[PATCH v2 03/20] mm: let _folio_nr_pages overlay memcg_data in first tail page

David Hildenbrand
2025-02-24T16:56:15Z

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.

Reviewed-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Signed-off-by: David Hildenbrand <david@redhat.com>
---
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(-)

diff --git a/include/linux/mm.h b/include/linux/mm.h
index 7c5a8fd29cfcd..f6b6373a864dd 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -1199,10 +1199,10 @@ static inline unsigned int folio_large_order(const struct folio *folio)
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)
diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
index 689b2a7461892..e81be20bbabc6 100644
--- a/include/linux/mm_types.h
+++ b/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) == \
diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 1e45064046a0f..13152313e77ce 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -3307,9 +3307,10 @@ bool can_split_folio(struct folio *folio, int caller_pins, int *pextra_pins)
* 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 nr_pages = 1 << old_order;
long new_nr_pages = 1 << new_order;
long index;

@@ -3529,12 +3530,21 @@ static int __split_unmapped_folio(struct folio *folio, int new_order,
}
}

+ /*
+ * 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:
/*
diff --git a/mm/internal.h b/mm/internal.h
index b07550db2bfd1..7f6d5def00fa0 100644
--- a/mm/internal.h
+++ b/mm/internal.h
@@ -684,8 +684,8 @@ static inline void folio_set_order(struct folio *folio, unsigned int order)
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
}

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 16dfcf7ade74a..bdfc954dab9aa 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -1174,8 +1174,12 @@ __always_inline bool free_pages_prepare(struct page *page,
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);
diff --git a/mm/page_owner.c b/mm/page_owner.c
index 2d6360eaccbb6..a409e2561a8fd 100644
--- a/mm/page_owner.c
+++ b/mm/page_owner.c
@@ -507,7 +507,7 @@ static inline int print_page_owner_memcg(char *kbuf, size_t count, int ret,

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)
--
2.48.1