@@ -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.
@@ -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
*/
@@ -67,9 +67,21 @@ 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);
+
+#ifdef CONFIG_PAGE_MAPCOUNT
+ mapcount = folio_precise_page_mapcount(folio, page);
+#else /* !CONFIG_PAGE_MAPCOUNT */
+ /*
+ * 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;
+#endif /* !CONFIG_PAGE_MAPCOUNT */
+ }
if (put_user(mapcount, out)) {
ret = -EFAULT;
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. Signed-off-by: David Hildenbrand <david@redhat.com> --- Documentation/admin-guide/mm/pagemap.rst | 7 +++++- fs/proc/internal.h | 31 ++++++++++++++++++++++++ fs/proc/page.c | 18 +++++++++++--- 3 files changed, 52 insertions(+), 4 deletions(-)