cgroup, memcg: implement css->id and convert css_from_id() to use it

Until now, cgroup->id has been used to identify all the associated
csses and css_from_id() takes cgroup ID and returns the matching css
by looking up the cgroup and then dereferencing the css associated
with it; however, now that the lifetimes of cgroup and css are
separate, this is incorrect and breaks on the unified hierarchy when a
controller is disabled and enabled back again before the previous
instance is released.

This patch adds css->id which is a subsystem-unique ID and converts
css_from_id() to look up by the new css->id instead.  memcg is the
only user of css_from_id() and also converted to use css->id instead.

For traditional hierarchies, this shouldn't make any functional
difference.

Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Michal Hocko <mhocko@suse.cz>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Jianyu Zhan <nasa4836@gmail.com>
Acked-by: Li Zefan <lizefan@huawei.com>

diff --git a/include/linux/cgroup.h b/include/linux/cgroup.h
index 793f70a48820ccfd7468159d0762713015b375c3..2dfabb3b749a49b885877d1d960aad1b22e22672 100644 (file)
--- a/include/linux/cgroup.h
+++ b/include/linux/cgroup.h
@@ -62,6 +62,12 @@ struct cgroup_subsys_state {
        /* the parent css */
        struct cgroup_subsys_state *parent;
 
+       /*
+        * Subsys-unique ID.  0 is unused and root is always 1.  The
+        * matching css can be looked up using css_from_id().
+        */
+       int id;
+
        unsigned int flags;
 
        /* percpu_ref killing and RCU release */
@@ -655,6 +661,9 @@ struct cgroup_subsys {
        /* link to parent, protected by cgroup_lock() */
        struct cgroup_root *root;
 
+       /* idr for css->id */
+       struct idr css_idr;
+
        /*
         * List of cftypes.  Each entry is the first entry of an array
         * terminated by zero length name.

diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index f1c98c527b2d7243e89883626a38787d3ee198b2..a1a20e8c973a2e350da265829bfd47dacf65095b 100644 (file)
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -100,8 +100,8 @@ static DECLARE_RWSEM(css_set_rwsem);
 #endif
 
 /*
- * Protects cgroup_idr so that IDs can be released without grabbing
- * cgroup_mutex.
+ * Protects cgroup_idr and css_idr so that IDs can be released without
+ * grabbing cgroup_mutex.
  */
 static DEFINE_SPINLOCK(cgroup_idr_lock);
 
@@ -1089,12 +1089,6 @@ static void cgroup_put(struct cgroup *cgrp)
        if (WARN_ON_ONCE(cgrp->parent && !cgroup_is_dead(cgrp)))
                return;
 
-       /*
-        * XXX: cgrp->id is only used to look up css's.  As cgroup and
-        * css's lifetimes will be decoupled, it should be made
-        * per-subsystem and moved to css->id so that lookups are
-        * successful until the target css is released.
-        */
        cgroup_idr_remove(&cgrp->root->cgroup_idr, cgrp->id);
        cgrp->id = -1;
 
@@ -4104,8 +4098,11 @@ static void css_release(struct percpu_ref *ref)
 {
        struct cgroup_subsys_state *css =
                container_of(ref, struct cgroup_subsys_state, refcnt);
+       struct cgroup_subsys *ss = css->ss;
+
+       RCU_INIT_POINTER(css->cgroup->subsys[ss->id], NULL);
+       cgroup_idr_remove(&ss->css_idr, css->id);
 
-       RCU_INIT_POINTER(css->cgroup->subsys[css->ss->id], NULL);
        call_rcu(&css->rcu_head, css_free_rcu_fn);
 }
 
@@ -4195,9 +4192,17 @@ static int create_css(struct cgroup *cgrp, struct cgroup_subsys *ss)
        if (err)
                goto err_free_css;
 
+       err = cgroup_idr_alloc(&ss->css_idr, NULL, 2, 0, GFP_NOWAIT);
+       if (err < 0)
+               goto err_free_percpu_ref;
+       css->id = err;
+
        err = cgroup_populate_dir(cgrp, 1 << ss->id);
        if (err)
-               goto err_free_percpu_ref;
+               goto err_free_id;
+
+       /* @css is ready to be brought online now, make it visible */
+       cgroup_idr_replace(&ss->css_idr, css, css->id);
 
        err = online_css(css);
        if (err)
@@ -4216,6 +4221,8 @@ static int create_css(struct cgroup *cgrp, struct cgroup_subsys *ss)
 
 err_clear_dir:
        cgroup_clear_dir(css->cgroup, 1 << css->ss->id);
+err_free_id:
+       cgroup_idr_remove(&ss->css_idr, css->id);
 err_free_percpu_ref:
        percpu_ref_cancel_init(&css->refcnt);
 err_free_css:
@@ -4642,7 +4649,7 @@ static struct kernfs_syscall_ops cgroup_kf_syscall_ops = {
        .rename                 = cgroup_rename,
 };
 
-static void __init cgroup_init_subsys(struct cgroup_subsys *ss)
+static void __init cgroup_init_subsys(struct cgroup_subsys *ss, bool early)
 {
        struct cgroup_subsys_state *css;
 
@@ -4651,6 +4658,7 @@ static void __init cgroup_init_subsys(struct cgroup_subsys *ss)
        mutex_lock(&cgroup_tree_mutex);
        mutex_lock(&cgroup_mutex);
 
+       idr_init(&ss->css_idr);
        INIT_LIST_HEAD(&ss->cfts);
 
        /* Create the root cgroup state for this subsystem */
@@ -4659,6 +4667,13 @@ static void __init cgroup_init_subsys(struct cgroup_subsys *ss)
        /* We don't handle early failures gracefully */
        BUG_ON(IS_ERR(css));
        init_and_link_css(css, ss, &cgrp_dfl_root.cgrp);
+       if (early) {
+               /* idr_alloc() can't be called safely during early init */
+               css->id = 1;
+       } else {
+               css->id = cgroup_idr_alloc(&ss->css_idr, css, 1, 2, GFP_KERNEL);
+               BUG_ON(css->id < 0);
+       }
 
        /* Update the init_css_set to contain a subsys
         * pointer to this state - since the subsystem is
@@ -4709,7 +4724,7 @@ int __init cgroup_init_early(void)
                ss->name = cgroup_subsys_name[i];
 
                if (ss->early_init)
-                       cgroup_init_subsys(ss);
+                       cgroup_init_subsys(ss, true);
        }
        return 0;
 }
@@ -4741,8 +4756,16 @@ int __init cgroup_init(void)
        mutex_unlock(&cgroup_tree_mutex);
 
        for_each_subsys(ss, ssid) {
-               if (!ss->early_init)
-                       cgroup_init_subsys(ss);
+               if (ss->early_init) {
+                       struct cgroup_subsys_state *css =
+                               init_css_set.subsys[ss->id];
+
+                       css->id = cgroup_idr_alloc(&ss->css_idr, css, 1, 2,
+                                                  GFP_KERNEL);
+                       BUG_ON(css->id < 0);
+               } else {
+                       cgroup_init_subsys(ss, false);
+               }
 
                list_add_tail(&init_css_set.e_cset_node[ssid],
                              &cgrp_dfl_root.cgrp.e_csets[ssid]);
@@ -5196,14 +5219,8 @@ struct cgroup_subsys_state *css_tryget_from_dir(struct dentry *dentry,
  */
 struct cgroup_subsys_state *css_from_id(int id, struct cgroup_subsys *ss)
 {
-       struct cgroup *cgrp;
-
        WARN_ON_ONCE(!rcu_read_lock_held());
-
-       cgrp = idr_find(&ss->root->cgroup_idr, id);
-       if (cgrp)
-               return cgroup_css(cgrp, ss);
-       return NULL;
+       return idr_find(&ss->css_idr, id);
 }
 
 #ifdef CONFIG_CGROUP_DEBUG

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 1d0b29715b73c7a3d2e542b9404bebaf7dc9d2ec..c3f82f69ef58f90621caeed859be501f44485b45 100644 (file)
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -527,7 +527,7 @@ static inline bool mem_cgroup_is_root(struct mem_cgroup *memcg)
 
 static inline unsigned short mem_cgroup_id(struct mem_cgroup *memcg)
 {
-       return memcg->css.cgroup->id;
+       return memcg->css.id;
 }
 
 static inline struct mem_cgroup *mem_cgroup_from_id(unsigned short id)
@@ -6401,7 +6401,7 @@ mem_cgroup_css_online(struct cgroup_subsys_state *css)
        struct mem_cgroup *memcg = mem_cgroup_from_css(css);
        struct mem_cgroup *parent = mem_cgroup_from_css(css_parent(css));
 
-       if (css->cgroup->id > MEM_CGROUP_ID_MAX)
+       if (css->id > MEM_CGROUP_ID_MAX)
                return -ENOSPC;
 
        if (!parent)