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

[Zi Yan <ziy@nvidia.com>]

On 24 Feb 2025, at 16:15, David Hildenbrand wrote:

> 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 are no longer maintained -- soon with CONFIG_NO_PAGE_MAPCOUNT.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.

Sure. Your current choice might be good enough for now. My intend of
adding a comment here is just to let people know the adjust can be
changed in the future. :)


Best Regards,
Yan, Zi