[PATCH -mm 1/3] memcg: do not abuse memcg_kmem_skip_account

[PATCH -mm 1/3] memcg: do not abuse memcg_kmem_skip_account

[Vladimir Davydov]

task_struct->memcg_kmem_skip_account was initially introduced to avoid
recursion during kmem cache creation: memcg_kmem_get_cache, which is
called by kmem_cache_alloc to determine the per-memcg cache to account
allocation to, may issue lazy cache creation if the needed cache doesn't
exist, which means issuing yet another kmem_cache_alloc. We can't just
pass a flag to the nested kmem_cache_alloc disabling kmem accounting,
because there are hidden allocations, e.g. in INIT_WORK. So we
introduced a flag on the task_struct, memcg_kmem_skip_account, making
memcg_kmem_get_cache return immediately.

By its nature, the flag may also be used to disable accounting for
allocations shared among different cgroups, and currently it is used
this way in memcg_activate_kmem. Using it like this looks like abusing
it to me. If we want to disable accounting for some allocatrons (which
we will definitely want one day), we should either add GFP_NO_MEMCG or
GFP_MEMCG flag in order to blacklist/whitelist some allocations.

For now, let's simply remove memcg_stop/resume_kmem_account from
memcg_activate_kmem.

Signed-off-by: Vladimir Davydov <vdavydov@parallels.com>
---
 mm/memcontrol.c |    7 -------
 1 file changed, 7 deletions(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index f61ecbc97d30..b3fe830fdb29 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -3428,12 +3428,6 @@ static int memcg_activate_kmem(struct mem_cgroup *memcg,
 		return 0;
 
 	/*
-	 * We are going to allocate memory for data shared by all memory
-	 * cgroups so let's stop accounting here.
-	 */
-	memcg_stop_kmem_account();
-
-	/*
 	 * For simplicity, we won't allow this to be disabled.  It also can't
 	 * be changed if the cgroup has children already, or if tasks had
 	 * already joined.
@@ -3475,7 +3469,6 @@ static int memcg_activate_kmem(struct mem_cgroup *memcg,
 	 */
 	memcg->kmemcg_id = memcg_id;
 out:
-	memcg_resume_kmem_account();
 	return err;
 }
 
-- 
1.7.10.4

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[PATCH -mm 2/3] 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 40497a2ed2d4..7b08b0240736 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;
@@ -1684,8 +1688,7 @@ 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 b3fe830fdb29..52d1e933bb9f 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -2575,37 +2575,6 @@ void memcg_update_array_size(int num)
 	memcg_limited_groups_array_size = num;
 }
 
-/*
- * 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--;
-}
-
 struct memcg_cache_create_work {
 	struct mem_cgroup *memcg;
 	struct kmem_cache *cachep;
@@ -2660,9 +2629,9 @@ static void memcg_schedule_cache_create(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_cache_create(memcg, cachep);
-	memcg_resume_kmem_account();
+	current->memcg_kmem_skip_account = 0;
 }
 
 /*
-- 
1.7.10.4

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[PATCH -mm 3/3] 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 in checking the flag in
memcg_kmem_newpage_charge or memcg_kmem_recharge_slab, because there's
no way how these two functions 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 |   31 -------------------------------
 1 file changed, 31 deletions(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 52d1e933bb9f..d7de40cb3c8e 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -2717,34 +2717,6 @@ __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)) {
@@ -2800,9 +2772,6 @@ int __memcg_kmem_recharge_slab(void *obj, gfp_t gfp)
 	int nr_pages;
 	int ret = 0;
 
-	if (current->memcg_kmem_skip_account)
-		goto out;
-
 	page = virt_to_head_page(obj);
 	page_memcg = ACCESS_ONCE(page->mem_cgroup);
 
-- 
1.7.10.4

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