Re: [PATCH v2 19/20] fs/proc/task_mmu: remove per-page mapcount dependency for smaps/smaps_rollup (CONFIG_NO_PAGE_MAPCOUNT)

Zi Yan
2025-02-24T20:54:08Z

On Mon Feb 24, 2025 at 11:56 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 no longer maintained -- soon with CONFIG_NO_PAGE_MAPCOUNT.
>
> When computing the output for smaps / smaps_rollups, in particular when
> calculating the USS (Unique Set Size) and the PSS (Proportional Set Size),
> we still rely on per-page mapcounts.
>
> To determine private vs. shared, we'll use folio_likely_mapped_shared(),
> similar to how we handle PM_MMAP_EXCLUSIVE. Similarly, we might now
> under-estimate the USS and count pages towards "shared" that are
> actually "private" ("exclusively mapped").
>
> When calculating the PSS, we'll now also use the average per-page
> mapcount for large folios: this can result in both, an over-estimation
> and an under-estimation of the PSS. The difference is not expected to
> matter much in practice, but we'll have to learn as we go.
>
> We can now provide folio_precise_page_mapcount() only with
> CONFIG_PAGE_MAPCOUNT, and remove one of the last users of per-page
> mapcounts when CONFIG_NO_PAGE_MAPCOUNT is enabled.
>
> Document the new behavior.
>
> Signed-off-by: David Hildenbrand <david@redhat.com>
> ---
> Documentation/filesystems/proc.rst | 13 +++++++++++++
> fs/proc/internal.h | 8 ++++++++
> fs/proc/task_mmu.c | 17 +++++++++++++++--
> 3 files changed, 36 insertions(+), 2 deletions(-)
>
> diff --git a/Documentation/filesystems/proc.rst b/Documentation/filesystems/proc.rst
> index 1aa190017f796..57d55274a1f42 100644
> --- a/Documentation/filesystems/proc.rst
> +++ b/Documentation/filesystems/proc.rst
> @@ -506,6 +506,19 @@ Note that even a page which is part of a MAP_SHARED mapping, but has only
> a single pte mapped, i.e. is currently used by only one process, is accounted
> as private and not as shared.
>
> +Note that in some kernel configurations, all pages part of a larger allocation
> +(e.g., THP) might be considered "shared" if the large allocation is
> +considered "shared": if not all pages are exclusive to the same process.
> +Further, some kernel configurations might consider larger allocations "shared",
> +if they were at one point considered "shared", even if they would now be
> +considered "exclusive".
> +
> +Some kernel configurations do not track the precise number of times a page part
> +of a larger allocation is mapped. In this case, when calculating the PSS, the
> +average number of mappings per page in this larger allocation might be used
> +as an approximation for the number of mappings of a page. The PSS calculation
> +will be imprecise in this case.
> +
> "Referenced" indicates the amount of memory currently marked as referenced or
> accessed.
>
> diff --git a/fs/proc/internal.h b/fs/proc/internal.h
> index 16aa1fd260771..70205425a2daa 100644
> --- a/fs/proc/internal.h
> +++ b/fs/proc/internal.h
> @@ -143,6 +143,7 @@ unsigned name_to_int(const struct qstr *qstr);
> /* Worst case buffer size needed for holding an integer. */
> #define PROC_NUMBUF 13
>
> +#ifdef CONFIG_PAGE_MAPCOUNT
> /**
> * folio_precise_page_mapcount() - Number of mappings of this folio page.
> * @folio: The folio.
> @@ -173,6 +174,13 @@ static inline int folio_precise_page_mapcount(struct folio *folio,
>
> return mapcount;
> }
> +#else /* !CONFIG_PAGE_MAPCOUNT */
> +static inline int folio_precise_page_mapcount(struct folio *folio,
> + struct page *page)
> +{
> + BUILD_BUG();
> +}
> +#endif /* CONFIG_PAGE_MAPCOUNT */
>
> /**
> * folio_average_page_mapcount() - Average number of mappings per page in this
> diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
> index d7ee842367f0f..7ca0bc3bf417d 100644
> --- a/fs/proc/task_mmu.c
> +++ b/fs/proc/task_mmu.c
> @@ -707,6 +707,8 @@ static void smaps_account(struct mem_size_stats *mss, struct page *page,
> struct folio *folio = page_folio(page);
> int i, nr = compound ? compound_nr(page) : 1;
> unsigned long size = nr * PAGE_SIZE;
> + bool exclusive;
> + int mapcount;
>
> /*
> * First accumulate quantities that depend only on |size| and the type
> @@ -747,18 +749,29 @@ static void smaps_account(struct mem_size_stats *mss, struct page *page,
> dirty, locked, present);
> return;
> }
> +
> + if (IS_ENABLED(CONFIG_NO_PAGE_MAPCOUNT)) {
> + mapcount = folio_average_page_mapcount(folio);

This seems inconsistent with how folio_average_page_mapcount() is used
in patch 16 and 18.

> + exclusive = !folio_maybe_mapped_shared(folio);
> + }
> +
> /*
> * We obtain a snapshot of the mapcount. Without holding the folio lock
> * this snapshot can be slightly wrong as we cannot always read the
> * mapcount atomically.
> */
> for (i = 0; i < nr; i++, page++) {
> - int mapcount = folio_precise_page_mapcount(folio, page);
> unsigned long pss = PAGE_SIZE << PSS_SHIFT;
> +
> + if (IS_ENABLED(CONFIG_PAGE_MAPCOUNT)) {
> + mapcount = folio_precise_page_mapcount(folio, page);
> + exclusive = mapcount < 2;
> + }
> +
> if (mapcount >= 2)
> pss /= mapcount;
> smaps_page_accumulate(mss, folio, PAGE_SIZE, pss,
> - dirty, locked, mapcount < 2);
> + dirty, locked, exclusive);
> }
> }
>

--
Best Regards,
Yan, Zi