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no longer maintained -- soon with CONFIG_NO_PAGE_MAPCOUNT.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@xxxxxxxxxx>
---
Documentation/filesystems/proc.rst | 13 +++++++++++++
fs/proc/internal.h | 2 ++
fs/proc/task_mmu.c | 17 +++++++++++++++--
3 files changed, 30 insertions(+), 2 deletions(-)
diff --git a/Documentation/filesystems/proc.rst b/Documentation/filesystems/proc.rst
index bed03e77c0f91..7cbab4135f244 100644
--- a/Documentation/filesystems/proc.rst
+++ b/Documentation/filesystems/proc.rst
@@ -504,6 +504,19 @@ 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 3c687f97e18c4..8c9ef19526d2b 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 +174,7 @@ static inline int folio_precise_page_mapcount(struct folio *folio,
return mapcount;
}
+#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 3d9fe99346478..30306e231ff04 100644
--- a/fs/proc/task_mmu.c
+++ b/fs/proc/task_mmu.c
@@ -734,6 +734,8 @@ 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
@@ -774,18 +776,29 @@ static void smaps_account(struct mem_size_stats *mss, struct page *page,
dirty, locked, present);
return;
}
+
+#ifndef CONFIG_PAGE_MAPCOUNT
+ mapcount = folio_average_page_mapcount(folio);
+ exclusive = !folio_likely_mapped_shared(folio);
+#endif
+
/*
* 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;
+
+#ifdef CONFIG_PAGE_MAPCOUNT
+ mapcount = folio_precise_page_mapcount(folio, page);
+ exclusive = mapcount < 2;
+#endif
+
if (mapcount >= 2)
pss /= mapcount;
smaps_page_accumulate(mss, folio, PAGE_SIZE, pss,
- dirty, locked, mapcount < 2);
+ dirty, locked, exclusive);
}
}
--
2.46.0
