Re: [PATCH v3 03/20] mm: let _folio_nr_pages overlay memcg_data in first tail page
From: David Hildenbrand
Date: Wed Mar 05 2025 - 05:30:37 EST
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@xxxxxxxxxxxxxxx>
Signed-off-by: David Hildenbrand <david@xxxxxxxxxx>
---
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@xxxxxxxxxx>
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@xxxxxxxxxx>
---
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