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

Zi Yan
2025-02-24T20:40:19Z

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 are no longer maintained -- soon with CONFIG_NO_PAGE_MAPCOUNT.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;

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.

> + return ((mapcount + adjust) >> folio_large_order(folio)) +
> + entire_mapcount;
> +}
> /*
> * array.c
> */
> diff --git a/fs/proc/page.c b/fs/proc/page.c
> index a55f5acefa974..4d3290cc69667 100644
> --- a/fs/proc/page.c
> +++ b/fs/proc/page.c
> @@ -67,9 +67,22 @@ static ssize_t kpagecount_read(struct file *file, char __user *buf,
> * memmaps that were actually initialized.
> */
> page = pfn_to_online_page(pfn);
> - if (page)
> - mapcount = folio_precise_page_mapcount(page_folio(page),
> - page);
> + if (page) {
> + struct folio *folio = page_folio(page);
> +
> + if (IS_ENABLED(CONFIG_PAGE_MAPCOUNT)) {
> + mapcount = folio_precise_page_mapcount(folio, page);
> + } else {
> + /*
> + * Indicate the per-page average, but at least "1" for
> + * mapped folios.
> + */
> + mapcount = folio_average_page_mapcount(folio);
> + if (!mapcount && folio_test_large(folio) &&
> + folio_mapped(folio))
> + mapcount = 1;

This should be part of folio_average_page_mapcount() right?
Otherwise, the comment on folio_average_page_mapcount() is not correct,
since it can return 0 when a folio is mapped to user space.

> + }
> + }
>
> if (put_user(mapcount, out)) {
> ret = -EFAULT;

--
Best Regards,
Yan, Zi