+ fs-proc-task_mmu-remove-per-page-mapcount-dependency-for-mapmax-config_no_page_mapcount.patch added to mm-unstable branch

+ fs-proc-task_mmu-remove-per-page-mapcount-dependency-for-mapmax-config_no_page_mapcount.patch added to mm-unstable branch

[Andrew Morton]

The patch titled
     Subject: fs/proc/task_mmu: remove per-page mapcount dependency for "mapmax" (CONFIG_NO_PAGE_MAPCOUNT)
has been added to the -mm mm-unstable branch.  Its filename is
     fs-proc-task_mmu-remove-per-page-mapcount-dependency-for-mapmax-config_no_page_mapcount.patch

This patch will shortly appear at
     https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/fs-proc-task_mmu-remove-per-page-mapcount-dependency-for-mapmax-config_no_page_mapcount.patch

This patch will later appear in the mm-unstable branch at
    git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm

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------------------------------------------------------
From: David Hildenbrand <david@redhat.com>
Subject: fs/proc/task_mmu: remove per-page mapcount dependency for "mapmax" (CONFIG_NO_PAGE_MAPCOUNT)
Date: Mon, 3 Mar 2025 17:30:11 +0100

Let's implement an alternative when per-page mapcounts in large folios are
no longer maintained -- soon with CONFIG_NO_PAGE_MAPCOUNT.

For calculating "mapmax", we now use the average per-page mapcount in a
large folio instead of the per-page mapcount.

For hugetlb folios and folios that are not partially mapped into MMs,
there is no change.

Likely, this change will not matter much in practice, and an alternative
might be to simple remove this stat with CONFIG_NO_PAGE_MAPCOUNT. 
However, there might be value to it, so let's keep it like that and
document the behavior.

Link: https://lkml.kernel.org/r/20250303163014.1128035-19-david@redhat.com
Signed-off-by: David Hildenbrand <david@redhat.com>
Cc: Andy Lutomirks^H^Hski <luto@kernel.org>
Cc: Borislav Betkov <bp@alien8.de>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Jann Horn <jannh@google.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Cc: Lance Yang <ioworker0@gmail.com>
Cc: Liam Howlett <liam.howlett@oracle.com>
Cc: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Cc: Matthew Wilcow (Oracle) <willy@infradead.org>
Cc: Michal Koutn <mkoutny@suse.com>
Cc: Muchun Song <muchun.song@linux.dev>
Cc: tejun heo <tj@kernel.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: Zefan Li <lizefan.x@bytedance.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

 Documentation/filesystems/proc.rst |    5 +++++
 fs/proc/task_mmu.c                 |    7 ++++++-
 2 files changed, 11 insertions(+), 1 deletion(-)

--- a/Documentation/filesystems/proc.rst~fs-proc-task_mmu-remove-per-page-mapcount-dependency-for-mapmax-config_no_page_mapcount
+++ a/Documentation/filesystems/proc.rst
@@ -686,6 +686,11 @@ Where:
 node locality page counters (N0 == node0, N1 == node1, ...) and the kernel page
 size, in KB, that is backing the mapping up.
 
+Note that some kernel configurations do not track the precise number of times
+a page part of a larger allocation (e.g., THP) is mapped. In these
+configurations, "mapmax" might corresponds to the average number of mappings
+per page in such a larger allocation instead.
+
 1.2 Kernel data
 ---------------
 
--- a/fs/proc/task_mmu.c~fs-proc-task_mmu-remove-per-page-mapcount-dependency-for-mapmax-config_no_page_mapcount
+++ a/fs/proc/task_mmu.c
@@ -2866,7 +2866,12 @@ static void gather_stats(struct page *pa
 			unsigned long nr_pages)
 {
 	struct folio *folio = page_folio(page);
-	int count = folio_precise_page_mapcount(folio, page);
+	int count;
+
+	if (IS_ENABLED(CONFIG_PAGE_MAPCOUNT))
+		count = folio_precise_page_mapcount(folio, page);
+	else
+		count = folio_average_page_mapcount(folio);
 
 	md->pages += nr_pages;
 	if (pte_dirty || folio_test_dirty(folio))
_

Patches currently in -mm which might be from david@redhat.com are

mm-gup-reject-foll_split_pmd-with-hugetlb-vmas.patch
mm-rmap-reject-hugetlb-folios-in-folio_make_device_exclusive.patch
mm-rmap-convert-make_device_exclusive_range-to-make_device_exclusive.patch
mm-rmap-convert-make_device_exclusive_range-to-make_device_exclusive-fix.patch
mm-rmap-implement-make_device_exclusive-using-folio_walk-instead-of-rmap-walk.patch
mm-memory-detect-writability-in-restore_exclusive_pte-through-can_change_pte_writable.patch
mm-use-single-swp_device_exclusive-entry-type.patch
mm-page_vma_mapped-device-exclusive-entries-are-not-migration-entries.patch
kernel-events-uprobes-handle-device-exclusive-entries-correctly-in-__replace_page.patch
mm-ksm-handle-device-exclusive-entries-correctly-in-write_protect_page.patch
mm-rmap-handle-device-exclusive-entries-correctly-in-try_to_unmap_one.patch
mm-rmap-handle-device-exclusive-entries-correctly-in-try_to_migrate_one.patch
mm-rmap-handle-device-exclusive-entries-correctly-in-page_vma_mkclean_one.patch
mm-page_idle-handle-device-exclusive-entries-correctly-in-page_idle_clear_pte_refs_one.patch
mm-damon-handle-device-exclusive-entries-correctly-in-damon_folio_young_one.patch
mm-damon-handle-device-exclusive-entries-correctly-in-damon_folio_mkold_one.patch
mm-rmap-keep-mapcount-untouched-for-device-exclusive-entries.patch
mm-rmap-avoid-ebusy-from-make_device_exclusive.patch
lib-test_hmm-make-dmirror_atomic_map-consume-a-single-page.patch
mm-memory-remove-pageanonexclusive-sanity-check-in-restore_exclusive_pte.patch
mm-memory-pass-folio-and-pte-to-restore_exclusive_pte.patch
mm-memory-document-restore_exclusive_pte.patch
mm-mmu_notifier-use-mmu_notify_clear-in-remove_device_exclusive_entry.patch
mm-factor-out-large-folio-handling-from-folio_order-into-folio_large_order.patch
mm-factor-out-large-folio-handling-from-folio_nr_pages-into-folio_large_nr_pages.patch
mm-let-_folio_nr_pages-overlay-memcg_data-in-first-tail-page.patch
mm-move-hugetlb-specific-things-in-folio-to-page.patch
mm-move-_pincount-in-folio-to-page-on-32bit.patch
mm-move-_entire_mapcount-in-folio-to-page-on-32bit.patch
mm-rmap-pass-dst_vma-to-folio_dup_file_rmap_pte-and-friends.patch
mm-rmap-pass-vma-to-__folio_add_rmap.patch
mm-rmap-abstract-large-mapcount-operations-for-large-folios-hugetlb.patch
bit_spinlock-__always_inline-unlock-functions.patch
mm-rmap-use-folio_large_nr_pages-in-add-remove-functions.patch
mm-rmap-basic-mm-owner-tracking-for-large-folios-hugetlb.patch
mm-copy-on-write-cow-reuse-support-for-pte-mapped-thp.patch
mm-convert-folio_likely_mapped_shared-to-folio_maybe_mapped_shared.patch
mm-config_no_page_mapcount-to-prepare-for-not-maintain-per-page-mapcounts-in-large-folios.patch
fs-proc-page-remove-per-page-mapcount-dependency-for-proc-kpagecount-config_no_page_mapcount.patch
fs-proc-task_mmu-remove-per-page-mapcount-dependency-for-pm_mmap_exclusive-config_no_page_mapcount.patch
fs-proc-task_mmu-remove-per-page-mapcount-dependency-for-mapmax-config_no_page_mapcount.patch
fs-proc-task_mmu-remove-per-page-mapcount-dependency-for-smaps-smaps_rollup-config_no_page_mapcount.patch
mm-stop-maintaining-the-per-page-mapcount-of-large-folios-config_no_page_mapcount.patch