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 22:44, Zi Yan wrote:
> On 24 Feb 2025, at 16:42, David Hildenbrand wrote:
> 
>> On 24.02.25 22:23, Zi Yan wrote:
>>> 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 are no longer maintained -- soon with CONFIG_NO_PAGE_MAPCOUNT.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. :)
>>
>> The following will make the callers easier to read, while keeping
>> the rounding to the next integer for the other cases untouched.
>>
>> +/**
>> + * 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).
>> + *
>> + * The average is calculated by rounding to the nearest integer; however,
>> + * if at least a single page is mapped, the average will be at least 1.
>> + *
>> + * Returns: The average number of mappings per page in this folio.
>> + */
>> +static inline int folio_average_page_mapcount(struct folio *folio)
>> +{
>> +       int mapcount, entire_mapcount, avg;
>> +
>> +       if (!folio_test_large(folio))
>> +               return atomic_read(&folio->_mapcount) + 1;
>> +
>> +       mapcount = folio_large_mapcount(folio);
>> +       if (unlikely(mapcount <= 0))
>> +               return 0;
>> +       entire_mapcount = folio_entire_mapcount(folio);
>> +       if (mapcount <= entire_mapcount)
>> +               return entire_mapcount;
>> +       mapcount -= entire_mapcount;
>> +
>> +       /* Round to closest integer ... */
>> +       avg = (mapcount + folio_large_nr_pages(folio) / 2) >> folio_large_order(folio);
>> +       avg += entire_mapcount;
>> +       /* ... but return at least 1. */
>> +       return max_t(int, avg, 1);
>> +}
> 
> LGTM. Thanks.

Thanks! BTW, I think I chose the "round to closest integer" primarily
to make the PSS estimate a bit better. But that is indeed something
that can be adjusted easily later.

BTW, as commented in the cover letter, being able to calculate the
avg without the entire_mapcount could clean this function up quite a bit
(and make it completely atomic), but that will require more work.

-- 
Cheers,

David / dhildenb