[PATCH 1/3] memcg: simplify charging kmem pages

[Vladimir Davydov]

Charging kmem pages proceeds in two steps. First, we try to charge the
allocation size to the memcg the current task belongs to, then we
allocate a page and "commit" the charge storing the pointer to the memcg
in the page struct.

Such a design looks overcomplicated, because there is no much sense in
trying charging the allocation before actually allocating a page: we
won't be able to consume much memory over the limit even if we charge
after doing the actual allocation, besides we already charge user pages
post factum, so being pedantic with kmem pages just looks pointless.

So this patch simplifies the design by merging the "charge" and the
"commit" steps into the same function, which takes the allocated page.

Also, rename the charge and uncharge methods to memcg_kmem_charge and
memcg_kmem_uncharge and make the charge method return error code instead
of bool to conform to mem_cgroup_try_charge.

Signed-off-by: Vladimir Davydov <vdavydov@xxxxxxxxxxxxx>
---
 include/linux/memcontrol.h | 69 +++++++++++++---------------------------------
 mm/memcontrol.c            | 39 +++-----------------------
 mm/page_alloc.c            | 18 ++++++------
 3 files changed, 32 insertions(+), 94 deletions(-)

diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
index a3e0296eb063..9e1f4d5efc56 100644
--- a/include/linux/memcontrol.h
+++ b/include/linux/memcontrol.h
@@ -752,11 +752,8 @@ static inline bool memcg_kmem_is_active(struct mem_cgroup *memcg)
  * conditions, but because they are pretty simple, they are expected to be
  * fast.
  */
-bool __memcg_kmem_newpage_charge(gfp_t gfp, struct mem_cgroup **memcg,
-					int order);
-void __memcg_kmem_commit_charge(struct page *page,
-				       struct mem_cgroup *memcg, int order);
-void __memcg_kmem_uncharge_pages(struct page *page, int order);
+int __memcg_kmem_charge(struct page *page, gfp_t gfp, int order);
+void __memcg_kmem_uncharge(struct page *page, int order);
 
 /*
  * helper for acessing a memcg's index. It will be used as an index in the
@@ -789,52 +786,30 @@ static inline bool __memcg_kmem_bypass(gfp_t gfp)
 }
 
 /**
- * memcg_kmem_newpage_charge: verify if a new kmem allocation is allowed.
- * @gfp: the gfp allocation flags.
- * @memcg: a pointer to the memcg this was charged against.
- * @order: allocation order.
+ * memcg_kmem_charge: charge a kmem page
+ * @page: page to charge
+ * @gfp: reclaim mode
+ * @order: allocation order
  *
- * returns true if the memcg where the current task belongs can hold this
- * allocation.
- *
- * We return true automatically if this allocation is not to be accounted to
- * any memcg.
+ * Returns 0 on success, an error code on failure.
  */
-static inline bool
-memcg_kmem_newpage_charge(gfp_t gfp, struct mem_cgroup **memcg, int order)
+static __always_inline int memcg_kmem_charge(struct page *page,
+					     gfp_t gfp, int order)
 {
 	if (__memcg_kmem_bypass(gfp))
-		return true;
-	return __memcg_kmem_newpage_charge(gfp, memcg, order);
+		return 0;
+	return __memcg_kmem_charge(page, gfp, order);
 }
 
 /**
- * memcg_kmem_uncharge_pages: uncharge pages from memcg
- * @page: pointer to struct page being freed
- * @order: allocation order.
+ * memcg_kmem_uncharge: uncharge a kmem page
+ * @page: page to uncharge
+ * @order: allocation order
  */
-static inline void
-memcg_kmem_uncharge_pages(struct page *page, int order)
+static __always_inline void memcg_kmem_uncharge(struct page *page, int order)
 {
 	if (memcg_kmem_enabled())
-		__memcg_kmem_uncharge_pages(page, order);
-}
-
-/**
- * memcg_kmem_commit_charge: embeds correct memcg in a page
- * @page: pointer to struct page recently allocated
- * @memcg: the memcg structure we charged against
- * @order: allocation order.
- *
- * Needs to be called after memcg_kmem_newpage_charge, regardless of success or
- * failure of the allocation. if @page is NULL, this function will revert the
- * charges. Otherwise, it will commit @page to @memcg.
- */
-static inline void
-memcg_kmem_commit_charge(struct page *page, struct mem_cgroup *memcg, int order)
-{
-	if (memcg_kmem_enabled() && memcg)
-		__memcg_kmem_commit_charge(page, memcg, order);
+		__memcg_kmem_uncharge(page, order);
 }
 
 /**
@@ -878,18 +853,12 @@ static inline bool memcg_kmem_is_active(struct mem_cgroup *memcg)
 	return false;
 }
 
-static inline bool
-memcg_kmem_newpage_charge(gfp_t gfp, struct mem_cgroup **memcg, int order)
-{
-	return true;
-}
-
-static inline void memcg_kmem_uncharge_pages(struct page *page, int order)
+static inline int memcg_kmem_charge(struct page *page, gfp_t gfp, int order)
 {
+	return 0;
 }
 
-static inline void
-memcg_kmem_commit_charge(struct page *page, struct mem_cgroup *memcg, int order)
+static inline void memcg_kmem_uncharge(struct page *page, int order)
 {
 }
 
diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 44706a17cddc..7c0af36fc8d0 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -2404,57 +2404,26 @@ void __memcg_kmem_put_cache(struct kmem_cache *cachep)
 		css_put(&cachep->memcg_params.memcg->css);
 }
 
-/*
- * We need to verify if the allocation against current->mm->owner's memcg is
- * possible for the given order. But the page is not allocated yet, so we'll
- * need a further commit step to do the final arrangements.
- *
- * It is possible for the task to switch cgroups in this mean time, so at
- * commit time, we can't rely on task conversion any longer.  We'll then use
- * the handle argument to return to the caller which cgroup we should commit
- * against. We could also return the memcg directly and avoid the pointer
- * passing, but a boolean return value gives better semantics considering
- * the compiled-out case as well.
- *
- * Returning true means the allocation is possible.
- */
-bool
-__memcg_kmem_newpage_charge(gfp_t gfp, struct mem_cgroup **_memcg, int order)
+int __memcg_kmem_charge(struct page *page, gfp_t gfp, int order)
 {
 	struct mem_cgroup *memcg;
 	int ret;
 
-	*_memcg = NULL;
-
 	memcg = get_mem_cgroup_from_mm(current->mm);
 
 	if (!memcg_kmem_is_active(memcg)) {
 		css_put(&memcg->css);
-		return true;
+		return 0;
 	}
 
 	ret = memcg_charge_kmem(memcg, gfp, 1 << order);
-	if (!ret)
-		*_memcg = memcg;
 
 	css_put(&memcg->css);
-	return (ret == 0);
-}
-
-void __memcg_kmem_commit_charge(struct page *page, struct mem_cgroup *memcg,
-			      int order)
-{
-	VM_BUG_ON(mem_cgroup_is_root(memcg));
-
-	/* The page allocation failed. Revert */
-	if (!page) {
-		memcg_uncharge_kmem(memcg, 1 << order);
-		return;
-	}
 	page->mem_cgroup = memcg;
+	return ret;
 }
 
-void __memcg_kmem_uncharge_pages(struct page *page, int order)
+void __memcg_kmem_uncharge(struct page *page, int order)
 {
 	struct mem_cgroup *memcg = page->mem_cgroup;
 
diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index e19132074404..91d1a1923eb8 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -3414,24 +3414,24 @@ EXPORT_SYMBOL(__free_page_frag);
 struct page *alloc_kmem_pages(gfp_t gfp_mask, unsigned int order)
 {
 	struct page *page;
-	struct mem_cgroup *memcg = NULL;
 
-	if (!memcg_kmem_newpage_charge(gfp_mask, &memcg, order))
-		return NULL;
 	page = alloc_pages(gfp_mask, order);
-	memcg_kmem_commit_charge(page, memcg, order);
+	if (page && memcg_kmem_charge(page, gfp_mask, order) != 0) {
+		__free_pages(page, order);
+		page = NULL;
+	}
 	return page;
 }
 
 struct page *alloc_kmem_pages_node(int nid, gfp_t gfp_mask, unsigned int order)
 {
 	struct page *page;
-	struct mem_cgroup *memcg = NULL;
 
-	if (!memcg_kmem_newpage_charge(gfp_mask, &memcg, order))
-		return NULL;
 	page = alloc_pages_node(nid, gfp_mask, order);
-	memcg_kmem_commit_charge(page, memcg, order);
+	if (page && memcg_kmem_charge(page, gfp_mask, order) != 0) {
+		__free_pages(page, order);
+		page = NULL;
+	}
 	return page;
 }
 
@@ -3441,7 +3441,7 @@ struct page *alloc_kmem_pages_node(int nid, gfp_t gfp_mask, unsigned int order)
  */
 void __free_kmem_pages(struct page *page, unsigned int order)
 {
-	memcg_kmem_uncharge_pages(page, order);
+	memcg_kmem_uncharge(page, order);
 	__free_pages(page, order);
 }
 
-- 
2.1.4

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