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Subject[RFC][PATCH 1/6] memcg: fix pre_destory handler
better name for new flag is welcome.

==
Because pre_destroy() handler is moved out to cgroup_lock() for
avoiding dead-lock, now, cgroup's rmdir() does following sequence.

cgroup_lock()
check children and tasks.
(A)
cgroup_unlock()
(B)
pre_destroy() for subsys;-----(1)
(C)
cgroup_lock();
(D)
Second check:check for -EBUSY again because we released the lock.
(E)
mark cgroup as removed.
(F)
unlink from lists.
cgroup_unlock();
dput()
=> when dentry's refcnt goes down to 0
destroy() handers for subsys
memcg marks itself as "obsolete" when pre_destroy() is called at (1)
But rmdir() can fail after pre_destroy(). So marking "obsolete" is bug.
I'd like to fix sanity of pre_destroy() in cgroup layer.

Considering above sequence, new tasks can be added while
(B) and (C)
swap-in recored can be charged back to a cgroup after pre_destroy()
at (C) and (D), (E)
(means cgrp's refcnt not comes from task but from other persistent objects.)

This patch adds "cgroup_is_being_removed()" check. (better name is welcome)
After this,

- cgroup is marked as CGRP_PRE_REMOVAL at (A)
- If Second check fails, CGRP_PRE_REMOVAL flag is removed.
- memcg's its own obsolete flag is removed.
- While CGROUP_PRE_REMOVAL, task attach will fail by -EBUSY.
(task attach via clone() will not hit the case.)

By this, we can trust pre_restroy()'s result.


Note: if CGRP_REMOVED can be set and cleared, it should be used instead of
CGRP_PRE_REMOVAL.


Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>

---
include/linux/cgroup.h | 5 +++++
kernel/cgroup.c | 18 +++++++++++++++++-
mm/memcontrol.c | 36 ++++++++++++++++++++++++++----------
3 files changed, 48 insertions(+), 11 deletions(-)
Index: mmotm-2.6.28-Dec08/mm/memcontrol.c
===================================================================
--- mmotm-2.6.28-Dec08.orig/mm/memcontrol.c
+++ mmotm-2.6.28-Dec08/mm/memcontrol.c
@@ -166,7 +166,6 @@ struct mem_cgroup {
*/
bool use_hierarchy;
unsigned long last_oom_jiffies;
- int obsolete;
atomic_t refcnt;

unsigned int swappiness;
@@ -211,6 +210,24 @@ pcg_default_flags[NR_CHARGE_TYPE] = {
static void mem_cgroup_get(struct mem_cgroup *mem);
static void mem_cgroup_put(struct mem_cgroup *mem);

+static bool memcg_is_obsolete(struct mem_cgroup *mem)
+{
+ struct cgroup *cg = mem->css.cgroup;
+ /*
+ * "Being Removed" means pre_destroy() handler is called.
+ * After "pre_destroy" handler is called, memcg should not
+ * have any additional charges.
+ * This means there are small races for mis-accounting. But this
+ * mis-accounting should happen only under swap-in opration.
+ * (Attachin new task will fail if cgroup is under rmdir()).
+ */
+
+ if (!cg || cgroup_is_removed(cg) || cgroup_is_being_removed(cg))
+ return true;
+ return false;
+}
+
+
static void mem_cgroup_charge_statistics(struct mem_cgroup *mem,
struct page_cgroup *pc,
bool charge)
@@ -597,7 +614,7 @@ mem_cgroup_get_first_node(struct mem_cgr
struct cgroup *cgroup;
struct mem_cgroup *ret;
bool obsolete = (root_mem->last_scanned_child &&
- root_mem->last_scanned_child->obsolete);
+ memcg_is_obsolete(root_mem->last_scanned_child);

/*
* Scan all children under the mem_cgroup mem
@@ -1070,7 +1087,7 @@ int mem_cgroup_try_charge_swapin(struct
ent.val = page_private(page);

mem = lookup_swap_cgroup(ent);
- if (!mem || mem->obsolete)
+ if (!mem || memcg_is_obsolete(mem))
goto charge_cur_mm;
*ptr = mem;
return __mem_cgroup_try_charge(NULL, mask, ptr, true);
@@ -1104,7 +1121,7 @@ int mem_cgroup_cache_charge_swapin(struc
ent.val = page_private(page);
if (do_swap_account) {
mem = lookup_swap_cgroup(ent);
- if (mem && mem->obsolete)
+ if (mem && memcg_is_obsolete(mem))
mem = NULL;
if (mem)
mm = NULL;
@@ -2050,9 +2067,6 @@ static struct mem_cgroup *mem_cgroup_all
* the number of reference from swap_cgroup and free mem_cgroup when
* it goes down to 0.
*
- * When mem_cgroup is destroyed, mem->obsolete will be set to 0 and
- * entry which points to this memcg will be ignore at swapin.
- *
* Removal of cgroup itself succeeds regardless of refs from swap.
*/

@@ -2081,7 +2095,7 @@ static void mem_cgroup_get(struct mem_cg
static void mem_cgroup_put(struct mem_cgroup *mem)
{
if (atomic_dec_and_test(&mem->refcnt)) {
- if (!mem->obsolete)
+ if (!memcg_is_obsolete(mem))
return;
mem_cgroup_free(mem);
}
@@ -2148,14 +2162,16 @@ static void mem_cgroup_pre_destroy(struc
struct cgroup *cont)
{
struct mem_cgroup *mem = mem_cgroup_from_cont(cont);
- mem->obsolete = 1;
mem_cgroup_force_empty(mem, false);
}

static void mem_cgroup_destroy(struct cgroup_subsys *ss,
struct cgroup *cont)
{
- mem_cgroup_free(mem_cgroup_from_cont(cont));
+ struct mem_cgroup *mem = mem_cgroup_from_cont(cont):
+ mem_cgroup_free(mem);
+ /* forget */
+ mem->css.cgroup = NULL;
}

static int mem_cgroup_populate(struct cgroup_subsys *ss,
Index: mmotm-2.6.28-Dec08/include/linux/cgroup.h
===================================================================
--- mmotm-2.6.28-Dec08.orig/include/linux/cgroup.h
+++ mmotm-2.6.28-Dec08/include/linux/cgroup.h
@@ -98,6 +98,8 @@ enum {
CGRP_RELEASABLE,
/* Control Group requires release notifications to userspace */
CGRP_NOTIFY_ON_RELEASE,
+ /* Control Group is preparing for death */
+ CGRP_PRE_REMOVAL,
};

struct cgroup {
@@ -303,8 +305,11 @@ int cgroup_add_files(struct cgroup *cgrp
const struct cftype cft[],
int count);

+
int cgroup_is_removed(const struct cgroup *cgrp);

+int cgroup_is_being_removed(const struct cgroup *cgrp);
+
int cgroup_path(const struct cgroup *cgrp, char *buf, int buflen);

int cgroup_task_count(const struct cgroup *cgrp);
Index: mmotm-2.6.28-Dec08/kernel/cgroup.c
===================================================================
--- mmotm-2.6.28-Dec08.orig/kernel/cgroup.c
+++ mmotm-2.6.28-Dec08/kernel/cgroup.c
@@ -123,6 +123,11 @@ inline int cgroup_is_removed(const struc
return test_bit(CGRP_REMOVED, &cgrp->flags);
}

+inline int cgroup_is_being_removed(const struct cgroup *cgrp)
+{
+ return test_bit(CGRP_PRE_REMOVAL, &cgrp->flags);
+}
+
/* bits in struct cgroupfs_root flags field */
enum {
ROOT_NOPREFIX, /* mounted subsystems have no named prefix */
@@ -1217,6 +1222,13 @@ int cgroup_attach_task(struct cgroup *cg
if (cgrp == oldcgrp)
return 0;

+ /*
+ * This cgroup is under rmdir() operation. Never fails here when this
+ * is called from clone().
+ */
+ if (cgroup_is_being_removed(cgrp))
+ return -EBUSY;
+
for_each_subsys(root, ss) {
if (ss->can_attach) {
retval = ss->can_attach(ss, cgrp, tsk);
@@ -2469,12 +2481,14 @@ static int cgroup_rmdir(struct inode *un
mutex_unlock(&cgroup_mutex);
return -EBUSY;
}
- mutex_unlock(&cgroup_mutex);

/*
* Call pre_destroy handlers of subsys. Notify subsystems
* that rmdir() request comes.
*/
+ set_bit(CGRP_PRE_REMOVAL, &cgrp->flags);
+ mutex_unlock(&cgroup_mutex);
+
cgroup_call_pre_destroy(cgrp);

mutex_lock(&cgroup_mutex);
@@ -2483,12 +2497,14 @@ static int cgroup_rmdir(struct inode *un
if (atomic_read(&cgrp->count)
|| !list_empty(&cgrp->children)
|| cgroup_has_css_refs(cgrp)) {
+ clear_bit(CGRP_PRE_REMOVAL, &cgrp->flags);
mutex_unlock(&cgroup_mutex);
return -EBUSY;
}

spin_lock(&release_list_lock);
set_bit(CGRP_REMOVED, &cgrp->flags);
+ clear_bit(CGRP_PRE_REMOVAL, &cgrp->flags);
if (!list_empty(&cgrp->release_list))
list_del(&cgrp->release_list);
spin_unlock(&release_list_lock);


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