[PATCH v3 19/20] fs/proc/task_mmu: remove per-page mapcount dependency for smaps/smaps_rollup (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no longer maintained -- soon with CONFIG_NO_PAGE_MAPCOUNT.

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 | 22 +++++++++++++++++++---
 fs/proc/internal.h                 |  8 ++++++++
 fs/proc/task_mmu.c                 | 17 +++++++++++++++--
 3 files changed, 42 insertions(+), 5 deletions(-)

diff --git a/Documentation/filesystems/proc.rst b/Documentation/filesystems/proc.rst
index 1aa190017f796..c9e62e8e0685e 100644
--- a/Documentation/filesystems/proc.rst
+++ b/Documentation/filesystems/proc.rst
@@ -502,9 +502,25 @@ process, its PSS will be 1500.  "Pss_Dirty" is the portion of PSS which
 consists of dirty pages.  ("Pss_Clean" is not included, but it can be
 calculated by subtracting "Pss_Dirty" from "Pss".)
 
-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.
+Traditionally, a page is accounted as "private" if it is mapped exactly once,
+and a page is accounted as "shared" when mapped multiple times, even when
+mapped in the same process multiple times. Note that this accounting is
+independent of MAP_SHARED.
+
+In some kernel configurations, the semantics of pages part of a larger
+allocation (e.g., THP) can differ: a page is accounted as "private" if all
+pages part of the corresponding large allocation are *certainly* mapped in the
+same process, even if the page is mapped multiple times in that process. A
+page is accounted as "shared" if any page page of the larger allocation
+is *maybe* mapped in a different process. In some cases, a large allocation
+might be treated as "maybe mapped by multiple processes" even though this
+is no longer the case.
+
+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 96ea58e843114..8c921bc8652d9 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 5043376ebd476..061f16b767118 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);
+		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);
 	}
 }
 
-- 
2.48.1