Re: [PATCH v2 16/20] fs/proc/page: remove per-page mapcount dependency for /proc/kpagecount (CONFIG_NO_PAGE_MAPCOUNT)

[David Hildenbrand <david@redhat.com>]

On 24.02.25 21:40, Zi Yan wrote:
> On Mon Feb 24, 2025 at 11:55 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 are no longer maintained -- soon with CONFIG_NO_PAGE_MAPCOUNT.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 large folios, we'll return the per-page average mapcount within the
>> folio, except when the average is 0 but the folio is mapped: then we
>> return 1.
>>
>> For hugetlb folios and for large folios that are fully mapped
>> into all address spaces, there is no change.
>>
>> As an alternative, we could simply return 0 for non-hugetlb large folios,
>> or disable this legacy interface with CONFIG_NO_PAGE_MAPCOUNT.
>>
>> But the information exposed by this interface can still be valuable, and
>> frequently we deal with fully-mapped large folios where the average
>> corresponds to the actual page mapcount. So we'll leave it like this for
>> now and document the new behavior.
>>
>> Note: this interface is likely not very relevant for performance. If
>> ever required, we could try doing a rather expensive rmap walk to collect
>> precisely how often this folio page is mapped.
>>
>> Signed-off-by: David Hildenbrand <david@redhat.com>
>> ---
>>   Documentation/admin-guide/mm/pagemap.rst |  7 +++++-
>>   fs/proc/internal.h                       | 31 ++++++++++++++++++++++++
>>   fs/proc/page.c                           | 19 ++++++++++++---
>>   3 files changed, 53 insertions(+), 4 deletions(-)
>>
>> diff --git a/Documentation/admin-guide/mm/pagemap.rst b/Documentation/admin-guide/mm/pagemap.rst
>> index caba0f52dd36c..49590306c61a0 100644
>> --- a/Documentation/admin-guide/mm/pagemap.rst
>> +++ b/Documentation/admin-guide/mm/pagemap.rst
>> @@ -42,7 +42,12 @@ There are four components to pagemap:
>>      skip over unmapped regions.
>>   
>>    * ``/proc/kpagecount``.  This file contains a 64-bit count of the number of
>> -   times each page is mapped, indexed by PFN.
>> +   times each page is mapped, indexed by PFN. 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, the average number of
>> +   mappings per page in this larger allocation is returned instead. However,
>> +   if any page of the large allocation is mapped, the returned value will
>> +   be at least 1.
>>   
>>   The page-types tool in the tools/mm directory can be used to query the
>>   number of times a page is mapped.
>> diff --git a/fs/proc/internal.h b/fs/proc/internal.h
>> index 1695509370b88..16aa1fd260771 100644
>> --- a/fs/proc/internal.h
>> +++ b/fs/proc/internal.h
>> @@ -174,6 +174,37 @@ static inline int folio_precise_page_mapcount(struct folio *folio,
>>   	return mapcount;
>>   }
>>   
>> +/**
>> + * folio_average_page_mapcount() - Average number of mappings per page in this
>> + *				   folio
>> + * @folio: The folio.
>> + *
>> + * The average number of present user page table entries that reference each
>> + * page in this folio as tracked via the RMAP: either referenced directly
>> + * (PTE) or as part of a larger area that covers this page (e.g., PMD).
>> + *
>> + * Returns: The average number of mappings per page in this folio. 0 for
>> + * folios that are not mapped to user space or are not tracked via the RMAP
>> + * (e.g., shared zeropage).
>> + */
>> +static inline int folio_average_page_mapcount(struct folio *folio)
>> +{
>> +	int mapcount, entire_mapcount;
>> +	unsigned int adjust;
>> +
>> +	if (!folio_test_large(folio))
>> +		return atomic_read(&folio->_mapcount) + 1;
>> +
>> +	mapcount = folio_large_mapcount(folio);
>> +	entire_mapcount = folio_entire_mapcount(folio);
>> +	if (mapcount <= entire_mapcount)
>> +		return entire_mapcount;
>> +	mapcount -= entire_mapcount;
>> +
>> +	adjust = folio_large_nr_pages(folio) / 2;

Thanks for the review!

> 
> Is there any reason for choosing this adjust number? A comment might be
> helpful in case people want to change it later, either with some reasoning
> or just saying it is chosen empirically.

We're dividing by folio_large_nr_pages(folio) (shifting by 
folio_large_order(folio)), so this is not a magic number at all.

So this should be "ordinary" rounding.

Assume nr_pages = 512.

With 255 we want to round down, with 256 we want to round up.

255 / 512 = 0 :)
256 / 512 = 0 :(

Compared to:

(255 + (512 / 2)) / 512 = (255 + 256) / 512 = 0 :)
(256 + (512 / 2)) / 512 = (256 + 256) / 512 = 1 :)

> 
>> +	return ((mapcount + adjust) >> folio_large_order(folio)) +
>> +		entire_mapcount;
>> +}
>>   /*
>>    * array.c
>>    */
>> diff --git a/fs/proc/page.c b/fs/proc/page.c
>> index a55f5acefa974..4d3290cc69667 100644
>> --- a/fs/proc/page.c
>> +++ b/fs/proc/page.c
>> @@ -67,9 +67,22 @@ static ssize_t kpagecount_read(struct file *file, char __user *buf,
>>   		 * memmaps that were actually initialized.
>>   		 */
>>   		page = pfn_to_online_page(pfn);
>> -		if (page)
>> -			mapcount = folio_precise_page_mapcount(page_folio(page),
>> -							       page);
>> +		if (page) {
>> +			struct folio *folio = page_folio(page);
>> +
>> +			if (IS_ENABLED(CONFIG_PAGE_MAPCOUNT)) {
>> +				mapcount = folio_precise_page_mapcount(folio, page);
>> +			} else {
>> +				/*
>> +				 * Indicate the per-page average, but at least "1" for
>> +				 * mapped folios.
>> +				 */
>> +				mapcount = folio_average_page_mapcount(folio);
>> +				if (!mapcount && folio_test_large(folio) &&
>> +				    folio_mapped(folio))
>> +					mapcount = 1;
> 
> This should be part of folio_average_page_mapcount() right?

No, that's not desired.

> Otherwise, the comment on folio_average_page_mapcount() is not correct,
> since it can return 0 when a folio is mapped to user space.

It's misleading. I'll clarify the comment, probably simply saying:

Returns: The average number of mappings per page in this folio.

Thanks!

-- 
Cheers,

David / dhildenb