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 22:10, Zi Yan wrote:
> On 24 Feb 2025, at 16:02, David Hildenbrand wrote:
> 
>> 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 are no longer maintained -- soon with CONFIG_NO_PAGE_MAPCOUNT.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.
> 
> I thought the rounding would be (mapcount + 511) / 512.

Yes, that's "rounding up".

> But
> that means if one subpage is mapped, the average will be 1.
> Your rounding means if at least half of the subpages is mapped,
> the average will be 1. Others might think 1/3 is mapped,
> the average will be 1. That is why I think adjust looks like
> a magic number.

I think all callers could tolerate (or benefit) from 
folio_average_page_mapcount() returning at least 1 in case any page is 
mapped.

There was a reason why I decided to round to the nearest integer instead.

Let me think about this once more, I went back and forth a couple of 
times on this.

-- 
Cheers,

David / dhildenb