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

David Hildenbrand
2025-02-24T16:56:43Z

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are no longer maintained -- soon with CONFIG_NO_PAGE_MAPCOUNT.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;
+ 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;
+ }
+ }

if (put_user(mapcount, out)) {
ret = -EFAULT;
--
2.48.1