @@ -747,18 +749,29 @@ static void smaps_account(struct mem_size_stats *mss, struct page *page,
>>   				      dirty, locked, present);
>>   		return;
>>   	}
>> +
>> +	if (IS_ENABLED(CONFIG_NO_PAGE_MAPCOUNT)) {
>> +		mapcount = folio_average_page_mapcount(folio);
> 
> This seems inconsistent with how folio_average_page_mapcount() is used
> in patch 16 and 18.

It only looks that way. Note that the code below only does

>>   		if (mapcount >= 2)
>>   			pss /= mapcount;

Having it as 0 or 1 doesn't matter in that regard.

Thanks!

-- 
Cheers,

David / dhildenb