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

[Matthew Wilcox <willy@infradead.org>]

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