[PATCH v2 20/20] mm: stop maintaining the per-page mapcount of large folios (CONFIG_NO_PAGE_MAPCOUNT)

David Hildenbrand
2025-02-24T16:56:51Z

Everything is in place to stop using the per-page mapcounts in large
folios: the mapcount of tail pages will always be logically 0 (-1 value),
just like it currently is for hugetlb folios already, and the page
mapcount of the head page is either 0 (-1 value) or contains a page type
(e.g., hugetlb).

Maintaining _nr_pages_mapped without per-page mapcounts is impossible,
so that one also has to go with CONFIG_NO_PAGE_MAPCOUNT.

There are two remaining implications:

(1) Per-node, per-cgroup and per-lruvec stats of "NR_ANON_MAPPED"
("mapped anonymous memory") and "NR_FILE_MAPPED"
("mapped file memory"):

As soon as any page of the folio is mapped -- folio_mapped() -- we
now account the complete folio as mapped. Once the last page is
unmapped -- !folio_mapped() -- we account the complete folio as
unmapped.

This implies that ...

* "AnonPages" and "Mapped" in /proc/meminfo and
/sys/devices/system/node/*/meminfo
* cgroup v2: "anon" and "file_mapped" in "memory.stat" and
"memory.numa_stat"
* cgroup v1: "rss" and "mapped_file" in "memory.stat" and
"memory.numa_stat

... can now appear higher than before. But note that these folios do
consume that memory, simply not all pages are actually currently
mapped.

It's worth nothing that other accounting in the kernel (esp. cgroup
charging on allocation) is not affected by this change.

[why oh why is "anon" called "rss" in cgroup v1]

(2) Detecting partial mappings

Detecting whether anon THPs are partially mapped gets a bit more
unreliable. As long as a single MM maps such a large folio
("exclusively mapped"), we can reliably detect it. Especially before
fork() / after a short-lived child process quit, we will detect
partial mappings reliably, which is the common case.

In essence, if the average per-page mapcount in an anon THP is < 1,
we know for sure that we have a partial mapping.

However, as soon as multiple MMs are involved, we might miss detecting
partial mappings: this might be relevant with long-lived child
processes. If we have a fully-mapped anon folio before fork(), once
our child processes and our parent all unmap (zap/COW) the same pages
(but not the complete folio), we might not detect the partial mapping.
However, once the child processes quit we would detect the partial
mapping.

How relevant this case is in practice remains to be seen.
Swapout/migration will likely mitigate this.

In the future, RMAP walkers could check for that for that case
(e.g., when collecting access bits during reclaim) and simply flag
them for deferred-splitting.

Signed-off-by: David Hildenbrand <david@redhat.com>
---
.../admin-guide/cgroup-v1/memory.rst | 4 +
Documentation/admin-guide/cgroup-v2.rst | 10 ++-
Documentation/filesystems/proc.rst | 10 ++-
Documentation/mm/transhuge.rst | 31 +++++--
include/linux/rmap.h | 35 ++++++--
mm/internal.h | 5 +-
mm/page_alloc.c | 3 +-
mm/rmap.c | 80 +++++++++++++++++--
8 files changed, 150 insertions(+), 28 deletions(-)

diff --git a/Documentation/admin-guide/cgroup-v1/memory.rst b/Documentation/admin-guide/cgroup-v1/memory.rst
index 286d16fc22ebb..53cf081b22e81 100644
--- a/Documentation/admin-guide/cgroup-v1/memory.rst
+++ b/Documentation/admin-guide/cgroup-v1/memory.rst
@@ -609,6 +609,10 @@ memory.stat file includes following statistics:

'rss + mapped_file" will give you resident set size of cgroup.

+ Note that some kernel configurations might account complete larger
+ allocations (e.g., THP) towards 'rss' and 'mapped_file', even if
+ only some, but not all that memory is mapped.
+
(Note: file and shmem may be shared among other cgroups. In that case,
mapped_file is accounted only when the memory cgroup is owner of page
cache.)
diff --git a/Documentation/admin-guide/cgroup-v2.rst b/Documentation/admin-guide/cgroup-v2.rst
index 175e9435ad5c1..53ada5c2620a7 100644
--- a/Documentation/admin-guide/cgroup-v2.rst
+++ b/Documentation/admin-guide/cgroup-v2.rst
@@ -1448,7 +1448,10 @@ The following nested keys are defined.

anon
Amount of memory used in anonymous mappings such as
- brk(), sbrk(), and mmap(MAP_ANONYMOUS)
+ brk(), sbrk(), and mmap(MAP_ANONYMOUS). Note that
+ some kernel configurations might account complete larger
+ allocations (e.g., THP) if only some, but not all the
+ memory of such an allocation is mapped anymore.

file
Amount of memory used to cache filesystem data,
@@ -1491,7 +1494,10 @@ The following nested keys are defined.
Amount of application memory swapped out to zswap.

file_mapped
- Amount of cached filesystem data mapped with mmap()
+ Amount of cached filesystem data mapped with mmap(). Note
+ that some kernel configurations might account complete
+ larger allocations (e.g., THP) if only some, but not
+ not all the memory of such an allocation is mapped.

file_dirty
Amount of cached filesystem data that was modified but
diff --git a/Documentation/filesystems/proc.rst b/Documentation/filesystems/proc.rst
index 57d55274a1f42..c5052acfa0747 100644
--- a/Documentation/filesystems/proc.rst
+++ b/Documentation/filesystems/proc.rst
@@ -1150,9 +1150,15 @@ Dirty
Writeback
Memory which is actively being written back to the disk
AnonPages
- Non-file backed pages mapped into userspace page tables
+ Non-file backed pages mapped into userspace page tables. Note that
+ some kernel configurations might consider all pages part of a
+ larger allocation (e.g., THP) as "mapped", as soon as a single
+ page is mapped.
Mapped
- files which have been mmapped, such as libraries
+ files which have been mmapped, such as libraries. Note that some
+ kernel configurations might consider all pages part of a larger
+ allocation (e.g., THP) as "mapped", as soon as a single page is
+ mapped.
Shmem
Total memory used by shared memory (shmem) and tmpfs
KReclaimable
diff --git a/Documentation/mm/transhuge.rst b/Documentation/mm/transhuge.rst
index baa17d718a762..0e7f8e4cd2e33 100644
--- a/Documentation/mm/transhuge.rst
+++ b/Documentation/mm/transhuge.rst
@@ -116,23 +116,28 @@ pages:
succeeds on tail pages.

- map/unmap of a PMD entry for the whole THP increment/decrement
- folio->_entire_mapcount, increment/decrement folio->_large_mapcount
- and also increment/decrement folio->_nr_pages_mapped by ENTIRELY_MAPPED
- when _entire_mapcount goes from -1 to 0 or 0 to -1.
+ folio->_entire_mapcount and folio->_large_mapcount.

We also maintain the two slots for tracking MM owners (MM ID and
corresponding mapcount), and the current status ("maybe mapped shared" vs.
"mapped exclusively").

+ With CONFIG_PAGE_MAPCOUNT, we also increment/decrement
+ folio->_nr_pages_mapped by ENTIRELY_MAPPED when _entire_mapcount goes
+ from -1 to 0 or 0 to -1.
+
- map/unmap of individual pages with PTE entry increment/decrement
- page->_mapcount, increment/decrement folio->_large_mapcount and also
- increment/decrement folio->_nr_pages_mapped when page->_mapcount goes
- from -1 to 0 or 0 to -1 as this counts the number of pages mapped by PTE.
+ folio->_large_mapcount.

We also maintain the two slots for tracking MM owners (MM ID and
corresponding mapcount), and the current status ("maybe mapped shared" vs.
"mapped exclusively").

+ With CONFIG_PAGE_MAPCOUNT, we also increment/decrement
+ page->_mapcount and increment/decrement folio->_nr_pages_mapped when
+ page->_mapcount goes from -1 to 0 or 0 to -1 as this counts the number
+ of pages mapped by PTE.
+
split_huge_page internally has to distribute the refcounts in the head
page to the tail pages before clearing all PG_head/tail bits from the page
structures. It can be done easily for refcounts taken by page table
@@ -159,8 +164,8 @@ clear where references should go after split: it will stay on the head page.
Note that split_huge_pmd() doesn't have any limitations on refcounting:
pmd can be split at any point and never fails.

-Partial unmap and deferred_split_folio()
-========================================
+Partial unmap and deferred_split_folio() (anon THP only)
+========================================================

Unmapping part of THP (with munmap() or other way) is not going to free
memory immediately. Instead, we detect that a subpage of THP is not in use
@@ -175,3 +180,13 @@ a THP crosses a VMA boundary.
The function deferred_split_folio() is used to queue a folio for splitting.
The splitting itself will happen when we get memory pressure via shrinker
interface.
+
+With CONFIG_PAGE_MAPCOUNT, we reliably detect partial mappings based on
+folio->_nr_pages_mapped.
+
+With CONFIG_NO_PAGE_MAPCOUNT, we detect partial mappings based on the
+average per-page mapcount in a THP: if the average is < 1, an anon THP is
+certainly partially mapped. As long as only a single process maps a THP,
+this detection is reliable. With long-running child processes, there can
+be scenarios where partial mappings can currently not be detected, and
+might need asynchronous detection during memory reclaim in the future.
diff --git a/include/linux/rmap.h b/include/linux/rmap.h
index c131b0efff0fa..6b82b618846ee 100644
--- a/include/linux/rmap.h
+++ b/include/linux/rmap.h
@@ -240,7 +240,7 @@ static __always_inline void folio_set_large_mapcount(struct folio *folio,
folio_set_mm_id(folio, 0, vma->vm_mm->mm_id);
}

-static __always_inline void folio_add_large_mapcount(struct folio *folio,
+static __always_inline int folio_add_return_large_mapcount(struct folio *folio,
int diff, struct vm_area_struct *vma)
{
const mm_id_t mm_id = vma->vm_mm->mm_id;
@@ -286,9 +286,11 @@ static __always_inline void folio_add_large_mapcount(struct folio *folio,
folio->_mm_ids |= FOLIO_MM_IDS_SHARED_BIT;
}
folio_unlock_large_mapcount(folio);
+ return new_mapcount_val + 1;
}
+#define folio_add_large_mapcount folio_add_return_large_mapcount

-static __always_inline void folio_sub_large_mapcount(struct folio *folio,
+static __always_inline int folio_sub_return_large_mapcount(struct folio *folio,
int diff, struct vm_area_struct *vma)
{
const mm_id_t mm_id = vma->vm_mm->mm_id;
@@ -331,7 +333,9 @@ static __always_inline void folio_sub_large_mapcount(struct folio *folio,
folio->_mm_ids &= ~FOLIO_MM_IDS_SHARED_BIT;
out:
folio_unlock_large_mapcount(folio);
+ return new_mapcount_val + 1;
}
+#define folio_sub_large_mapcount folio_sub_return_large_mapcount
#else /* !CONFIG_MM_ID */
/*
* See __folio_rmap_sanity_checks(), we might map large folios even without
@@ -350,17 +354,33 @@ static inline void folio_add_large_mapcount(struct folio *folio,
atomic_add(diff, &folio->_large_mapcount);
}

+static inline int folio_add_return_large_mapcount(struct folio *folio,
+ int diff, struct vm_area_struct *vma)
+{
+ BUILD_BUG();
+}
+
static inline void folio_sub_large_mapcount(struct folio *folio,
int diff, struct vm_area_struct *vma)
{
atomic_sub(diff, &folio->_large_mapcount);
}
+
+static inline int folio_sub_return_large_mapcount(struct folio *folio,
+ int diff, struct vm_area_struct *vma)
+{
+ BUILD_BUG();
+}
#endif /* CONFIG_MM_ID */

#define folio_inc_large_mapcount(folio, vma) \
folio_add_large_mapcount(folio, 1, vma)
+#define folio_inc_return_large_mapcount(folio, vma) \
+ folio_add_return_large_mapcount(folio, 1, vma)
#define folio_dec_large_mapcount(folio, vma) \
folio_sub_large_mapcount(folio, 1, vma)
+#define folio_dec_return_large_mapcount(folio, vma) \
+ folio_sub_return_large_mapcount(folio, 1, vma)

/* RMAP flags, currently only relevant for some anon rmap operations. */
typedef int __bitwise rmap_t;
@@ -538,9 +558,11 @@ static __always_inline void __folio_dup_file_rmap(struct folio *folio,
break;
}

- do {
- atomic_inc(&page->_mapcount);
- } while (page++, --nr_pages > 0);
+ if (IS_ENABLED(CONFIG_PAGE_MAPCOUNT)) {
+ do {
+ atomic_inc(&page->_mapcount);
+ } while (page++, --nr_pages > 0);
+ }
folio_add_large_mapcount(folio, orig_nr_pages, dst_vma);
break;
case RMAP_LEVEL_PMD:
@@ -638,7 +660,8 @@ static __always_inline int __folio_try_dup_anon_rmap(struct folio *folio,
do {
if (PageAnonExclusive(page))
ClearPageAnonExclusive(page);
- atomic_inc(&page->_mapcount);
+ if (IS_ENABLED(CONFIG_PAGE_MAPCOUNT))
+ atomic_inc(&page->_mapcount);
} while (page++, --nr_pages > 0);
folio_add_large_mapcount(folio, orig_nr_pages, dst_vma);
break;
diff --git a/mm/internal.h b/mm/internal.h
index 7303ddd9dac04..7a4f81a6edd66 100644
--- a/mm/internal.h
+++ b/mm/internal.h
@@ -84,6 +84,8 @@ void page_writeback_init(void);
*/
static inline int folio_nr_pages_mapped(const struct folio *folio)
{
+ if (IS_ENABLED(CONFIG_NO_PAGE_MAPCOUNT))
+ return -1;
return atomic_read(&folio->_nr_pages_mapped) & FOLIO_PAGES_MAPPED;
}

@@ -721,7 +723,8 @@ static inline void prep_compound_head(struct page *page, unsigned int order)

folio_set_order(folio, order);
atomic_set(&folio->_large_mapcount, -1);
- atomic_set(&folio->_nr_pages_mapped, 0);
+ if (IS_ENABLED(CONFIG_PAGE_MAPCOUNT))
+ atomic_set(&folio->_nr_pages_mapped, 0);
if (IS_ENABLED(CONFIG_MM_ID)) {
folio->_mm_ids = 0;
folio->_mm_id_mapcount[0] = -1;
diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 08caa92553998..4402672b5d838 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -951,7 +951,8 @@ static int free_tail_page_prepare(struct page *head_page, struct page *page)
bad_page(page, "nonzero large_mapcount");
goto out;
}
- if (unlikely(atomic_read(&folio->_nr_pages_mapped))) {
+ if (IS_ENABLED(CONFIG_PAGE_MAPCOUNT) &&
+ unlikely(atomic_read(&folio->_nr_pages_mapped))) {
bad_page(page, "nonzero nr_pages_mapped");
goto out;
}
diff --git a/mm/rmap.c b/mm/rmap.c
index 8de415157bc8d..67bb273dfb80d 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -1258,6 +1258,16 @@ static __always_inline unsigned int __folio_add_rmap(struct folio *folio,
break;
}

+ if (IS_ENABLED(CONFIG_NO_PAGE_MAPCOUNT)) {
+ nr = folio_add_return_large_mapcount(folio, orig_nr_pages, vma);
+ if (nr == orig_nr_pages)
+ /* Was completely unmapped. */
+ nr = folio_large_nr_pages(folio);
+ else
+ nr = 0;
+ break;
+ }
+
do {
first += atomic_inc_and_test(&page->_mapcount);
} while (page++, --nr_pages > 0);
@@ -1271,6 +1281,18 @@ static __always_inline unsigned int __folio_add_rmap(struct folio *folio,
case RMAP_LEVEL_PMD:
case RMAP_LEVEL_PUD:
first = atomic_inc_and_test(&folio->_entire_mapcount);
+ if (IS_ENABLED(CONFIG_NO_PAGE_MAPCOUNT)) {
+ if (level == RMAP_LEVEL_PMD && first)
+ *nr_pmdmapped = folio_large_nr_pages(folio);
+ nr = folio_inc_return_large_mapcount(folio, vma);
+ if (nr == 1)
+ /* Was completely unmapped. */
+ nr = folio_large_nr_pages(folio);
+ else
+ nr = 0;
+ break;
+ }
+
if (first) {
nr = atomic_add_return_relaxed(ENTIRELY_MAPPED, mapped);
if (likely(nr < ENTIRELY_MAPPED + ENTIRELY_MAPPED)) {
@@ -1436,13 +1458,23 @@ static __always_inline void __folio_add_anon_rmap(struct folio *folio,
break;
}
}
+
+ VM_WARN_ON_FOLIO(!folio_test_large(folio) && PageAnonExclusive(page) &&
+ atomic_read(&folio->_mapcount) > 0, folio);
for (i = 0; i < nr_pages; i++) {
struct page *cur_page = page + i;

- /* While PTE-mapping a THP we have a PMD and a PTE mapping. */
- VM_WARN_ON_FOLIO((atomic_read(&cur_page->_mapcount) > 0 ||
- (folio_test_large(folio) &&
- folio_entire_mapcount(folio) > 1)) &&
+ VM_WARN_ON_FOLIO(folio_test_large(folio) &&
+ folio_entire_mapcount(folio) > 1 &&
+ PageAnonExclusive(cur_page), folio);
+ if (IS_ENABLED(CONFIG_NO_PAGE_MAPCOUNT))
+ continue;
+
+ /*
+ * While PTE-mapping a THP we have a PMD and a PTE
+ * mapping.
+ */
+ VM_WARN_ON_FOLIO(atomic_read(&cur_page->_mapcount) > 0 &&
PageAnonExclusive(cur_page), folio);
}

@@ -1548,20 +1580,23 @@ void folio_add_new_anon_rmap(struct folio *folio, struct vm_area_struct *vma,
for (i = 0; i < nr; i++) {
struct page *page = folio_page(folio, i);

- /* increment count (starts at -1) */
- atomic_set(&page->_mapcount, 0);
+ if (IS_ENABLED(CONFIG_PAGE_MAPCOUNT))
+ /* increment count (starts at -1) */
+ atomic_set(&page->_mapcount, 0);
if (exclusive)
SetPageAnonExclusive(page);
}

folio_set_large_mapcount(folio, nr, vma);
- atomic_set(&folio->_nr_pages_mapped, nr);
+ if (IS_ENABLED(CONFIG_PAGE_MAPCOUNT))
+ atomic_set(&folio->_nr_pages_mapped, nr);
} else {
nr = folio_large_nr_pages(folio);
/* increment count (starts at -1) */
atomic_set(&folio->_entire_mapcount, 0);
folio_set_large_mapcount(folio, 1, vma);
- atomic_set(&folio->_nr_pages_mapped, ENTIRELY_MAPPED);
+ if (IS_ENABLED(CONFIG_PAGE_MAPCOUNT))
+ atomic_set(&folio->_nr_pages_mapped, ENTIRELY_MAPPED);
if (exclusive)
SetPageAnonExclusive(&folio->page);
nr_pmdmapped = nr;
@@ -1665,6 +1700,19 @@ static __always_inline void __folio_remove_rmap(struct folio *folio,
break;
}

+ if (IS_ENABLED(CONFIG_NO_PAGE_MAPCOUNT)) {
+ nr = folio_sub_return_large_mapcount(folio, nr_pages, vma);
+ if (!nr) {
+ /* Now completely unmapped. */
+ nr = folio_nr_pages(folio);
+ } else {
+ partially_mapped = nr < folio_large_nr_pages(folio) &&
+ !folio_entire_mapcount(folio);
+ nr = 0;
+ }
+ break;
+ }
+
folio_sub_large_mapcount(folio, nr_pages, vma);
do {
last += atomic_add_negative(-1, &page->_mapcount);
@@ -1678,6 +1726,22 @@ static __always_inline void __folio_remove_rmap(struct folio *folio,
break;
case RMAP_LEVEL_PMD:
case RMAP_LEVEL_PUD:
+ if (IS_ENABLED(CONFIG_NO_PAGE_MAPCOUNT)) {
+ last = atomic_add_negative(-1, &folio->_entire_mapcount);
+ if (level == RMAP_LEVEL_PMD && last)
+ nr_pmdmapped = folio_large_nr_pages(folio);
+ nr = folio_dec_return_large_mapcount(folio, vma);
+ if (!nr) {
+ /* Now completely unmapped. */
+ nr = folio_large_nr_pages(folio);
+ } else {
+ partially_mapped = last &&
+ nr < folio_large_nr_pages(folio);
+ nr = 0;
+ }
+ break;
+ }
+
folio_dec_large_mapcount(folio, vma);
last = atomic_add_negative(-1, &folio->_entire_mapcount);
if (last) {
--
2.48.1