[PATCH v3 0/8] memcg, cgroup: kill css_id

[PATCH v3 0/8] memcg, cgroup: kill css_id

[Li Zefan]

This patchset converts memcg to use cgroup->id, and then we can remove
cgroup css_id.

As we've removed memcg's own refcnt, converting memcg to use cgroup->id
is very straight-forward.

The patchset is based on Tejun's cgroup tree.


v2->v3:
- some minor cleanups suggested by Michal.
- fixed the call to idr_alloc() in cgroup_init() in the first patch.

Li Zefan (8):
      cgroup: convert cgroup_ida to cgroup_idr
      cgroup: document how cgroup IDs are assigned
      cgroup: implement cgroup_from_id()
      memcg: convert to use cgroup_is_descendant()
      memcg: convert to use cgroup id
      memcg: fail to create cgroup if the cgroup id is too big
      memcg: stop using css id
      cgroup: kill css_id
--
 include/linux/cgroup.h |  49 ++--------
 kernel/cgroup.c        | 296 ++++++++---------------------------------------------------
 mm/memcontrol.c        |  68 ++++++++------
 3 files changed, 91 insertions(+), 322 deletions(-)

[PATCH v3 1/8] cgroup: convert cgroup_ida to cgroup_idr

[Li Zefan]

This enables us to lookup a cgroup by its id.

v3:
- on success, idr_alloc() returns the id but not 0, so fix the BUG_ON()
  in cgroup_init().
- pass the right value to idr_alloc() so that the id for dummy cgroup is 0.

Signed-off-by: Li Zefan <lizefan@huawei.com>
Reviewed-by: Michal Hocko <mhocko@suse.cz>
---
 include/linux/cgroup.h |  4 ++--
 kernel/cgroup.c        | 28 ++++++++++++++++++++++------
 2 files changed, 24 insertions(+), 8 deletions(-)

diff --git a/include/linux/cgroup.h b/include/linux/cgroup.h
index 297462b..2bd052d 100644
--- a/include/linux/cgroup.h
+++ b/include/linux/cgroup.h
@@ -161,7 +161,7 @@ struct cgroup_name {
 struct cgroup {
 	unsigned long flags;		/* "unsigned long" so bitops work */
 
-	int id;				/* ida allocated in-hierarchy ID */
+	int id;				/* idr allocated in-hierarchy ID */
 
 	/*
 	 * We link our 'sibling' struct into our parent's 'children'.
@@ -322,7 +322,7 @@ struct cgroupfs_root {
 	unsigned long flags;
 
 	/* IDs for cgroups in this hierarchy */
-	struct ida cgroup_ida;
+	struct idr cgroup_idr;
 
 	/* The path to use for release notifications. */
 	char release_agent_path[PATH_MAX];
diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index 345fac8..b7c7c68 100644
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -866,8 +866,6 @@ static void cgroup_free_fn(struct work_struct *work)
 	 */
 	dput(cgrp->parent->dentry);
 
-	ida_simple_remove(&cgrp->root->cgroup_ida, cgrp->id);
-
 	/*
 	 * Drop the active superblock reference that we took when we
 	 * created the cgroup. This will free cgrp->root, if we are
@@ -1379,6 +1377,7 @@ static void init_cgroup_root(struct cgroupfs_root *root)
 	cgrp->root = root;
 	RCU_INIT_POINTER(cgrp->name, &root_cgroup_name);
 	init_cgroup_housekeeping(cgrp);
+	idr_init(&root->cgroup_idr);
 }
 
 static int cgroup_init_root_id(struct cgroupfs_root *root, int start, int end)
@@ -1451,7 +1450,6 @@ static struct cgroupfs_root *cgroup_root_from_opts(struct cgroup_sb_opts *opts)
 	 */
 	root->subsys_mask = opts->subsys_mask;
 	root->flags = opts->flags;
-	ida_init(&root->cgroup_ida);
 	if (opts->release_agent)
 		strcpy(root->release_agent_path, opts->release_agent);
 	if (opts->name)
@@ -1467,7 +1465,7 @@ static void cgroup_free_root(struct cgroupfs_root *root)
 		/* hierarhcy ID shoulid already have been released */
 		WARN_ON_ONCE(root->hierarchy_id);
 
-		ida_destroy(&root->cgroup_ida);
+		idr_destroy(&root->cgroup_idr);
 		kfree(root);
 	}
 }
@@ -1582,6 +1580,11 @@ static struct dentry *cgroup_mount(struct file_system_type *fs_type,
 		mutex_lock(&cgroup_mutex);
 		mutex_lock(&cgroup_root_mutex);
 
+		root_cgrp->id = idr_alloc(&root->cgroup_idr, root_cgrp,
+					   0, 1, GFP_KERNEL);
+		if (root_cgrp->id < 0)
+			goto unlock_drop;
+
 		/* Check for name clashes with existing mounts */
 		ret = -EBUSY;
 		if (strlen(root->name))
@@ -4273,7 +4276,11 @@ static long cgroup_create(struct cgroup *parent, struct dentry *dentry,
 		goto err_free_cgrp;
 	rcu_assign_pointer(cgrp->name, name);
 
-	cgrp->id = ida_simple_get(&root->cgroup_ida, 1, 0, GFP_KERNEL);
+	/*
+	 * Temporarily set the pointer to NULL, so idr_find() won't return
+	 * a half-baked cgroup.
+	 */
+	cgrp->id = idr_alloc(&root->cgroup_idr, NULL, 1, 0, GFP_KERNEL);
 	if (cgrp->id < 0)
 		goto err_free_name;
 
@@ -4371,6 +4378,8 @@ static long cgroup_create(struct cgroup *parent, struct dentry *dentry,
 		}
 	}
 
+	idr_replace(&root->cgroup_idr, cgrp, cgrp->id);
+
 	err = cgroup_addrm_files(cgrp, NULL, cgroup_base_files, true);
 	if (err)
 		goto err_destroy;
@@ -4397,7 +4406,7 @@ err_free_all:
 	/* Release the reference count that we took on the superblock */
 	deactivate_super(sb);
 err_free_id:
-	ida_simple_remove(&root->cgroup_ida, cgrp->id);
+	idr_remove(&root->cgroup_idr, cgrp->id);
 err_free_name:
 	kfree(rcu_dereference_raw(cgrp->name));
 err_free_cgrp:
@@ -4590,6 +4599,9 @@ static void cgroup_offline_fn(struct work_struct *work)
 	/* delete this cgroup from parent->children */
 	list_del_rcu(&cgrp->sibling);
 
+	if (cgrp->id)
+		idr_remove(&cgrp->root->cgroup_idr, cgrp->id);
+
 	dput(d);
 
 	set_bit(CGRP_RELEASABLE, &parent->flags);
@@ -4915,6 +4927,10 @@ int __init cgroup_init(void)
 
 	BUG_ON(cgroup_init_root_id(&cgroup_dummy_root, 0, 1));
 
+	err = idr_alloc(&cgroup_dummy_root.cgroup_idr, cgroup_dummy_top,
+			0, 1, GFP_KERNEL);
+	BUG_ON(err < 0);
+
 	mutex_unlock(&cgroup_root_mutex);
 	mutex_unlock(&cgroup_mutex);
 
-- 
1.8.0.2

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[PATCH v3 2/8] cgroup: document how cgroup IDs are assigned

[Li Zefan]

As cgroup id has been used in netprio cgroup and will be used in memcg,
it's important to make it clear how a cgroup id is allocated.

For example, in netprio cgroup, the id is used as index of anarray.

Signed-off-by: Li Zefan <lizefan@huwei.com>
Reviewed-by: Michal Hocko <mhocko@suse.cz>
---
 include/linux/cgroup.h | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/include/linux/cgroup.h b/include/linux/cgroup.h
index 2bd052d..8c107e9 100644
--- a/include/linux/cgroup.h
+++ b/include/linux/cgroup.h
@@ -161,7 +161,13 @@ struct cgroup_name {
 struct cgroup {
 	unsigned long flags;		/* "unsigned long" so bitops work */
 
-	int id;				/* idr allocated in-hierarchy ID */
+	/*
+	 * idr allocated in-hierarchy ID.
+	 *
+	 * The ID of the root cgroup is always 0, and a new cgroup
+	 * will be assigned with a smallest available ID.
+	 */
+	int id;
 
 	/*
 	 * We link our 'sibling' struct into our parent's 'children'.
-- 
1.8.0.2

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[PATCH v3 3/8] cgroup: implement cgroup_from_id()

[Li Zefan]

This will be used as a replacement for css_lookup().

There's a difference with cgroup id and css id. cgroup id starts with 0,
while css id starts with 1.

Signed-off-by: Li Zefan <lizefan@huawei.com>
Reviewed-by: Michal Hocko <mhocko@suse.cz>
---
 include/linux/cgroup.h |  2 ++
 kernel/cgroup.c        | 16 ++++++++++++++++
 2 files changed, 18 insertions(+)

diff --git a/include/linux/cgroup.h b/include/linux/cgroup.h
index 8c107e9..e8eb361 100644
--- a/include/linux/cgroup.h
+++ b/include/linux/cgroup.h
@@ -553,6 +553,8 @@ int task_cgroup_path_from_hierarchy(struct task_struct *task, int hierarchy_id,
 
 int cgroup_task_count(const struct cgroup *cgrp);
 
+struct cgroup *cgroup_from_id(struct cgroup_subsys *ss, int id);
+
 /*
  * Control Group taskset, used to pass around set of tasks to cgroup_subsys
  * methods.
diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index b7c7c68..dc4a749 100644
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -5536,6 +5536,22 @@ struct cgroup_subsys_state *cgroup_css_from_dir(struct file *f, int id)
 	return css ? css : ERR_PTR(-ENOENT);
 }
 
+/**
+ * cgroup_from_id - lookup cgroup by id
+ * @ss: cgroup subsys to be looked into
+ * @id: the cgroup id
+ *
+ * Returns the cgroup if there's valid one with @id, otherwise returns NULL.
+ * Should be called under rcu_readlock().
+ */
+struct cgroup *cgroup_from_id(struct cgroup_subsys *ss, int id)
+{
+	rcu_lockdep_assert(rcu_read_lock_held(),
+			   "cgroup_from_id() needs rcu_read_lock()"
+			   " protection");
+	return idr_find(&ss->root->cgroup_idr, id);
+}
+
 #ifdef CONFIG_CGROUP_DEBUG
 static struct cgroup_subsys_state *debug_css_alloc(struct cgroup *cgrp)
 {
-- 
1.8.0.2

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[PATCH v3 4/8] memcg: convert to use cgroup_is_descendant()

[Li Zefan]

This is a preparation to kill css_id.

Signed-off-by: Li Zefan <lizefan@huawei.com>
Acked-by: Michal Hocko <mhocko@suse.cz>
---
 mm/memcontrol.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index d12ca6f..626c426 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -1434,7 +1434,7 @@ bool __mem_cgroup_same_or_subtree(const struct mem_cgroup *root_memcg,
 		return true;
 	if (!root_memcg->use_hierarchy || !memcg)
 		return false;
-	return css_is_ancestor(&memcg->css, &root_memcg->css);
+	return cgroup_is_descendant(memcg->css.cgroup, root_memcg->css.cgroup);
 }
 
 static bool mem_cgroup_same_or_subtree(const struct mem_cgroup *root_memcg,
-- 
1.8.0.2

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[PATCH v3 5/8] memcg: convert to use cgroup id

[Li Zefan]

Use cgroup id instead of css id. This is a preparation to kill css id.

Note, as memcg treat 0 as an invalid id, while cgroup id starts with 0,
we define memcg_id == cgroup_id + 1.

v3:
- add a helper mem_cgroup_from_id(), suggested by Michal.

Signed-off-by: Li Zefan <lizefan@huawei.com>
Acked-by: Michal Hocko <mhocko@suse.cz>
---
 mm/memcontrol.c | 34 ++++++++++++++++++++++++----------
 1 file changed, 24 insertions(+), 10 deletions(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 626c426..62541f5 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -512,6 +512,25 @@ static inline bool mem_cgroup_is_root(struct mem_cgroup *memcg)
 	return (memcg == root_mem_cgroup);
 }
 
+static inline unsigned short mem_cgroup_id(struct mem_cgroup *memcg)
+{
+	/*
+	 * The ID of the root cgroup is 0, but memcg treat 0 as an
+	 * invalid ID, so we return (cgroup_id + 1).
+	 */
+	return memcg->css.cgroup->id + 1;
+}
+
+static inline struct mem_cgroup *mem_cgroup_from_id(unsigned short id)
+{
+	struct cgroup *cgrp;
+
+	cgrp = cgroup_from_id(&mem_cgroup_subsys, id - 1);
+	if (!cgrp)
+		return NULL;
+	return mem_cgroup_from_cont(cgrp);
+}
+
 /* Writing them here to avoid exposing memcg's inner layout */
 #if defined(CONFIG_INET) && defined(CONFIG_MEMCG_KMEM)
 
@@ -2821,15 +2840,10 @@ static void __mem_cgroup_cancel_local_charge(struct mem_cgroup *memcg,
  */
 static struct mem_cgroup *mem_cgroup_lookup(unsigned short id)
 {
-	struct cgroup_subsys_state *css;
-
 	/* ID 0 is unused ID */
 	if (!id)
 		return NULL;
-	css = css_lookup(&mem_cgroup_subsys, id);
-	if (!css)
-		return NULL;
-	return mem_cgroup_from_css(css);
+	return mem_cgroup_from_id(id);
 }
 
 struct mem_cgroup *try_get_mem_cgroup_from_page(struct page *page)
@@ -4328,7 +4342,7 @@ mem_cgroup_uncharge_swapcache(struct page *page, swp_entry_t ent, bool swapout)
 	 * css_get() was called in uncharge().
 	 */
 	if (do_swap_account && swapout && memcg)
-		swap_cgroup_record(ent, css_id(&memcg->css));
+		swap_cgroup_record(ent, mem_cgroup_id(memcg));
 }
 #endif
 
@@ -4380,8 +4394,8 @@ static int mem_cgroup_move_swap_account(swp_entry_t entry,
 {
 	unsigned short old_id, new_id;
 
-	old_id = css_id(&from->css);
-	new_id = css_id(&to->css);
+	old_id = mem_cgroup_id(from);
+	new_id = mem_cgroup_id(to);
 
 	if (swap_cgroup_cmpxchg(entry, old_id, new_id) == old_id) {
 		mem_cgroup_swap_statistics(from, false);
@@ -6542,7 +6556,7 @@ static enum mc_target_type get_mctgt_type(struct vm_area_struct *vma,
 	}
 	/* There is a swap entry and a page doesn't exist or isn't charged */
 	if (ent.val && !ret &&
-			css_id(&mc.from->css) == lookup_swap_cgroup_id(ent)) {
+	    mem_cgroup_id(mc.from) == lookup_swap_cgroup_id(ent)) {
 		ret = MC_TARGET_SWAP;
 		if (target)
 			target->ent = ent;
-- 
1.8.0.2

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[PATCH v3 6/8] memcg: fail to create cgroup if the cgroup id is too big

[Li Zefan]

memcg requires the cgroup id to be smaller than 65536.

This is a preparation to kill css id.

Signed-off-by: Li Zefan <lizefan@huawei.com>
Acked-by: Michal Hocko <mhocko@suse.cz>
---
 mm/memcontrol.c | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 62541f5..ad7d7c9 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -512,6 +512,12 @@ static inline bool mem_cgroup_is_root(struct mem_cgroup *memcg)
 	return (memcg == root_mem_cgroup);
 }
 
+/*
+ * We restrict the id in the range of [1, 65535], so it can fit into
+ * an unsigned short.
+ */
+#define MEM_CGROUP_ID_MAX	USHRT_MAX
+
 static inline unsigned short mem_cgroup_id(struct mem_cgroup *memcg)
 {
 	/*
@@ -6248,6 +6254,9 @@ mem_cgroup_css_alloc(struct cgroup *cont)
 	long error = -ENOMEM;
 	int node;
 
+	if (cont->id > MEM_CGROUP_ID_MAX)
+		return ERR_PTR(-ENOSPC);
+
 	memcg = mem_cgroup_alloc();
 	if (!memcg)
 		return ERR_PTR(error);
-- 
1.8.0.2

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[PATCH v3 7/8] memcg: stop using css id

[Li Zefan]

Now memcg uses cgroup id instead of css id. Update some comments and
set mem_cgroup_subsys->use_id to 0.

Signed-off-by: Li Zefan <lizefan@huawei.com>
Acked-by: Michal Hocko <mhocko@suse.cz>
---
 mm/memcontrol.c | 23 ++++++++---------------
 1 file changed, 8 insertions(+), 15 deletions(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index ad7d7c9..6b77ab6 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -608,16 +608,11 @@ static void disarm_sock_keys(struct mem_cgroup *memcg)
 #ifdef CONFIG_MEMCG_KMEM
 /*
  * This will be the memcg's index in each cache's ->memcg_params->memcg_caches.
- * There are two main reasons for not using the css_id for this:
- *  1) this works better in sparse environments, where we have a lot of memcgs,
- *     but only a few kmem-limited. Or also, if we have, for instance, 200
- *     memcgs, and none but the 200th is kmem-limited, we'd have to have a
- *     200 entry array for that.
- *
- *  2) In order not to violate the cgroup API, we would like to do all memory
- *     allocation in ->create(). At that point, we haven't yet allocated the
- *     css_id. Having a separate index prevents us from messing with the cgroup
- *     core for this
+ * The main reason for not using cgroup id for this:
+ *  this works better in sparse environments, where we have a lot of memcgs,
+ *  but only a few kmem-limited. Or also, if we have, for instance, 200
+ *  memcgs, and none but the 200th is kmem-limited, we'd have to have a
+ *  200 entry array for that.
  *
  * The current size of the caches array is stored in
  * memcg_limited_groups_array_size.  It will double each time we have to
@@ -632,14 +627,14 @@ int memcg_limited_groups_array_size;
  * cgroups is a reasonable guess. In the future, it could be a parameter or
  * tunable, but that is strictly not necessary.
  *
- * MAX_SIZE should be as large as the number of css_ids. Ideally, we could get
+ * MAX_SIZE should be as large as the number of cgrp_ids. Ideally, we could get
  * this constant directly from cgroup, but it is understandable that this is
  * better kept as an internal representation in cgroup.c. In any case, the
- * css_id space is not getting any smaller, and we don't have to necessarily
+ * cgrp_id space is not getting any smaller, and we don't have to necessarily
  * increase ours as well if it increases.
  */
 #define MEMCG_CACHES_MIN_SIZE 4
-#define MEMCG_CACHES_MAX_SIZE 65535
+#define MEMCG_CACHES_MAX_SIZE MEM_CGROUP_ID_MAX
 
 /*
  * A lot of the calls to the cache allocation functions are expected to be
@@ -6188,7 +6183,6 @@ static void __mem_cgroup_free(struct mem_cgroup *memcg)
 	size_t size = memcg_size();
 
 	mem_cgroup_remove_from_trees(memcg);
-	free_css_id(&mem_cgroup_subsys, &memcg->css);
 
 	for_each_node(node)
 		free_mem_cgroup_per_zone_info(memcg, node);
@@ -6985,7 +6979,6 @@ struct cgroup_subsys mem_cgroup_subsys = {
 	.bind = mem_cgroup_bind,
 	.base_cftypes = mem_cgroup_files,
 	.early_init = 0,
-	.use_id = 1,
 };
 
 #ifdef CONFIG_MEMCG_SWAP
-- 
1.8.0.2

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[PATCH v3 8/8] cgroup: kill css_id

[Li Zefan]

The only user of css_id was memcg, and it has been convered to use
cgroup->id, so kill css_id.

Signed-off-by: Li Zefan <lizefan@huwei.com>
Reviewed-by: Michal Hocko <mhocko@suse.cz>
---
 include/linux/cgroup.h |  37 --------
 kernel/cgroup.c        | 252 +------------------------------------------------
 2 files changed, 1 insertion(+), 288 deletions(-)

diff --git a/include/linux/cgroup.h b/include/linux/cgroup.h
index e8eb361..ec8c380 100644
--- a/include/linux/cgroup.h
+++ b/include/linux/cgroup.h
@@ -599,11 +599,6 @@ struct cgroup_subsys {
 	int subsys_id;
 	int disabled;
 	int early_init;
-	/*
-	 * True if this subsys uses ID. ID is not available before cgroup_init()
-	 * (not available in early_init time.)
-	 */
-	bool use_id;
 
 	/*
 	 * If %false, this subsystem is properly hierarchical -
@@ -629,9 +624,6 @@ struct cgroup_subsys {
 	 */
 	struct cgroupfs_root *root;
 	struct list_head sibling;
-	/* used when use_id == true */
-	struct idr idr;
-	spinlock_t id_lock;
 
 	/* list of cftype_sets */
 	struct list_head cftsets;
@@ -858,35 +850,6 @@ int cgroup_scan_tasks(struct cgroup_scanner *scan);
 int cgroup_attach_task_all(struct task_struct *from, struct task_struct *);
 int cgroup_transfer_tasks(struct cgroup *to, struct cgroup *from);
 
-/*
- * CSS ID is ID for cgroup_subsys_state structs under subsys. This only works
- * if cgroup_subsys.use_id == true. It can be used for looking up and scanning.
- * CSS ID is assigned at cgroup allocation (create) automatically
- * and removed when subsys calls free_css_id() function. This is because
- * the lifetime of cgroup_subsys_state is subsys's matter.
- *
- * Looking up and scanning function should be called under rcu_read_lock().
- * Taking cgroup_mutex is not necessary for following calls.
- * But the css returned by this routine can be "not populated yet" or "being
- * destroyed". The caller should check css and cgroup's status.
- */
-
-/*
- * Typically Called at ->destroy(), or somewhere the subsys frees
- * cgroup_subsys_state.
- */
-void free_css_id(struct cgroup_subsys *ss, struct cgroup_subsys_state *css);
-
-/* Find a cgroup_subsys_state which has given ID */
-
-struct cgroup_subsys_state *css_lookup(struct cgroup_subsys *ss, int id);
-
-/* Returns true if root is ancestor of cg */
-bool css_is_ancestor(struct cgroup_subsys_state *cg,
-		     const struct cgroup_subsys_state *root);
-
-/* Get id and depth of css */
-unsigned short css_id(struct cgroup_subsys_state *css);
 struct cgroup_subsys_state *cgroup_css_from_dir(struct file *f, int id);
 
 #else /* !CONFIG_CGROUPS */
diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index dc4a749..26caaa1 100644
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -123,38 +123,6 @@ struct cfent {
 };
 
 /*
- * CSS ID -- ID per subsys's Cgroup Subsys State(CSS). used only when
- * cgroup_subsys->use_id != 0.
- */
-#define CSS_ID_MAX	(65535)
-struct css_id {
-	/*
-	 * The css to which this ID points. This pointer is set to valid value
-	 * after cgroup is populated. If cgroup is removed, this will be NULL.
-	 * This pointer is expected to be RCU-safe because destroy()
-	 * is called after synchronize_rcu(). But for safe use, css_tryget()
-	 * should be used for avoiding race.
-	 */
-	struct cgroup_subsys_state __rcu *css;
-	/*
-	 * ID of this css.
-	 */
-	unsigned short id;
-	/*
-	 * Depth in hierarchy which this ID belongs to.
-	 */
-	unsigned short depth;
-	/*
-	 * ID is freed by RCU. (and lookup routine is RCU safe.)
-	 */
-	struct rcu_head rcu_head;
-	/*
-	 * Hierarchy of CSS ID belongs to.
-	 */
-	unsigned short stack[0]; /* Array of Length (depth+1) */
-};
-
-/*
  * cgroup_event represents events which userspace want to receive.
  */
 struct cgroup_event {
@@ -364,9 +332,6 @@ struct cgrp_cset_link {
 static struct css_set init_css_set;
 static struct cgrp_cset_link init_cgrp_cset_link;
 
-static int cgroup_init_idr(struct cgroup_subsys *ss,
-			   struct cgroup_subsys_state *css);
-
 /* css_set_lock protects the list of css_set objects, and the
  * chain of tasks off each css_set.  Nests outside task->alloc_lock
  * due to cgroup_iter_start() */
@@ -815,9 +780,6 @@ static struct backing_dev_info cgroup_backing_dev_info = {
 	.capabilities	= BDI_CAP_NO_ACCT_AND_WRITEBACK,
 };
 
-static int alloc_css_id(struct cgroup_subsys *ss,
-			struct cgroup *parent, struct cgroup *child);
-
 static struct inode *cgroup_new_inode(umode_t mode, struct super_block *sb)
 {
 	struct inode *inode = new_inode(sb);
@@ -4160,20 +4122,6 @@ static int cgroup_populate_dir(struct cgroup *cgrp, unsigned long subsys_mask)
 		}
 	}
 
-	/* This cgroup is ready now */
-	for_each_root_subsys(cgrp->root, ss) {
-		struct cgroup_subsys_state *css = cgrp->subsys[ss->subsys_id];
-		struct css_id *id = rcu_dereference_protected(css->id, true);
-
-		/*
-		 * Update id->css pointer and make this css visible from
-		 * CSS ID functions. This pointer will be dereferened
-		 * from RCU-read-side without locks.
-		 */
-		if (id)
-			rcu_assign_pointer(id->css, css);
-	}
-
 	return 0;
 err:
 	cgroup_clear_dir(cgrp, subsys_mask);
@@ -4202,7 +4150,6 @@ static void init_cgroup_css(struct cgroup_subsys_state *css,
 {
 	css->cgroup = cgrp;
 	css->flags = 0;
-	css->id = NULL;
 	if (cgrp == cgroup_dummy_top)
 		css->flags |= CSS_ROOT;
 	BUG_ON(cgrp->subsys[ss->subsys_id]);
@@ -4331,12 +4278,6 @@ static long cgroup_create(struct cgroup *parent, struct dentry *dentry,
 			goto err_free_all;
 
 		init_cgroup_css(css, ss, cgrp);
-
-		if (ss->use_id) {
-			err = alloc_css_id(ss, parent, cgrp);
-			if (err)
-				goto err_free_all;
-		}
 	}
 
 	/*
@@ -4741,12 +4682,6 @@ int __init_or_module cgroup_load_subsys(struct cgroup_subsys *ss)
 
 	/* our new subsystem will be attached to the dummy hierarchy. */
 	init_cgroup_css(css, ss, cgroup_dummy_top);
-	/* init_idr must be after init_cgroup_css because it sets css->id. */
-	if (ss->use_id) {
-		ret = cgroup_init_idr(ss, css);
-		if (ret)
-			goto err_unload;
-	}
 
 	/*
 	 * Now we need to entangle the css into the existing css_sets. unlike
@@ -4812,9 +4747,6 @@ void cgroup_unload_subsys(struct cgroup_subsys *ss)
 
 	offline_css(ss, cgroup_dummy_top);
 
-	if (ss->use_id)
-		idr_destroy(&ss->idr);
-
 	/* deassign the subsys_id */
 	cgroup_subsys[ss->subsys_id] = NULL;
 
@@ -4841,8 +4773,7 @@ void cgroup_unload_subsys(struct cgroup_subsys *ss)
 	/*
 	 * remove subsystem's css from the cgroup_dummy_top and free it -
 	 * need to free before marking as null because ss->css_free needs
-	 * the cgrp->subsys pointer to find their state. note that this
-	 * also takes care of freeing the css_id.
+	 * the cgrp->subsys pointer to find their state.
 	 */
 	ss->css_free(cgroup_dummy_top);
 	cgroup_dummy_top->subsys[ss->subsys_id] = NULL;
@@ -4913,8 +4844,6 @@ int __init cgroup_init(void)
 	for_each_builtin_subsys(ss, i) {
 		if (!ss->early_init)
 			cgroup_init_subsys(ss);
-		if (ss->use_id)
-			cgroup_init_idr(ss, init_css_set.subsys[ss->subsys_id]);
 	}
 
 	/* allocate id for the dummy hierarchy */
@@ -5335,185 +5264,6 @@ static int __init cgroup_disable(char *str)
 __setup("cgroup_disable=", cgroup_disable);
 
 /*
- * Functons for CSS ID.
- */
-
-/* to get ID other than 0, this should be called when !cgroup_is_dead() */
-unsigned short css_id(struct cgroup_subsys_state *css)
-{
-	struct css_id *cssid;
-
-	/*
-	 * This css_id() can return correct value when somone has refcnt
-	 * on this or this is under rcu_read_lock(). Once css->id is allocated,
-	 * it's unchanged until freed.
-	 */
-	cssid = rcu_dereference_raw(css->id);
-
-	if (cssid)
-		return cssid->id;
-	return 0;
-}
-EXPORT_SYMBOL_GPL(css_id);
-
-/**
- *  css_is_ancestor - test "root" css is an ancestor of "child"
- * @child: the css to be tested.
- * @root: the css supporsed to be an ancestor of the child.
- *
- * Returns true if "root" is an ancestor of "child" in its hierarchy. Because
- * this function reads css->id, the caller must hold rcu_read_lock().
- * But, considering usual usage, the csses should be valid objects after test.
- * Assuming that the caller will do some action to the child if this returns
- * returns true, the caller must take "child";s reference count.
- * If "child" is valid object and this returns true, "root" is valid, too.
- */
-
-bool css_is_ancestor(struct cgroup_subsys_state *child,
-		    const struct cgroup_subsys_state *root)
-{
-	struct css_id *child_id;
-	struct css_id *root_id;
-
-	child_id  = rcu_dereference(child->id);
-	if (!child_id)
-		return false;
-	root_id = rcu_dereference(root->id);
-	if (!root_id)
-		return false;
-	if (child_id->depth < root_id->depth)
-		return false;
-	if (child_id->stack[root_id->depth] != root_id->id)
-		return false;
-	return true;
-}
-
-void free_css_id(struct cgroup_subsys *ss, struct cgroup_subsys_state *css)
-{
-	struct css_id *id = rcu_dereference_protected(css->id, true);
-
-	/* When this is called before css_id initialization, id can be NULL */
-	if (!id)
-		return;
-
-	BUG_ON(!ss->use_id);
-
-	rcu_assign_pointer(id->css, NULL);
-	rcu_assign_pointer(css->id, NULL);
-	spin_lock(&ss->id_lock);
-	idr_remove(&ss->idr, id->id);
-	spin_unlock(&ss->id_lock);
-	kfree_rcu(id, rcu_head);
-}
-EXPORT_SYMBOL_GPL(free_css_id);
-
-/*
- * This is called by init or create(). Then, calls to this function are
- * always serialized (By cgroup_mutex() at create()).
- */
-
-static struct css_id *get_new_cssid(struct cgroup_subsys *ss, int depth)
-{
-	struct css_id *newid;
-	int ret, size;
-
-	BUG_ON(!ss->use_id);
-
-	size = sizeof(*newid) + sizeof(unsigned short) * (depth + 1);
-	newid = kzalloc(size, GFP_KERNEL);
-	if (!newid)
-		return ERR_PTR(-ENOMEM);
-
-	idr_preload(GFP_KERNEL);
-	spin_lock(&ss->id_lock);
-	/* Don't use 0. allocates an ID of 1-65535 */
-	ret = idr_alloc(&ss->idr, newid, 1, CSS_ID_MAX + 1, GFP_NOWAIT);
-	spin_unlock(&ss->id_lock);
-	idr_preload_end();
-
-	/* Returns error when there are no free spaces for new ID.*/
-	if (ret < 0)
-		goto err_out;
-
-	newid->id = ret;
-	newid->depth = depth;
-	return newid;
-err_out:
-	kfree(newid);
-	return ERR_PTR(ret);
-
-}
-
-static int __init_or_module cgroup_init_idr(struct cgroup_subsys *ss,
-					    struct cgroup_subsys_state *rootcss)
-{
-	struct css_id *newid;
-
-	spin_lock_init(&ss->id_lock);
-	idr_init(&ss->idr);
-
-	newid = get_new_cssid(ss, 0);
-	if (IS_ERR(newid))
-		return PTR_ERR(newid);
-
-	newid->stack[0] = newid->id;
-	RCU_INIT_POINTER(newid->css, rootcss);
-	RCU_INIT_POINTER(rootcss->id, newid);
-	return 0;
-}
-
-static int alloc_css_id(struct cgroup_subsys *ss, struct cgroup *parent,
-			struct cgroup *child)
-{
-	int subsys_id, i, depth = 0;
-	struct cgroup_subsys_state *parent_css, *child_css;
-	struct css_id *child_id, *parent_id;
-
-	subsys_id = ss->subsys_id;
-	parent_css = parent->subsys[subsys_id];
-	child_css = child->subsys[subsys_id];
-	parent_id = rcu_dereference_protected(parent_css->id, true);
-	depth = parent_id->depth + 1;
-
-	child_id = get_new_cssid(ss, depth);
-	if (IS_ERR(child_id))
-		return PTR_ERR(child_id);
-
-	for (i = 0; i < depth; i++)
-		child_id->stack[i] = parent_id->stack[i];
-	child_id->stack[depth] = child_id->id;
-	/*
-	 * child_id->css pointer will be set after this cgroup is available
-	 * see cgroup_populate_dir()
-	 */
-	rcu_assign_pointer(child_css->id, child_id);
-
-	return 0;
-}
-
-/**
- * css_lookup - lookup css by id
- * @ss: cgroup subsys to be looked into.
- * @id: the id
- *
- * Returns pointer to cgroup_subsys_state if there is valid one with id.
- * NULL if not. Should be called under rcu_read_lock()
- */
-struct cgroup_subsys_state *css_lookup(struct cgroup_subsys *ss, int id)
-{
-	struct css_id *cssid = NULL;
-
-	BUG_ON(!ss->use_id);
-	cssid = idr_find(&ss->idr, id);
-
-	if (unlikely(!cssid))
-		return NULL;
-
-	return rcu_dereference(cssid->css);
-}
-EXPORT_SYMBOL_GPL(css_lookup);
-
-/*
  * get corresponding css from file open on cgroupfs directory
  */
 struct cgroup_subsys_state *cgroup_css_from_dir(struct file *f, int id)
-- 
1.8.0.2

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Re: [PATCH v3 2/8] cgroup: document how cgroup IDs are assigned

[Tejun Heo]

On Mon, Jul 29, 2013 at 03:08:04PM +0800, Li Zefan wrote:
> As cgroup id has been used in netprio cgroup and will be used in memcg,
> it's important to make it clear how a cgroup id is allocated.
> 
> For example, in netprio cgroup, the id is used as index of anarray.
> 
> Signed-off-by: Li Zefan <lizefan@huwei.com>
> Reviewed-by: Michal Hocko <mhocko@suse.cz>

We can merge this into the first patch?

Thanks.

-- 
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Re: [PATCH v3 1/8] cgroup: convert cgroup_ida to cgroup_idr

[Tejun Heo]

Hello,

On Mon, Jul 29, 2013 at 03:07:48PM +0800, Li Zefan wrote:
> @@ -4590,6 +4599,9 @@ static void cgroup_offline_fn(struct work_struct *work)
>  	/* delete this cgroup from parent->children */
>  	list_del_rcu(&cgrp->sibling);
>  
> +	if (cgrp->id)
> +		idr_remove(&cgrp->root->cgroup_idr, cgrp->id);
> +

Yeap, if we're gonna allow lookups, removal should happen here but can
we please add short comment explaining why that is?  Also, do we want
to clear cgrp->id?

Thanks.

-- 
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Re: [PATCH v3 3/8] cgroup: implement cgroup_from_id()

[Tejun Heo]

On Mon, Jul 29, 2013 at 03:08:15PM +0800, Li Zefan wrote:
> +struct cgroup *cgroup_from_id(struct cgroup_subsys *ss, int id)
> +{
> +	rcu_lockdep_assert(rcu_read_lock_held(),
> +			   "cgroup_from_id() needs rcu_read_lock()"
> +			   " protection");

Maybe we want to add notation for &cgroup_mutex for completeness?

> +	return idr_find(&ss->root->cgroup_idr, id);
> +}

And maybe inline it?

Thanks.

-- 
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Re: [PATCH v3 2/8] cgroup: document how cgroup IDs are assigned

[Li Zefan]

On 2013/7/30 2:26, Tejun Heo wrote:
> On Mon, Jul 29, 2013 at 03:08:04PM +0800, Li Zefan wrote:
>> As cgroup id has been used in netprio cgroup and will be used in memcg,
>> it's important to make it clear how a cgroup id is allocated.
>>
>> For example, in netprio cgroup, the id is used as index of anarray.
>>
>> Signed-off-by: Li Zefan <lizefan@huwei.com>
>> Reviewed-by: Michal Hocko <mhocko@suse.cz>
> 
> We can merge this into the first patch?
> 

The first patch just changes ida to idr, it doesn't change how IDs are
allocated, so I prefer make this a standalone patch.

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Re: [PATCH v3 1/8] cgroup: convert cgroup_ida to cgroup_idr

[Li Zefan]

On 2013/7/30 2:28, Tejun Heo wrote:
> Hello,
> 
> On Mon, Jul 29, 2013 at 03:07:48PM +0800, Li Zefan wrote:
>> @@ -4590,6 +4599,9 @@ static void cgroup_offline_fn(struct work_struct *work)
>>  	/* delete this cgroup from parent->children */
>>  	list_del_rcu(&cgrp->sibling);
>>  
>> +	if (cgrp->id)
>> +		idr_remove(&cgrp->root->cgroup_idr, cgrp->id);
>> +
> 
> Yeap, if we're gonna allow lookups, removal should happen here but can
> we please add short comment explaining why that is?

sure

> Also, do we want to clear cgrp->id?
> 

Set cgrp->id to 0? No, 0 is a valid id. The if is here because at first
I called idr_alloc() very late in cgroup_create(), so cgroup_offline_fn()
can be called while cgrp->id hasn't been initialized. Now I can remove
this check.

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Re: [PATCH v3 1/8] cgroup: convert cgroup_ida to cgroup_idr

[Tejun Heo]

On Tue, Jul 30, 2013 at 09:10:19AM +0800, Li Zefan wrote:
> Set cgrp->id to 0? No, 0 is a valid id. The if is here because at first

I don't know.  -1 then?

> I called idr_alloc() very late in cgroup_create(), so cgroup_offline_fn()
> can be called while cgrp->id hasn't been initialized. Now I can remove
> this check.

I'm just a bit apprehensive as IDs will be recycled very fast and
controllers would keep accessing the css and cgroup after offline
until all refs are drained, so it'd be nice if there's some mechanism
to prevent / detect stale ID usages.

Thanks.

-- 
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Re: [PATCH v3 2/8] cgroup: document how cgroup IDs are assigned

[Tejun Heo]

On Tue, Jul 30, 2013 at 09:08:14AM +0800, Li Zefan wrote:
> On 2013/7/30 2:26, Tejun Heo wrote:
> > On Mon, Jul 29, 2013 at 03:08:04PM +0800, Li Zefan wrote:
> >> As cgroup id has been used in netprio cgroup and will be used in memcg,
> >> it's important to make it clear how a cgroup id is allocated.
> >>
> >> For example, in netprio cgroup, the id is used as index of anarray.
> >>
> >> Signed-off-by: Li Zefan <lizefan@huwei.com>
> >> Reviewed-by: Michal Hocko <mhocko@suse.cz>
> > 
> > We can merge this into the first patch?
> > 
> 
> The first patch just changes ida to idr, it doesn't change how IDs are
> allocated, so I prefer make this a standalone patch.

Hmmm... I'd just add "while at it, add a comment explaining ..." to
the description and fold it into the prev patch but no biggie.  Please
do as you see fit.

Thanks.

-- 
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