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

[David Hildenbrand <david@redhat.com>]

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are no longer maintained -- soon with CONFIG_NO_PAGE_MAPCOUNT.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