[PATCH -mm 1/2] memcg: only check memcg_kmem_skip_account in __memcg_kmem_get_cache

[PATCH -mm 1/2] memcg: only check memcg_kmem_skip_account in __memcg_kmem_get_cache

[Vladimir Davydov]

__memcg_kmem_get_cache can recurse if it calls kmalloc (which it does if
the cgroup's kmem cache doesn't exist), because kmalloc may call
__memcg_kmem_get_cache internally again. To avoid the recursion, we use
the task_struct->memcg_kmem_skip_account flag.

However, there's no need checking the flag in memcg_kmem_newpage_charge,
because there's no way how this function could result in recursion, if
called from memcg_kmem_get_cache. So let's remove the redundant code.

Signed-off-by: Vladimir Davydov <vdavydov@parallels.com>
---
 mm/memcontrol.c |   28 ----------------------------
 1 file changed, 28 deletions(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index bb8c237026cc..d9fab72da52e 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -2905,34 +2905,6 @@ __memcg_kmem_newpage_charge(gfp_t gfp, struct mem_cgroup **_memcg, int order)
 
 	*_memcg = NULL;
 
-	/*
-	 * Disabling accounting is only relevant for some specific memcg
-	 * internal allocations. Therefore we would initially not have such
-	 * check here, since direct calls to the page allocator that are
-	 * accounted to kmemcg (alloc_kmem_pages and friends) only happen
-	 * outside memcg core. We are mostly concerned with cache allocations,
-	 * and by having this test at memcg_kmem_get_cache, we are already able
-	 * to relay the allocation to the root cache and bypass the memcg cache
-	 * altogether.
-	 *
-	 * There is one exception, though: the SLUB allocator does not create
-	 * large order caches, but rather service large kmallocs directly from
-	 * the page allocator. Therefore, the following sequence when backed by
-	 * the SLUB allocator:
-	 *
-	 *	memcg_stop_kmem_account();
-	 *	kmalloc(<large_number>)
-	 *	memcg_resume_kmem_account();
-	 *
-	 * would effectively ignore the fact that we should skip accounting,
-	 * since it will drive us directly to this function without passing
-	 * through the cache selector memcg_kmem_get_cache. Such large
-	 * allocations are extremely rare but can happen, for instance, for the
-	 * cache arrays. We bring this test here.
-	 */
-	if (current->memcg_kmem_skip_account)
-		return true;
-
 	memcg = get_mem_cgroup_from_mm(current->mm);
 
 	if (!memcg_kmem_is_active(memcg)) {
-- 
1.7.10.4

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[PATCH -mm 2/2] memcg: turn memcg_kmem_skip_account into a bit field

[Vladimir Davydov]

It isn't supposed to stack, so turn it into a bit-field to save 4 bytes
on the task_struct.

Also, remove the memcg_stop/resume_kmem_account helpers - it is clearer
to set/clear the flag inline. Regarding the overwhelming comment to the
helpers, which is removed by this patch too, we already have a compact
yet accurate explanation in memcg_schedule_cache_create, no need in yet
another one.

Signed-off-by: Vladimir Davydov <vdavydov@parallels.com>
---
 include/linux/sched.h |    7 +++++--
 mm/memcontrol.c       |   35 ++---------------------------------
 2 files changed, 7 insertions(+), 35 deletions(-)

diff --git a/include/linux/sched.h b/include/linux/sched.h
index ffd1444b087c..8db31ef98d2f 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -1364,6 +1364,10 @@ struct task_struct {
 	unsigned sched_reset_on_fork:1;
 	unsigned sched_contributes_to_load:1;
 
+#ifdef CONFIG_MEMCG_KMEM
+	unsigned memcg_kmem_skip_account:1;
+#endif
+
 	unsigned long atomic_flags; /* Flags needing atomic access. */
 
 	pid_t pid;
@@ -1679,8 +1683,7 @@ struct task_struct {
 	/* bitmask and counter of trace recursion */
 	unsigned long trace_recursion;
 #endif /* CONFIG_TRACING */
-#ifdef CONFIG_MEMCG /* memcg uses this to do batch job */
-	unsigned int memcg_kmem_skip_account;
+#ifdef CONFIG_MEMCG
 	struct memcg_oom_info {
 		struct mem_cgroup *memcg;
 		gfp_t gfp_mask;
diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index d9fab72da52e..11cbfde4dc6d 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -2673,37 +2673,6 @@ static void memcg_unregister_cache(struct kmem_cache *cachep)
 	css_put(&memcg->css);
 }
 
-/*
- * During the creation a new cache, we need to disable our accounting mechanism
- * altogether. This is true even if we are not creating, but rather just
- * enqueing new caches to be created.
- *
- * This is because that process will trigger allocations; some visible, like
- * explicit kmallocs to auxiliary data structures, name strings and internal
- * cache structures; some well concealed, like INIT_WORK() that can allocate
- * objects during debug.
- *
- * If any allocation happens during memcg_kmem_get_cache, we will recurse back
- * to it. This may not be a bounded recursion: since the first cache creation
- * failed to complete (waiting on the allocation), we'll just try to create the
- * cache again, failing at the same point.
- *
- * memcg_kmem_get_cache is prepared to abort after seeing a positive count of
- * memcg_kmem_skip_account. So we enclose anything that might allocate memory
- * inside the following two functions.
- */
-static inline void memcg_stop_kmem_account(void)
-{
-	VM_BUG_ON(!current->mm);
-	current->memcg_kmem_skip_account++;
-}
-
-static inline void memcg_resume_kmem_account(void)
-{
-	VM_BUG_ON(!current->mm);
-	current->memcg_kmem_skip_account--;
-}
-
 int __memcg_cleanup_cache_params(struct kmem_cache *s)
 {
 	struct kmem_cache *c;
@@ -2798,9 +2767,9 @@ static void memcg_schedule_register_cache(struct mem_cgroup *memcg,
 	 * this point we can't allow ourselves back into memcg_kmem_get_cache,
 	 * the safest choice is to do it like this, wrapping the whole function.
 	 */
-	memcg_stop_kmem_account();
+	current->memcg_kmem_skip_account = 1;
 	__memcg_schedule_register_cache(memcg, cachep);
-	memcg_resume_kmem_account();
+	current->memcg_kmem_skip_account = 0;
 }
 
 int __memcg_charge_slab(struct kmem_cache *cachep, gfp_t gfp, int order)
-- 
1.7.10.4

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