[PATCH v4 4/4] mm: convert page kmemcg type to a page memcg flag

[Roman Gushchin <guro@fb.com>]

PageKmemcg flag is currently defined as a page type (like buddy,
offline, table and guard). Semantically it means that the page
was accounted as a kernel memory by the page allocator and has
to be uncharged on the release.

As a side effect of defining the flag as a page type, the accounted
page can't be mapped to userspace (look at page_has_type() and
comments above). In particular, this blocks the accounting of
vmalloc-backed memory used by some bpf maps, because these maps
do map the memory to userspace.

One option is to fix it by complicating the access to page->mapcount,
which provides some free bits for page->page_type.

But it's way better to move this flag into page->memcg_data flags.
Indeed, the flag makes no sense without enabled memory cgroups
and memory cgroup pointer set in particular.

This commit replaces PageKmemcg() and __SetPageKmemcg() with
PageMemcgKmem() and an open-coded OR operation setting the memcg
pointer with the MEMCG_DATA_KMEM bit. __ClearPageKmemcg() can be
simple deleted, as the whole memcg_data is zeroed at once.

As a bonus, on !CONFIG_MEMCG build the PageMemcgKmem() check will
be compiled out.

Signed-off-by: Roman Gushchin <guro@fb.com>
---
 include/linux/memcontrol.h | 39 ++++++++++++++++++++++++++++++++------
 include/linux/page-flags.h | 11 ++---------
 mm/memcontrol.c            | 16 +++++-----------
 mm/page_alloc.c            |  4 ++--
 4 files changed, 42 insertions(+), 28 deletions(-)

diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
index 5e05599e1f74..8c15978294b2 100644
--- a/include/linux/memcontrol.h
+++ b/include/linux/memcontrol.h
@@ -346,8 +346,10 @@ 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),
+	/* page has been accounted as a non-slab kernel page */
+	MEMCG_DATA_KMEM = (1UL << 1),
 	/* the next bit after the last actual flag */
-	__NR_MEMCG_DATA_FLAGS  = (1UL << 1),
+	__NR_MEMCG_DATA_FLAGS  = (1UL << 2),
 };
 
 #define MEMCG_DATA_FLAGS_MASK (__NR_MEMCG_DATA_FLAGS - 1)
@@ -369,8 +371,12 @@ enum page_memcg_data_flags {
  */
 static inline struct mem_cgroup *page_memcg(struct page *page)
 {
+	unsigned long memcg_data = page->memcg_data;
+
 	VM_BUG_ON_PAGE(PageSlab(page), page);
-	return (struct mem_cgroup *)page->memcg_data;
+	VM_BUG_ON_PAGE(memcg_data & MEMCG_DATA_OBJCGS, page);
+
+	return (struct mem_cgroup *)(memcg_data & ~MEMCG_DATA_FLAGS_MASK);
 }
 
 /*
@@ -416,7 +422,7 @@ static inline struct mem_cgroup *page_memcg_check(struct page *page)
 	if (memcg_data & MEMCG_DATA_OBJCGS)
 		return NULL;
 
-	return (struct mem_cgroup *)memcg_data;
+	return (struct mem_cgroup *)(memcg_data & ~MEMCG_DATA_FLAGS_MASK);
 }
 
 /*
@@ -438,6 +444,20 @@ static inline void set_page_memcg(struct page *page, struct mem_cgroup *memcg)
 }
 
 
+/*
+ * PageMemcgKmem - check if the page has MemcgKmem flag set
+ * @page: a pointer to the page struct
+ *
+ * Checks if the page has MemcgKmem flag set. The caller must ensure that
+ * the page has an associated memory cgroup. It's not safe to call this function
+ * against some types of pages, e.g. slab pages.
+ */
+static inline bool PageMemcgKmem(struct page *page)
+{
+	VM_BUG_ON_PAGE(page->memcg_data & MEMCG_DATA_OBJCGS, page);
+	return page->memcg_data & MEMCG_DATA_KMEM;
+}
+
 #ifdef CONFIG_MEMCG_KMEM
 /*
  * page_objcgs - get the object cgroups vector associated with a page
@@ -453,8 +473,8 @@ static inline struct obj_cgroup **page_objcgs(struct page *page)
 {
 	unsigned long memcg_data = READ_ONCE(page->memcg_data);
 
-	VM_BUG_ON_PAGE(memcg_data &&
-		       !(memcg_data & MEMCG_DATA_OBJCGS), page);
+	VM_BUG_ON_PAGE(memcg_data && !(memcg_data & MEMCG_DATA_OBJCGS), page);
+	VM_BUG_ON_PAGE(memcg_data & MEMCG_DATA_KMEM, page);
 
 	return (struct obj_cgroup **)(memcg_data & ~MEMCG_DATA_FLAGS_MASK);
 }
@@ -471,9 +491,11 @@ 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 & MEMCG_DATA_OBJCGS))
+	if (memcg_data && (memcg_data & MEMCG_DATA_OBJCGS)) {
+		VM_BUG_ON_PAGE(memcg_data & MEMCG_DATA_KMEM, page);
 		return (struct obj_cgroup **)
 			(memcg_data & ~MEMCG_DATA_FLAGS_MASK);
+	}
 
 	return NULL;
 }
@@ -1138,6 +1160,11 @@ static inline void set_page_memcg(struct page *page,
 {
 }
 
+static inline bool PageMemcgKmem(struct page *page)
+{
+	return false;
+}
+
 static inline bool mem_cgroup_is_root(struct mem_cgroup *memcg)
 {
 	return true;
diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
index 4f6ba9379112..fc0e1bd48e73 100644
--- a/include/linux/page-flags.h
+++ b/include/linux/page-flags.h
@@ -715,9 +715,8 @@ PAGEFLAG_FALSE(DoubleMap)
 #define PAGE_MAPCOUNT_RESERVE	-128
 #define PG_buddy	0x00000080
 #define PG_offline	0x00000100
-#define PG_kmemcg	0x00000200
-#define PG_table	0x00000400
-#define PG_guard	0x00000800
+#define PG_table	0x00000200
+#define PG_guard	0x00000400
 
 #define PageType(page, flag)						\
 	((page->page_type & (PAGE_TYPE_BASE | flag)) == PAGE_TYPE_BASE)
@@ -768,12 +767,6 @@ PAGE_TYPE_OPS(Buddy, buddy)
  */
 PAGE_TYPE_OPS(Offline, offline)
 
-/*
- * If kmemcg is enabled, the buddy allocator will set PageKmemcg() on
- * pages allocated with __GFP_ACCOUNT. It gets cleared on page free.
- */
-PAGE_TYPE_OPS(Kmemcg, kmemcg)
-
 /*
  * Marks pages in use as page tables.
  */
diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 7e690424a917..ff9bb3e8333a 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -3090,8 +3090,8 @@ int __memcg_kmem_charge_page(struct page *page, gfp_t gfp, int order)
 	if (memcg && !mem_cgroup_is_root(memcg)) {
 		ret = __memcg_kmem_charge(memcg, gfp, 1 << order);
 		if (!ret) {
-			set_page_memcg(page, memcg);
-			__SetPageKmemcg(page);
+			page->memcg_data = (unsigned long)memcg |
+				MEMCG_DATA_KMEM;
 			return 0;
 		}
 		css_put(&memcg->css);
@@ -3116,10 +3116,6 @@ void __memcg_kmem_uncharge_page(struct page *page, int order)
 	__memcg_kmem_uncharge(memcg, nr_pages);
 	page->memcg_data = 0;
 	css_put(&memcg->css);
-
-	/* slab pages do not have PageKmemcg flag set */
-	if (PageKmemcg(page))
-		__ClearPageKmemcg(page);
 }
 
 static bool consume_obj_stock(struct obj_cgroup *objcg, unsigned int nr_bytes)
@@ -6863,12 +6859,10 @@ static void uncharge_page(struct page *page, struct uncharge_gather *ug)
 	nr_pages = compound_nr(page);
 	ug->nr_pages += nr_pages;
 
-	if (!PageKmemcg(page)) {
-		ug->pgpgout++;
-	} else {
+	if (PageMemcgKmem(page))
 		ug->nr_kmem += nr_pages;
-		__ClearPageKmemcg(page);
-	}
+	else
+		ug->pgpgout++;
 
 	ug->dummy_page = page;
 	page->memcg_data = 0;
diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index e9f0fe4a143e..5ebd50183d93 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -1179,7 +1179,7 @@ static __always_inline bool free_pages_prepare(struct page *page,
 		 * Do not let hwpoison pages hit pcplists/buddy
 		 * Untie memcg state and reset page's owner
 		 */
-		if (memcg_kmem_enabled() && PageKmemcg(page))
+		if (memcg_kmem_enabled() && PageMemcgKmem(page))
 			__memcg_kmem_uncharge_page(page, order);
 		reset_page_owner(page, order);
 		return false;
@@ -1209,7 +1209,7 @@ static __always_inline bool free_pages_prepare(struct page *page,
 	}
 	if (PageMappingFlags(page))
 		page->mapping = NULL;
-	if (memcg_kmem_enabled() && PageKmemcg(page))
+	if (memcg_kmem_enabled() && PageMemcgKmem(page))
 		__memcg_kmem_uncharge_page(page, order);
 	if (check_free)
 		bad += check_free_page(page);
-- 
2.26.2