[PATCH v2 19/20] fs/proc/task_mmu: remove per-page mapcount dependency for smaps/smaps_rollup (CONFIG_NO_PAGE_MAPCOUNT)

David Hildenbrand
2025-02-24T16:56:49Z

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no longer maintained -- soon with CONFIG_NO_PAGE_MAPCOUNT.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 | 13 +++++++++++++
fs/proc/internal.h | 8 ++++++++
fs/proc/task_mmu.c | 17 +++++++++++++++--
3 files changed, 36 insertions(+), 2 deletions(-)

diff --git a/Documentation/filesystems/proc.rst b/Documentation/filesystems/proc.rst
index 1aa190017f796..57d55274a1f42 100644
--- a/Documentation/filesystems/proc.rst
+++ b/Documentation/filesystems/proc.rst
@@ -506,6 +506,19 @@ 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.

+Note that in some kernel configurations, all pages part of a larger allocation
+(e.g., THP) might be considered "shared" if the large allocation is
+considered "shared": if not all pages are exclusive to the same process.
+Further, some kernel configurations might consider larger allocations "shared",
+if they were at one point considered "shared", even if they would now be
+considered "exclusive".
+
+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 16aa1fd260771..70205425a2daa 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 d7ee842367f0f..7ca0bc3bf417d 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