+ mm-let-_folio_nr_pages-overlay-memcg_data-in-first-tail-page.patch added to mm-unstable branch

+ mm-let-_folio_nr_pages-overlay-memcg_data-in-first-tail-page.patch added to mm-unstable branch

[Andrew Morton]

The patch titled
     Subject: mm: let _folio_nr_pages overlay memcg_data in first tail page
has been added to the -mm mm-unstable branch.  Its filename is
     mm-let-_folio_nr_pages-overlay-memcg_data-in-first-tail-page.patch

This patch will shortly appear at
     https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/mm-let-_folio_nr_pages-overlay-memcg_data-in-first-tail-page.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: mm: let _folio_nr_pages overlay memcg_data in first tail page
Date: Mon, 3 Mar 2025 17:29:56 +0100

Let's free up some more of the "unconditionally available on 64BIT" space
in order-1 folios by letting _folio_nr_pages overlay memcg_data in the
first tail page (second folio page).  Consequently, we have the
optimization now whenever we have CONFIG_MEMCG, independent of 64BIT.

We have to make sure that page->memcg on tail pages does not return
"surprises".  page_memcg_check() already properly refuses PageTail(). 
Let's do that earlier in print_page_owner_memcg() to avoid printing wrong
"Slab cache page" information.  No other code should touch that field on
tail pages of compound pages.

Reset the "_nr_pages" to 0 when splitting folios, or when freeing them
back to the buddy (to avoid false page->memcg_data "bad page" reports).

Note that in __split_huge_page(), folio_nr_pages() would stop working
already as soon as we start messing with the subpages.

Most kernel configs should have at least CONFIG_MEMCG enabled, even if
disabled at runtime.  64byte "struct memmap" is what we usually have on
64BIT.

While at it, rename "_folio_nr_pages" to "_nr_pages".

Hopefully memdescs / dynamically allocating "strut folio" in the future
will further clean this up, e.g., making _nr_pages available in all
configs and maybe even in small folios.  Doing that should be fairly easy
on top of this change.

Link: https://lkml.kernel.org/r/20250303163014.1128035-4-david@redhat.com
Signed-off-by: David Hildenbrand <david@redhat.com>
Reviewed-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.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: 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>
---

 include/linux/mm.h       |    4 ++--
 include/linux/mm_types.h |   30 ++++++++++++++++++++++--------
 mm/huge_memory.c         |   16 +++++++++++++---
 mm/internal.h            |    4 ++--
 mm/page_alloc.c          |    6 +++++-
 mm/page_owner.c          |    2 +-
 6 files changed, 45 insertions(+), 17 deletions(-)

--- a/include/linux/mm.h~mm-let-_folio_nr_pages-overlay-memcg_data-in-first-tail-page
+++ a/include/linux/mm.h
@@ -1199,10 +1199,10 @@ static inline unsigned int folio_large_o
 	return folio->_flags_1 & 0xff;
 }
 
-#ifdef CONFIG_64BIT
+#ifdef NR_PAGES_IN_LARGE_FOLIO
 static inline long folio_large_nr_pages(const struct folio *folio)
 {
-	return folio->_folio_nr_pages;
+	return folio->_nr_pages;
 }
 #else
 static inline long folio_large_nr_pages(const struct folio *folio)
--- a/include/linux/mm_types.h~mm-let-_folio_nr_pages-overlay-memcg_data-in-first-tail-page
+++ a/include/linux/mm_types.h
@@ -287,6 +287,11 @@ typedef struct {
 	unsigned long val;
 } swp_entry_t;
 
+#if defined(CONFIG_MEMCG) || defined(CONFIG_SLAB_OBJ_EXT)
+/* We have some extra room after the refcount in tail pages. */
+#define NR_PAGES_IN_LARGE_FOLIO
+#endif
+
 /**
  * struct folio - Represents a contiguous set of bytes.
  * @flags: Identical to the page flags.
@@ -312,7 +317,7 @@ typedef struct {
  * @_large_mapcount: Do not use directly, call folio_mapcount().
  * @_nr_pages_mapped: Do not use outside of rmap and debug code.
  * @_pincount: Do not use directly, call folio_maybe_dma_pinned().
- * @_folio_nr_pages: Do not use directly, call folio_nr_pages().
+ * @_nr_pages: Do not use directly, call folio_nr_pages().
  * @_hugetlb_subpool: Do not use directly, use accessor in hugetlb.h.
  * @_hugetlb_cgroup: Do not use directly, use accessor in hugetlb_cgroup.h.
  * @_hugetlb_cgroup_rsvd: Do not use directly, use accessor in hugetlb_cgroup.h.
@@ -377,13 +382,20 @@ struct folio {
 			unsigned long _flags_1;
 			unsigned long _head_1;
 	/* public: */
-			atomic_t _large_mapcount;
-			atomic_t _entire_mapcount;
-			atomic_t _nr_pages_mapped;
-			atomic_t _pincount;
-#ifdef CONFIG_64BIT
-			unsigned int _folio_nr_pages;
-#endif
+			union {
+				struct {
+					atomic_t _large_mapcount;
+					atomic_t _entire_mapcount;
+					atomic_t _nr_pages_mapped;
+					atomic_t _pincount;
+				};
+				unsigned long _usable_1[4];
+			};
+			atomic_t _mapcount_1;
+			atomic_t _refcount_1;
+#ifdef NR_PAGES_IN_LARGE_FOLIO
+			unsigned int _nr_pages;
+#endif /* NR_PAGES_IN_LARGE_FOLIO */
 	/* private: the union with struct page is transitional */
 		};
 		struct page __page_1;
@@ -435,6 +447,8 @@ FOLIO_MATCH(_last_cpupid, _last_cpupid);
 			offsetof(struct page, pg) + sizeof(struct page))
 FOLIO_MATCH(flags, _flags_1);
 FOLIO_MATCH(compound_head, _head_1);
+FOLIO_MATCH(_mapcount, _mapcount_1);
+FOLIO_MATCH(_refcount, _refcount_1);
 #undef FOLIO_MATCH
 #define FOLIO_MATCH(pg, fl)						\
 	static_assert(offsetof(struct folio, fl) ==			\
--- a/mm/huge_memory.c~mm-let-_folio_nr_pages-overlay-memcg_data-in-first-tail-page
+++ a/mm/huge_memory.c
@@ -3307,10 +3307,11 @@ bool can_split_folio(struct folio *folio
  * It splits @folio into @new_order folios and copies the @folio metadata to
  * all the resulting folios.
  */
-static void __split_folio_to_order(struct folio *folio, int new_order)
+static void __split_folio_to_order(struct folio *folio, int old_order,
+		int new_order)
 {
-	long nr_pages = folio_nr_pages(folio);
 	long new_nr_pages = 1 << new_order;
+	long nr_pages = 1 << old_order;
 	long index;
 
 	/*
@@ -3528,12 +3529,21 @@ static int __split_unmapped_folio(struct
 			}
 		}
 
+		/*
+		 * Reset any memcg data overlay in the tail pages.
+		 * folio_nr_pages() is unreliable until prep_compound_page()
+		 * was called again.
+		 */
+#ifdef NR_PAGES_IN_LARGE_FOLIO
+		folio->_nr_pages = 0;
+#endif
+
 		/* complete memcg works before add pages to LRU */
 		split_page_memcg(&folio->page, old_order, split_order);
 		split_page_owner(&folio->page, old_order, split_order);
 		pgalloc_tag_split(folio, old_order, split_order);
 
-		__split_folio_to_order(folio, split_order);
+		__split_folio_to_order(folio, old_order, split_order);
 
 after_split:
 		/*
--- a/mm/internal.h~mm-let-_folio_nr_pages-overlay-memcg_data-in-first-tail-page
+++ a/mm/internal.h
@@ -682,8 +682,8 @@ static inline void folio_set_order(struc
 		return;
 
 	folio->_flags_1 = (folio->_flags_1 & ~0xffUL) | order;
-#ifdef CONFIG_64BIT
-	folio->_folio_nr_pages = 1U << order;
+#ifdef NR_PAGES_IN_LARGE_FOLIO
+	folio->_nr_pages = 1U << order;
 #endif
 }
 
--- a/mm/page_alloc.c~mm-let-_folio_nr_pages-overlay-memcg_data-in-first-tail-page
+++ a/mm/page_alloc.c
@@ -1178,8 +1178,12 @@ __always_inline bool free_pages_prepare(
 	if (unlikely(order)) {
 		int i;
 
-		if (compound)
+		if (compound) {
 			page[1].flags &= ~PAGE_FLAGS_SECOND;
+#ifdef NR_PAGES_IN_LARGE_FOLIO
+			folio->_nr_pages = 0;
+#endif
+		}
 		for (i = 1; i < (1 << order); i++) {
 			if (compound)
 				bad += free_tail_page_prepare(page, page + i);
--- a/mm/page_owner.c~mm-let-_folio_nr_pages-overlay-memcg_data-in-first-tail-page
+++ a/mm/page_owner.c
@@ -507,7 +507,7 @@ static inline int print_page_owner_memcg
 
 	rcu_read_lock();
 	memcg_data = READ_ONCE(page->memcg_data);
-	if (!memcg_data)
+	if (!memcg_data || PageTail(page))
 		goto out_unlock;
 
 	if (memcg_data & MEMCG_DATA_OBJEXTS)
_

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