Re: [PATCH v3 03/20] mm: let _folio_nr_pages overlay memcg_data in first tail page

David Hildenbrand
2025-03-05T10:30:00Z

On 03.03.25 17:29, David Hildenbrand wrote:
> 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 a743321dc1a5d..694704217df8a 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 */

@Andrew

The following on top to make htmldoc happy.

There will be two conflicts to be resolved in two patches because of
the added "/* private: " under "_pincount".

It's fairly straight forward to resolve (just keep "/* private:" below
any changes), but let me know if I should resend a v4 instead for this.

From 9d9ff38c2ea14f1b4dab29f099ec0f6be683f3fe Mon Sep 17 00:00:00 2001
From: David Hildenbrand <david@redhat.com>
Date: Wed, 5 Mar 2025 11:14:52 +0100
Subject: [PATCH] fixup: mm: let _folio_nr_pages overlay memcg_data in first
tail page

Make "make htmldoc" happy by marking non-private placeholder entries
private.

Signed-off-by: David Hildenbrand <david@redhat.com>
---
include/linux/mm_types.h | 4 +++-
1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
index e81be20bbabc..6062c12c3871 100644
--- a/include/linux/mm_types.h
+++ b/include/linux/mm_types.h
@@ -381,18 +381,20 @@ struct folio {
struct {
unsigned long _flags_1;
unsigned long _head_1;
- /* public: */
union {
struct {
+ /* public: */
atomic_t _large_mapcount;
atomic_t _entire_mapcount;
atomic_t _nr_pages_mapped;
atomic_t _pincount;
+ /* private: the union with struct page is transitional */
};
unsigned long _usable_1[4];
};
atomic_t _mapcount_1;
atomic_t _refcount_1;
+ /* public: */
#ifdef NR_PAGES_IN_LARGE_FOLIO
unsigned int _nr_pages;
#endif /* NR_PAGES_IN_LARGE_FOLIO */
--
2.48.1

--
Cheers,

David / dhildenb