Re: [PATCH v9 2/8] mm/huge_memory: add two new (not yet used) functions for folio_split()

Matthew Wilcox
2025-02-27T15:14:29Z

On Thu, Feb 27, 2025 at 05:55:43AM +0000, Matthew Wilcox wrote:
> On Wed, Feb 26, 2025 at 04:00:25PM -0500, Zi Yan wrote:
> > +static int __split_unmapped_folio(struct folio *folio, int new_order,
> > + struct page *split_at, struct page *lock_at,
> > + struct list_head *list, pgoff_t end,
> > + struct xa_state *xas, struct address_space *mapping,
> > + bool uniform_split)
> > +{
> [...]
> > + /* 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);
>
> At least split_page_memcg() needs to become aware of 'uniform_split'.
>
> if (folio_memcg_kmem(folio))
> obj_cgroup_get_many(__folio_objcg(folio), old_nr / new_nr - 1);
>
> If we're doing uniform_split, that calculation should be
> old_order - new_order - 1

umm, old_order - new_order. Anyway, here's a patch I've done on top of
your work, but it probably needs to be massaged slightly and placed
before your work?

From 190e13ed77e562eb59fa1fa4bfefdefe5d0416ed Mon Sep 17 00:00:00 2001
From: "Matthew Wilcox (Oracle)" <willy@infradead.org>
Date: Mon, 28 Oct 2024 16:23:30 -0400
Subject: [PATCH] mm: Separate folio_split_memcg() from split_page_memcg()

Folios always use memcg_data to refer to the mem_cgroup while pages
allocated with GFP_ACCOUNT have a pointer to the obj_cgroup. Since the
caller already knows what it has, split the function into two and then
we don't need to check.

Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
---
include/linux/memcontrol.h | 7 +++++++
mm/huge_memory.c | 6 ++++--
mm/memcontrol.c | 18 +++++++++++++++---
3 files changed, 26 insertions(+), 5 deletions(-)

diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
index 57664e2a8fb7..155c3f81f4df 100644
--- a/include/linux/memcontrol.h
+++ b/include/linux/memcontrol.h
@@ -1039,6 +1039,8 @@ static inline void memcg_memory_event_mm(struct mm_struct *mm,
}

void split_page_memcg(struct page *head, int old_order, int new_order);
+void folio_split_memcg(struct folio *folio, unsigned old_order,
+ unsigned new_order, bool uniform_split);

static inline u64 cgroup_id_from_mm(struct mm_struct *mm)
{
@@ -1463,6 +1465,11 @@ static inline void split_page_memcg(struct page *head, int old_order, int new_or
{
}

+static inline void folio_split_memcg(struct folio *folio, unsigned old_order,
+ unsigned new_order, bool uniform)
+{
+}
+
static inline u64 cgroup_id_from_mm(struct mm_struct *mm)
{
return 0;
diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 1e45064046a0..75fa9c9d9ec9 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -3401,6 +3401,9 @@ static void __split_folio_to_order(struct folio *folio, int new_order)
folio_set_young(new_folio);
if (folio_test_idle(folio))
folio_set_idle(new_folio);
+#ifdef CONFIG_MEMCG
+ new_folio->memcg_data = folio->memcg_data;
+#endif

folio_xchg_last_cpupid(new_folio, folio_last_cpupid(folio));
}
@@ -3529,8 +3532,7 @@ static int __split_unmapped_folio(struct folio *folio, int new_order,
}
}

- /* complete memcg works before add pages to LRU */
- split_page_memcg(&folio->page, old_order, split_order);
+ folio_split_memcg(folio, old_order, split_order, uniform_split);
split_page_owner(&folio->page, old_order, split_order);
pgalloc_tag_split(folio, old_order, split_order);

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 16f3bdbd37d8..c2d41e1337cb 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -3064,10 +3064,22 @@ void split_page_memcg(struct page *head, int old_order, int new_order)
for (i = new_nr; i < old_nr; i += new_nr)
folio_page(folio, i)->memcg_data = folio->memcg_data;

- if (folio_memcg_kmem(folio))
- obj_cgroup_get_many(__folio_objcg(folio), old_nr / new_nr - 1);
+ obj_cgroup_get_many(__folio_objcg(folio), old_nr / new_nr - 1);
+}
+
+void folio_split_memcg(struct folio *folio, unsigned old_order,
+ unsigned new_order, bool uniform_split)
+{
+ unsigned new_refs;
+
+ if (mem_cgroup_disabled() || !folio_memcg_charged(folio))
+ return;
+
+ if (uniform_split)
+ new_refs = (1 << (old_order - new_order)) - 1;
else
- css_get_many(&folio_memcg(folio)->css, old_nr / new_nr - 1);
+ new_refs = old_order - new_order;
+ css_get_many(&__folio_memcg(folio)->css, new_refs);
}

unsigned long mem_cgroup_usage(struct mem_cgroup *memcg, bool swap)
--
2.47.2