[PATCH v3 19/20] fs/proc/task_mmu: remove per-page mapcount dependency for smaps/smaps_rollup (CONFIG_NO_PAGE_MAPCOUNT)

David Hildenbrand
2025-03-03T16:31:09Z

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no longer maintained -- soon with CONFIG_NO_PAGE_MAPCOUNT.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