2020-10-01 01:14:55

by Roman Gushchin

Subject: [PATCH v4 3/4] mm: introduce page memcg flags

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 <[email protected]>
Reviewed-by: Shakeel Butt <[email protected]>
---
include/linux/memcontrol.h | 35 ++++++++++++++++++++++++-----------
1 file changed, 24 insertions(+), 11 deletions(-)

diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
index 7e3da7cdc1ba..5e05599e1f74 100644
--- a/include/linux/memcontrol.h
+++ b/include/linux/memcontrol.h
@@ -343,6 +343,15 @@ struct mem_cgroup {

extern struct mem_cgroup *root_mem_cgroup;

+enum page_memcg_data_flags {
+ /* page->memcg_data is a pointer to an objcgs vector */
+ MEMCG_DATA_OBJCGS = (1UL << 0),
+ /* the next bit after the last actual flag */
+ __NR_MEMCG_DATA_FLAGS = (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(struct page *page)
*/
unsigned long memcg_data = READ_ONCE(page->memcg_data);

- /*
- * 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;

return (struct mem_cgroup *)memcg_data;
@@ -448,7 +451,12 @@ static inline void set_page_memcg(struct page *page, struct mem_cgroup *memcg)
*/
static inline struct obj_cgroup **page_objcgs(struct page *page)
{
- return (struct obj_cgroup **)(READ_ONCE(page->memcg_data) & ~0x1UL);
+ unsigned long memcg_data = 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);
}

/*
@@ -463,8 +471,9 @@ static inline struct obj_cgroup **page_objcgs_check(struct page *page)
{
unsigned long memcg_data = READ_ONCE(page->memcg_data);

- if (memcg_data && (memcg_data & 0x1UL))
- return (struct obj_cgroup **)(memcg_data & ~0x1UL);
+ if (memcg_data && (memcg_data & MEMCG_DATA_OBJCGS))
+ return (struct obj_cgroup **)
+ (memcg_data & ~MEMCG_DATA_FLAGS_MASK);

return NULL;
}
@@ -479,7 +488,11 @@ static inline struct obj_cgroup **page_objcgs_check(struct 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);
+ unsigned long memcg_data = (unsigned long)objcgs;
+
+ memcg_data |= MEMCG_DATA_OBJCGS;
+
+ return !cmpxchg(&page->memcg_data, 0, memcg_data);
}
#else
static inline struct obj_cgroup **page_objcgs(struct page *page)
--
2.26.2

2020-10-01 13:50:26

by Johannes Weiner

Subject: Re: [PATCH v4 3/4] mm: introduce page memcg flags

On Wed, Sep 30, 2020 at 05:27:09PM -0700, Roman Gushchin wrote:
> @@ -448,7 +451,12 @@ static inline void set_page_memcg(struct page *page, struct mem_cgroup *memcg)
> */
> static inline struct obj_cgroup **page_objcgs(struct page *page)
> {
> - return (struct obj_cgroup **)(READ_ONCE(page->memcg_data) & ~0x1UL);
> + unsigned long memcg_data = READ_ONCE(page->memcg_data);
> +
> + VM_BUG_ON_PAGE(memcg_data &&
> + !(memcg_data & MEMCG_DATA_OBJCGS), page);

This fits on a single line.

> + return (struct obj_cgroup **)(memcg_data & ~MEMCG_DATA_FLAGS_MASK);
> }
>
> /*
> @@ -463,8 +471,9 @@ static inline struct obj_cgroup **page_objcgs_check(struct page *page)
> {
> unsigned long memcg_data = READ_ONCE(page->memcg_data);
>
> - if (memcg_data && (memcg_data & 0x1UL))
> - return (struct obj_cgroup **)(memcg_data & ~0x1UL);
> + if (memcg_data && (memcg_data & MEMCG_DATA_OBJCGS))
> + return (struct obj_cgroup **)
> + (memcg_data & ~MEMCG_DATA_FLAGS_MASK);

if (!memcg_data || (memcg_data & MEMCG_DATA_OBJCGS))
return NULL;

return (struct obj_cgroup **)(memcg_data & ~MEMCG_DATA_FLAGS_MASK);

> @@ -479,7 +488,11 @@ static inline struct obj_cgroup **page_objcgs_check(struct 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);
> + unsigned long memcg_data = (unsigned long)objcgs;
> +
> + memcg_data |= MEMCG_DATA_OBJCGS;
> +
> + return !cmpxchg(&page->memcg_data, 0, memcg_data);

return !cmpxchg(&page->memcg_data, 0, memcg_data | MEMCG_DATA_OBJCGS);