[PATCH RFC 02/13] fs/proc/task_mmu: remove CONFIG_PAGE_MAPCOUNT handling for "mapmax"

["David Hildenbrand (Arm)" <david@kernel.org>]

In preparation for removing CONFIG_PAGE_MAPCOUNT, let's always use a
folio's average page mapcount instead of the precise page mapcount when
calculating "mapmax".

Update the doc to state that this behavior no longer depends on the
kernel config. While at it, make it clearer what "mapmax" actually
expresses.

For small folios, or large folios that are mostly fully-mapped, there is
no change at all.

Signed-off-by: David Hildenbrand (Arm) <david@kernel.org>
---
 Documentation/filesystems/proc.rst |  8 ++++----
 fs/proc/task_mmu.c                 | 11 +++--------
 2 files changed, 7 insertions(+), 12 deletions(-)

diff --git a/Documentation/filesystems/proc.rst b/Documentation/filesystems/proc.rst
index 628364b0f69f..1224dc73e089 100644
--- a/Documentation/filesystems/proc.rst
+++ b/Documentation/filesystems/proc.rst
@@ -699,10 +699,10 @@ Where:
 node locality page counters (N0 == node0, N1 == node1, ...) and the kernel page
 size, in KB, that is backing the mapping up.
 
-Note that 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, "mapmax" might corresponds to the average number of mappings
-per page in such a larger allocation instead.
+"mapmax" is the maximum page mapcount of any page in the mapping, i.e.,
+the highest sharing level observed. For pages that are part of larger
+allocations (e.g., THP), it is derived from the average mapcount per page
+in the allocation, since precise per-page mapcounts are not available.
 
 1.2 Kernel data
 ---------------
diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
index e091931d7ca1..ad0989d101ab 100644
--- a/fs/proc/task_mmu.c
+++ b/fs/proc/task_mmu.c
@@ -3137,12 +3137,7 @@ static void gather_stats(struct page *page, struct numa_maps *md, int pte_dirty,
 			unsigned long nr_pages)
 {
 	struct folio *folio = page_folio(page);
-	int count;
-
-	if (IS_ENABLED(CONFIG_PAGE_MAPCOUNT))
-		count = folio_precise_page_mapcount(folio, page);
-	else
-		count = folio_average_page_mapcount(folio);
+	const int mapcount = folio_average_page_mapcount(folio);
 
 	md->pages += nr_pages;
 	if (pte_dirty || folio_test_dirty(folio))
@@ -3160,8 +3155,8 @@ static void gather_stats(struct page *page, struct numa_maps *md, int pte_dirty,
 	if (folio_test_anon(folio))
 		md->anon += nr_pages;
 
-	if (count > md->mapcount_max)
-		md->mapcount_max = count;
+	if (mapcount > md->mapcount_max)
+		md->mapcount_max = mapcount;
 
 	md->node[folio_nid(folio)] += nr_pages;
 }

-- 
2.43.0