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 22:23, 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 are no longer maintained -- soon with CONFIG_NO_PAGE_MAPCOUNT.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);
+}


-- 
Cheers,

David / dhildenb