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

Zi Yan
2025-02-24T21:44:48Z

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 are no longer maintained -- soon with CONFIG_NO_PAGE_MAPCOUNT.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.

Best Regards,
Yan, Zi