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From:   Roman Gushchin <guro@fb.com>
To:     Andrew Morton <akpm@linux-foundation.org>
CC:     Shakeel Butt <shakeelb@google.com>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Michal Hocko <mhocko@kernel.org>,
        <linux-kernel@vger.kernel.org>, <linux-mm@kvack.org>,
        <kernel-team@fb.com>, Roman Gushchin <guro@fb.com>
Subject: [PATCH v5 3/4] mm: introduce page memcg flags
Date:   Fri, 2 Oct 2020 10:25:58 -0700
Message-ID: <20201002172559.4000748-4-guro@fb.com>
In-Reply-To: <20201002172559.4000748-1-guro@fb.com>
References: <20201002172559.4000748-1-guro@fb.com>
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The lowest bit in page->memcg_data is used to distinguish between
struct memory_cgroup pointer and a pointer to a objcgs array.
All checks and modifications of this bit are open-coded.

Let's formalize it using page memcg flags, defined in enum
page_memcg_data_flags.

Additional flags might be added later.

Signed-off-by: Roman Gushchin <guro@fb.com>
Reviewed-by: Shakeel Butt <shakeelb@google.com>
---
 include/linux/memcontrol.h | 32 ++++++++++++++++++++------------
 1 file changed, 20 insertions(+), 12 deletions(-)

diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
index c7ac0a5b8989..99a4841d658b 100644
--- a/include/linux/memcontrol.h
+++ b/include/linux/memcontrol.h
@@ -343,6 +343,15 @@ struct mem_cgroup {
=20
 extern struct mem_cgroup *root_mem_cgroup;
=20
+enum page_memcg_data_flags {
+	/* page->memcg_data is a pointer to an objcgs vector */
+	MEMCG_DATA_OBJCGS =3D (1UL << 0),
+	/* the next bit after the last actual flag */
+	__NR_MEMCG_DATA_FLAGS  =3D (1UL << 1),
+};
+
+#define MEMCG_DATA_FLAGS_MASK (__NR_MEMCG_DATA_FLAGS - 1)
+
 /*
  * page_memcg - get the memory cgroup associated with a page
  * @page: a pointer to the page struct
@@ -404,13 +413,7 @@ static inline struct mem_cgroup *page_memcg_check(st=
ruct page *page)
 	 */
 	unsigned long memcg_data =3D READ_ONCE(page->memcg_data);
=20
-	/*
-	 * The lowest bit set means that memcg isn't a valid
-	 * memcg pointer, but a obj_cgroups pointer.
-	 * In this case the page is shared and doesn't belong
-	 * to any specific memory cgroup.
-	 */
-	if (memcg_data & 0x1UL)
+	if (memcg_data & MEMCG_DATA_OBJCGS)
 		return NULL;
=20
 	return (struct mem_cgroup *)memcg_data;
@@ -429,7 +432,11 @@ static inline struct mem_cgroup *page_memcg_check(st=
ruct page *page)
  */
 static inline struct obj_cgroup **page_objcgs(struct page *page)
 {
-	return (struct obj_cgroup **)(READ_ONCE(page->memcg_data) & ~0x1UL);
+	unsigned long memcg_data =3D READ_ONCE(page->memcg_data);
+
+	VM_BUG_ON_PAGE(memcg_data && !(memcg_data & MEMCG_DATA_OBJCGS), page);
+
+	return (struct obj_cgroup **)(memcg_data & ~MEMCG_DATA_FLAGS_MASK);
 }
=20
 /*
@@ -444,10 +451,10 @@ static inline struct obj_cgroup **page_objcgs_check=
(struct page *page)
 {
 	unsigned long memcg_data =3D READ_ONCE(page->memcg_data);
=20
-	if (memcg_data && (memcg_data & 0x1UL))
-		return (struct obj_cgroup **)(memcg_data & ~0x1UL);
+	if (!memcg_data || !(memcg_data & MEMCG_DATA_OBJCGS))
+		return NULL;
=20
-	return NULL;
+	return (struct obj_cgroup **)(memcg_data & ~MEMCG_DATA_FLAGS_MASK);
 }
=20
 /*
@@ -460,7 +467,8 @@ static inline struct obj_cgroup **page_objcgs_check(s=
truct page *page)
 static inline bool set_page_objcgs(struct page *page,
 					struct obj_cgroup **objcgs)
 {
-	return !cmpxchg(&page->memcg_data, 0, (unsigned long)objcgs | 0x1UL);
+	return !cmpxchg(&page->memcg_data, 0, (unsigned long)objcgs |
+			MEMCG_DATA_OBJCGS);
 }
 #else
 static inline struct obj_cgroup **page_objcgs(struct page *page)
--=20
2.26.2