Currently, while migrating tasks from one cgroup to another,
cgroup_attach_task() builds a flex array of all target tasks;
unfortunately, this has a couple issues.

* Flex array has size limit.  On 64bit, struct task_and_cgroup is
  24bytes making the flex element limit around 87k.  It is a high
  number but not impossible to hit.  This means that the current
  cgroup implementation can't migrate a process with more than 87k
  threads.

* Process migration involves memory allocation whose size is dependent
  on the number of threads the process has.  This means that cgroup
  core can't guarantee success or failure of multi-process migrations
  as memory allocation failure can happen in the middle.  This is in
  part because cgroup can't grab threadgroup locks of multiple
  processes at the same time, so when there are multiple processes to
  migrate, it is imposible to tell how many tasks are to be migrated
  beforehand.

  Note that this already affects cgroup_transfer_tasks().  cgroup
  currently cannot guarantee atomic success or failure of the
  operation.  It may fail in the middle and after such failure cgroup
  doesn't have enough information to roll back properly.  It just
  aborts with some tasks migrated and others not.

To resolve the situation, this patch updates the migration path to use
task->cg_list to track target tasks.  The previous patch already added
css_set->mg_tasks and updated iterations in non-migration paths to
include them during task migration.  This patch updates migration path
to actually make use of it.

Instead of putting onto a flex_array, each target task is moved from
its css_set->tasks list to css_set->mg_tasks and the migration path
keeps trace of all the source css_sets and the associated cgroups.
Once all source css_sets are determined, the destination css_set for
each is determined, linked to the matching source css_set and put on a
separate list.

To iterate the target tasks, migration path just needs to iterat
through either the source or target css_sets, depending on whether
migration has been committed or not, and the tasks on their ->mg_tasks
lists.  cgroup_taskset is updated to contain the list_heads for source
and target css_sets and the iteration cursor.  cgroup_taskset_*() are
accordingly updated to walk through css_sets and their ->mg_tasks.

This resolves the above listed issues with moderate additional
complexity.
Signed-off-by: NTejun Heo <tj@kernel.org>
Acked-by: NLi Zefan <lizefan@huawei.com>

Currently, while migrating tasks from one cgroup to another,
cgroup_attach_task() builds a flex array of all target tasks;
unfortunately, this has a couple issues.

* Flex array has size limit.  On 64bit, struct task_and_cgroup is
  24bytes making the flex element limit around 87k.  It is a high
  number but not impossible to hit.  This means that the current
  cgroup implementation can't migrate a process with more than 87k
  threads.

* Process migration involves memory allocation whose size is dependent
  on the number of threads the process has.  This means that cgroup
  core can't guarantee success or failure of multi-process migrations
  as memory allocation failure can happen in the middle.  This is in
  part because cgroup can't grab threadgroup locks of multiple
  processes at the same time, so when there are multiple processes to
  migrate, it is imposible to tell how many tasks are to be migrated
  beforehand.

  Note that this already affects cgroup_transfer_tasks().  cgroup
  currently cannot guarantee atomic success or failure of the
  operation.  It may fail in the middle and after such failure cgroup
  doesn't have enough information to roll back properly.  It just
  aborts with some tasks migrated and others not.

To resolve the situation, we're going to use task->cg_list during
migration too.  Instead of building a separate array, target tasks
will be linked into a dedicated migration list_head on the owning
css_set.  Tasks on the migration list are treated the same as tasks on
the usual tasks list; however, being on a separate list allows cgroup
migration code path to keep track of the target tasks by simply
keeping the list of css_sets with tasks being migrated, making
unpredictable dynamic allocation unnecessary.

In prepartion of such migration path update, this patch introduces
css_set->mg_tasks list and updates css_set task iterations so that
they walk both css_set->tasks and ->mg_tasks.  Note that ->mg_tasks
isn't used yet.
Signed-off-by: NTejun Heo <tj@kernel.org>
Acked-by: NLi Zefan <lizefan@huawei.com>

Pull in for-3.14-fixes to receive 532de3fc ("cgroup: update
cgroup_enable_task_cg_lists() to grab siglock") which conflicts with
afeb0f9f ("cgroup: relocate cgroup_enable_task_cg_lists()") and
the following cg_lists updates.  This is likely to cause further
conflicts down the line too, so let's merge it early.

As cgroup_enable_task_cg_lists() is relocated in for-3.15, this merge
causes conflict in the original position.  It's resolved by applying
siglock changes to the updated version in the new location.

Conflicts:
	kernel/cgroup.c
Signed-off-by: NTejun Heo <tj@kernel.org>

Currently, there's nothing preventing cgroup_enable_task_cg_lists()
from missing set PF_EXITING and race against cgroup_exit().  Depending
on the timing, cgroup_exit() may finish with the task still linked on
css_set leading to list corruption.  Fix it by grabbing siglock in
cgroup_enable_task_cg_lists() so that PF_EXITING is guaranteed to be
visible.

This whole on-demand cg_list optimization is extremely fragile and has
ample possibility to lead to bugs which can cause things like
once-a-year oops during boot.  I'm wondering whether the better
approach would be just adding "cgroup_disable=all" handling which
disables the whole cgroup rather than tempting fate with this
on-demand craziness.
Signed-off-by: NTejun Heo <tj@kernel.org>
Acked-by: NLi Zefan <lizefan@huawei.com>
Cc: stable@vger.kernel.org

Fengguang reported this bug:

BUG: unable to handle kernel NULL pointer dereference at 0000003c
IP: [<cc90b4ad>] cgroup_cfts_commit+0x27/0x1c1
...
Call Trace:
  [<cc9d1129>] ? kmem_cache_alloc_trace+0x33f/0x3b7
  [<cc90c6fc>] cgroup_add_cftypes+0x8f/0xca
  [<cd78b646>] cgroup_init+0x6a/0x26a
  [<cd764d7d>] start_kernel+0x4d7/0x57a
  [<cd7642ef>] i386_start_kernel+0x92/0x96

This happens in a corner case. If CGROUP_SCHED=y but CFS_BANDWIDTH=n &&
FAIR_GROUP_SCHED=n && RT_GROUP_SCHED=n, we have:

cpu_files[] = {
	{ }	/* terminate */
}

When we pass cpu_files to cgroup_apply_cftypes(), as cpu_files[0].ss
is NULL, we'll access NULL pointer.

The bug was introduced by commit de00ffa5
("cgroup: make cgroup_subsys->base_cftypes use cgroup_add_cftypes()").
Reported-by: NFengguang Wu <fengguang.wu@intel.com>
Signed-off-by: NLi Zefan <lizefan@huawei.com>
Signed-off-by: NTejun Heo <tj@kernel.org>

My kernel fails to boot, because blkcg calls cgroup_path() while
cgroupfs is not mounted.

Fix both cgroup_name() and cgroup_path().
Signed-off-by: NLi Zefan <lizefan@huawei.com>
Signed-off-by: NTejun Heo <tj@kernel.org>

We should free the memory allocated in parse_cgroupfs_options() before
calling this function again.
Signed-off-by: NLi Zefan <lizefan@huawei.com>
Signed-off-by: NTejun Heo <tj@kernel.org>

css_set_lock has been converted to css_set_rwsem, and rwsem can't nest
inside rcu_read_lock.
Signed-off-by: NLi Zefan <lizefan@huawei.com>
Signed-off-by: NTejun Heo <tj@kernel.org>

kernel/cgroup.c:2256:1-3: WARNING: PTR_RET can be used

 Use PTR_ERR_OR_ZERO rather than if(IS_ERR(...)) + PTR_ERR

Generated by: coccinelle/api/ptr_ret.cocci
Signed-off-by: NFengguang Wu <fengguang.wu@intel.com>
Signed-off-by: NTejun Heo <tj@kernel.org>

To fix:

arch/sparc/kernel/leon_pci_grpci2.c: In function 'grpci2_of_probe':
arch/sparc/kernel/leon_pci_grpci2.c:720:2: error: implicit declaration of function 'kzalloc' [-Werror=implicit-function-declaration]
arch/sparc/kernel/leon_pci_grpci2.c:720:20: error: assignment makes pointer from integer without a cast [-Werror]
arch/sparc/kernel/leon_pci_grpci2.c:882:2: error: implicit declaration of function 'kfree' [-Werror=implicit-function-declaration]
cc1: all warnings being treated as errors
make[2]: *** [arch/sparc/kernel/leon_pci_grpci2.o] Error 1

According to Stephen, these types of failures are caused by commit
2bd59d48 ("cgroup: convert to kernfs") which was being included
implicitly via cgroup.h's inclusion of xattr.h (which has now been
removed).
Signed-off-by: NPaul Gortmaker <paul.gortmaker@windriver.com>
Signed-off-by: NTejun Heo <tj@kernel.org>
Acked-by: NDavid S. Miller <davem@davemloft.net>
Cc: Stephen Rothwell <sfr@canb.auug.org.au>

With module support gone, a lot of functions no longer need to be
exported.  Unexport them.
Signed-off-by: NTejun Heo <tj@kernel.org>
Acked-by: NLi Zefan <lizefan@huawei.com>

cgroup_attach_task() is planned to go through restructuring.  Let's
tidy it up a bit in preparation.

* Update cgroup_attach_task() to receive the target task argument in
  @leader instead of @tsk.

* Rename @tsk to @task.

* Rename @retval to @ret.

This is purely cosmetic.

v2: get_nr_threads() was using uninitialized @task instead of @leader.
    Fixed.  Reported by Dan Carpenter.
Signed-off-by: NTejun Heo <tj@kernel.org>
Acked-by: NLi Zefan <lizefan@huawei.com>
Cc: Dan Carpenter <dan.carpenter@oracle.com>

The two functions don't have any users left.  Remove them along with
cgroup_taskset->cur_cgrp.
Signed-off-by: NTejun Heo <tj@kernel.org>
Acked-by: NLi Zefan <lizefan@huawei.com>

cgroup_taskset_cur_css() will be removed during the planned
resturcturing of migration path.  The only use of
cgroup_taskset_cur_css() is finding out the old cgroup_subsys_state of
the leader in cpuset_attach().  This usage can easily be removed by
remembering the old value from cpuset_can_attach().
Signed-off-by: NTejun Heo <tj@kernel.org>
Acked-by: NLi Zefan <lizefan@huawei.com>

If !NULL, @skip_css makes cgroup_taskset_for_each() skip the matching
css.  The intention of the interface is to make it easy to skip css's
(cgroup_subsys_states) which already match the migration target;
however, this is entirely unnecessary as migration taskset doesn't
include tasks which are already in the target cgroup.  Drop @skip_css
from cgroup_taskset_for_each().
Signed-off-by: NTejun Heo <tj@kernel.org>
Acked-by: NLi Zefan <lizefan@huawei.com>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Arnaldo Carvalho de Melo <acme@ghostprotocols.net>
Cc: Daniel Borkmann <dborkman@redhat.com>

Instead of repeatedly locking and unlocking css_set_rwsem inside
cgroup_task_migrate(), update cgroup_attach_task() to grab it outside
of the loop and update cgroup_task_migrate() to use
put_css_set_locked().
Signed-off-by: NTejun Heo <tj@kernel.org>
Acked-by: NLi Zefan <lizefan@huawei.com>

put_css_set() is performed in two steps - it first tries to put
without grabbing css_set_rwsem if such put wouldn't make the count
zero.  If that fails, it puts after write-locking css_set_rwsem.  This
patch separates out the second phase into put_css_set_locked() which
should be called with css_set_rwsem locked.

Also, put_css_set_taskexit() is droped and put_css_set() is made to
take @taskexit.  There are only a handful users of these functions.
No point in providing different variants.

put_css_locked() will be used by later changes.  This patch doesn't
introduce any functional changes.
Signed-off-by: NTejun Heo <tj@kernel.org>
Acked-by: NLi Zefan <lizefan@huawei.com>

css_scan_tasks() doesn't have any user left.  Remove it.
Signed-off-by: NTejun Heo <tj@kernel.org>
Acked-by: NLi Zefan <lizefan@huawei.com>

Now that css_task_iter_start/next_end() supports blocking while
iterating, there's no reason to use css_scan_tasks() which is more
cumbersome to use and scheduled to be removed.

Convert all css_scan_tasks() usages in cpuset to
css_task_iter_start/next/end().  This simplifies the code by removing
heap allocation and callbacks.
Signed-off-by: NTejun Heo <tj@kernel.org>
Acked-by: NLi Zefan <lizefan@huawei.com>

Currently there are two ways to walk tasks of a cgroup -
css_task_iter_start/next/end() and css_scan_tasks().  The latter
builds on the former but allows blocking while iterating.
Unfortunately, the way css_scan_tasks() is implemented is rather
nasty, it uses a priority heap of pointers to extract some number of
tasks in task creation order and loops over them invoking the callback
and repeats that until it reaches the end.  It requires either
preallocated heap or may fail under memory pressure, while unlikely to
be problematic, the complexity is O(N^2), and in general just nasty.

We're gonna convert all css_scan_users() to
css_task_iter_start/next/end() and remove css_scan_users().  As
css_scan_tasks() users may block, let's convert css_set_lock to a
rwsem so that tasks can block during css_task_iter_*() is in progress.

While this does increase the chance of possible deadlock scenarios,
given the current usage, the probability is relatively low, and even
if that happens, the right thing to do is updating the iteration in
the similar way to css iterators so that it can handle blocking.

Most conversions are trivial; however, task_cgroup_path() now expects
to be called with css_set_rwsem locked instead of locking itself.
This is because the function is called with RCU read lock held and
rwsem locking should nest outside RCU read lock.
Signed-off-by: NTejun Heo <tj@kernel.org>
Acked-by: NLi Zefan <lizefan@huawei.com>

Reimplement cgroup_transfer_tasks() so that it repeatedly fetches the
first task in the cgroup and then tranfers it.  This achieves the same
result without using css_scan_tasks() which is scheduled to be
removed.
Signed-off-by: NTejun Heo <tj@kernel.org>
Acked-by: NLi Zefan <lizefan@huawei.com>

cgroup_task_count() read-locks css_set_lock and walks all tasks to
count them and then returns the result.  The only thing all the users
want is determining whether the cgroup is empty or not.  This patch
implements cgroup_has_tasks() which tests whether cgroup->cset_links
is empty, replaces all cgroup_task_count() usages and unexports it.

Note that the test isn't synchronized.  This is the same as before.
The test has always been racy.

This will help planned css_set locking update.
Signed-off-by: NTejun Heo <tj@kernel.org>
Acked-by: NLi Zefan <lizefan@huawei.com>
Acked-by: NMichal Hocko <mhocko@suse.cz>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Balbir Singh <bsingharora@gmail.com>
Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>

Move it above so that prototype isn't necessary.  Let's also move the
definition of use_task_css_set_links next to it.

This is purely cosmetic.
Signed-off-by: NTejun Heo <tj@kernel.org>
Acked-by: NLi Zefan <lizefan@huawei.com>

Tasks are not linked on their css_sets until cgroup task iteration is
actually used.  This is to avoid incurring overhead on the fork and
exit paths for systems which have cgroup compiled in but don't use it.
     
This lazy binding also affects the task migration path.  It has to be
careful so that it doesn't link tasks to css_sets when task_cg_lists
linking is not enabled yet.  Unfortunately, this conditional linking
in the migration path interferes with planned migration updates.

This patch moves the lazy binding a bit earlier, to the first cgroup
mount.  It's a clear indication that cgroup is being used on the
system and task_cg_lists linking is highly likely to be enabled soon
anyway through "tasks" and "cgroup.procs" files.

This allows cgroup_task_migrate() to always link @tsk->cg_list.  Note
that it may still race with cgroup_post_fork() but who wins that race
is inconsequential.

While at it, make use_task_css_set_links a bool, add sanity checks in
cgroup_enable_task_cg_lists() and css_task_iter_start(), and update
the former so that it's guaranteed and assumes to run only once.
Signed-off-by: NTejun Heo <tj@kernel.org>
Acked-by: NLi Zefan <lizefan@huawei.com>

Before kernfs conversion, due to the way super_block lookup works,
cgroup roots were created and made visible before being fully
initialized.  This in turn required a special flag to mark that the
root hasn't been fully initialized so that the destruction path can
tell fully bound ones from half initialized.

That flag is CGRP_ROOT_SUBSYS_BOUND and no longer necessary after the
kernfs conversion as the lookup and creation of new root are atomic
w.r.t. cgroup_mutex.  This patch removes the flag and passes the
requests subsystem mask to cgroup_setup_root() so that it can set the
respective mask bits as subsystems are bound.
Signed-off-by: NTejun Heo <tj@kernel.org>
Acked-by: NLi Zefan <lizefan@huawei.com>

Disallow more mount options if sane_behavior.  Note that xattr used to
generate warning.

While at it, simplify option check in cgroup_mount() and update
sane_behavior comment in cgroup.h.
Signed-off-by: NTejun Heo <tj@kernel.org>
Acked-by: NLi Zefan <lizefan@huawei.com>

This reverts commit ab3f5faa.
Explanation from Hugh:

  It's because more thorough testing, by others here, found that it
  wasn't always solving the problem: so I asked Tejun privately to
  hold off from sending it in, until we'd worked out why not.

  Most of our testing being on a v3,11-based kernel, it was perfectly
  possible that the problem was merely our own e.g. missing Tejun's
  8a2b7538 ("workqueue: fix ordered workqueues in NUMA setups").

  But that turned out not to be enough to fix it either. Then Filipe
  pointed out how percpu_ref_kill_and_confirm() uses call_rcu_sched()
  before we ever get to put the offline on to the workqueue: by the
  time we get to the workqueue, the ordering has already been lost.

  So, thanks for the Acks, but I'm afraid that this ordered workqueue
  solution is just not good enough: we should simply forget that patch
  and provide a different answer."
Signed-off-by: NTejun Heo <tj@kernel.org>
Cc: Hugh Dickins <hughd@google.com>

This was being included implicitly via cgroup.h's inclusion of xattr.h
(which has now been removed).
Signed-off-by: NStephen Rothwell <sfr@canb.auug.org.au>
Acked-by: NSam Ravnborg <sam@ravnborg.org>
Signed-off-by: NTejun Heo <tj@kernel.org>

Currently, cgroupfs_root and its ->top_cgroup are separated reference
counted and the latter's is ignored.  There's no reason to do this
separately.  This patch removes cgroupfs_root->refcnt and destroys
cgroupfs_root when the top_cgroup is released.

* cgroup_put() updated to ignore cgroup_is_dead() test for top
  cgroups.  cgroup_free_fn() updated to handle root destruction when
  releasing a top cgroup.

* As root destruction is now bounced through cgroup destruction, it is
  asynchronous.  Update cgroup_mount() so that it waits for pending
  release which is currently implemented using msleep().  Converting
  this to proper wait_queue isn't hard but likely unnecessary.
Signed-off-by: NTejun Heo <tj@kernel.org>
Acked-by: NLi Zefan <lizefan@huawei.com>

root->number_of_cgroups is currently an integer protected with
cgroup_mutex.  Except for sanity checks and proc reporting, the only
place it's used is to check whether the root has any child during
remount; however, this is a bit flawed as the counter is not
decremented when the cgroup is unlinked but when it's released,
meaning that there could be an extended period where all cgroups are
removed but remount is still not allowed because some internal objects
are lingering.  While not perfect either, it'd be better to use
emptiness test on root->top_cgroup.children.

This patch updates cgroup_remount() to test top_cgroup's children
instead, which makes number_of_cgroups only actual usage statistics
printing in proc implemented in proc_cgroupstats_show().  Let's
shorten its name and make it an atomic_t so that we don't have to
worry about its synchronization.  It's purely auxiliary at this point.
Signed-off-by: NTejun Heo <tj@kernel.org>
Acked-by: NLi Zefan <lizefan@huawei.com>

cgroup->name handling became quite complicated over time involving
dedicated struct cgroup_name for RCU protection.  Now that cgroup is
on kernfs, we can drop all of it and simply use kernfs_name/path() and
friends.  Replace cgroup->name and all related code with kernfs
name/path constructs.

* Reimplement cgroup_name() and cgroup_path() as thin wrappers on top
  of kernfs counterparts, which involves semantic changes.
  pr_cont_cgroup_name() and pr_cont_cgroup_path() added.

* cgroup->name handling dropped from cgroup_rename().

* All users of cgroup_name/path() updated to the new semantics.  Users
  which were formatting the string just to printk them are converted
  to use pr_cont_cgroup_name/path() instead, which simplifies things
  quite a bit.  As cgroup_name() no longer requires RCU read lock
  around it, RCU lockings which were protecting only cgroup_name() are
  removed.

v2: Comment above oom_info_lock updated as suggested by Michal.

v3: dummy_top doesn't have a kn associated and
    pr_cont_cgroup_name/path() ended up calling the matching kernfs
    functions with NULL kn leading to oops.  Test for NULL kn and
    print "/" if so.  This issue was reported by Fengguang Wu.

v4: Rebased on top of 0ab02ca8 ("cgroup: protect modifications to
    cgroup_idr with cgroup_mutex").
Signed-off-by: NTejun Heo <tj@kernel.org>
Acked-by: NPeter Zijlstra <peterz@infradead.org>
Acked-by: NMichal Hocko <mhocko@suse.cz>
Acked-by: NLi Zefan <lizefan@huawei.com>
Cc: Fengguang Wu <fengguang.wu@intel.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Balbir Singh <bsingharora@gmail.com>
Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>

cgroup currently releases its kernfs_node when it gets removed.  While
not buggy, this makes cgroup->kn access rules complicated than
necessary and leads to things like get/put protection around
kernfs_remove() in cgroup_destroy_locked().  In addition, we want to
use kernfs_name/path() and friends but also want to be able to
determine a cgroup's name between removal and release.

This patch makes cgroup hold onto its kernfs_node until freed so that
cgroup->kn is always accessible.
Signed-off-by: NTejun Heo <tj@kernel.org>
Acked-by: NLi Zefan <lizefan@huawei.com>

Dynamic cftype addition and removal using cgroup_add/rm_cftypes()
respectively has been quite hairy due to vfs i_mutex.  As i_mutex
nests outside cgroup_mutex, cgroup_mutex has to be released and
regrabbed on each iteration through the hierarchy complicating the
process.  Now that i_mutex is no longer in play, it can be simplified.

* Just holding cgroup_tree_mutex is enough.  No need to meddle with
  cgroup_mutex.

* No reason to play the unlock - relock - check serial_nr dancing.
  Everything can be atomically while holding cgroup_tree_mutex.

* cgroup_cfts_prepare() is replaced with direct locking of
  cgroup_tree_mutex.

* cgroup_cfts_commit() no longer fiddles with locking.  It just
  applies the cftypes change to the existing cgroups in the hierarchy.
  Renamed to cgroup_cfts_apply().
Signed-off-by: NTejun Heo <tj@kernel.org>
Acked-by: NLi Zefan <lizefan@huawei.com>

cftype_set was added primarily to allow registering the same cftype
array more than once for different subsystems.  Nobody uses or needs
such thing and it's already broken because each cftype has ->ss
pointer which is initialized during registration.

Let's add list_head ->node to cftype and use the first cftype entry in
the array to link them instead of allocating separate cftype_set.
While at it, trigger WARN if cft seems previously initialized during
registration.

This simplifies cftype handling a bit.
Signed-off-by: NTejun Heo <tj@kernel.org>
Acked-by: NLi Zefan <lizefan@huawei.com>

cftype handling is about to be revamped.  Relocate cgroup_rm_cftypes()
above cgroup_add_cftypes() in preparation.  This is pure relocation.
Signed-off-by: NTejun Heo <tj@kernel.org>
Acked-by: NLi Zefan <lizefan@huawei.com>

Mount option "xattr" is no longer necessary as it's enabled by default
on kernfs.  Warn if "xattr" is specified with "sane_behavior" so that
the option can be removed in the future.
Signed-off-by: NTejun Heo <tj@kernel.org>
Acked-by: NLi Zefan <lizefan@huawei.com>

cgroup filesystem code was derived from the original sysfs
implementation which was heavily intertwined with vfs objects and
locking with the goal of re-using the existing vfs infrastructure.
That experiment turned out rather disastrous and sysfs switched, a
long time ago, to distributed filesystem model where a separate
representation is maintained which is queried by vfs.  Unfortunately,
cgroup stuck with the failed experiment all these years and
accumulated even more problems over time.

Locking and object lifetime management being entangled with vfs is
probably the most egregious.  vfs is never designed to be misused like
this and cgroup ends up jumping through various convoluted dancing to
make things work.  Even then, operations across multiple cgroups can't
be done safely as it'll deadlock with rename locking.

Recently, kernfs is separated out from sysfs so that it can be used by
users other than sysfs.  This patch converts cgroup to use kernfs,
which will bring the following benefits.

* Separation from vfs internals.  Locking and object lifetime
  management is contained in cgroup proper making things a lot
  simpler.  This removes significant amount of locking convolutions,
  hairy object lifetime rules and the restriction on multi-cgroup
  operations.

* Can drop a lot of code to implement filesystem interface as most are
  provided by kernfs.

* Proper "severing" semantics, which allows controllers to not worry
  about lingering file accesses after offline.

While the preceding patches did as much as possible to make the
transition less painful, large part of the conversion has to be one
discrete step making this patch rather large.  The rest of the commit
message lists notable changes in different areas.

Overall
-------

* vfs constructs replaced with kernfs ones.  cgroup->dentry w/ ->kn,
  cgroupfs_root->sb w/ ->kf_root.

* All dentry accessors are removed.  Helpers to map from kernfs
  constructs are added.

* All vfs plumbing around dentry, inode and bdi removed.

* cgroup_mount() now directly looks for matching root and then
  proceeds to create a new one if not found.

Synchronization and object lifetime
-----------------------------------

* vfs inode locking removed.  Among other things, this removes the
  need for the convolution in cgroup_cfts_commit().  Future patches
  will further simplify it.

* vfs refcnting replaced with cgroup internal ones.  cgroup->refcnt,
  cgroupfs_root->refcnt added.  cgroup_put_root() now directly puts
  root->refcnt and when it reaches zero proceeds to destroy it thus
  merging cgroup_put_root() and the former cgroup_kill_sb().
  Simliarly, cgroup_put() now directly schedules cgroup_free_rcu()
  when refcnt reaches zero.

* Unlike before, kernfs objects don't hold onto cgroup objects.  When
  cgroup destroys a kernfs node, all existing operations are drained
  and the association is broken immediately.  The same for
  cgroupfs_roots and mounts.

* All operations which come through kernfs guarantee that the
  associated cgroup is and stays valid for the duration of operation;
  however, there are two paths which need to find out the associated
  cgroup from dentry without going through kernfs -
  css_tryget_from_dir() and cgroupstats_build().  For these two,
  kernfs_node->priv is RCU managed so that they can dereference it
  under RCU read lock.

File and directory handling
---------------------------

* File and directory operations converted to kernfs_ops and
  kernfs_syscall_ops.

* xattrs is implicitly supported by kernfs.  No need to worry about it
  from cgroup.  This means that "xattr" mount option is no longer
  necessary.  A future patch will add a deprecated warning message
  when sane_behavior.

* When cftype->max_write_len > PAGE_SIZE, it's necessary to make a
  private copy of one of the kernfs_ops to set its atomic_write_len.
  cftype->kf_ops is added and cgroup_init/exit_cftypes() are updated
  to handle it.

* cftype->lockdep_key added so that kernfs lockdep annotation can be
  per cftype.

* Inidividual file entries and open states are now managed by kernfs.
  No need to worry about them from cgroup.  cfent, cgroup_open_file
  and their friends are removed.

* kernfs_nodes are created deactivated and kernfs_activate()
  invocations added to places where creation of new nodes are
  committed.

* cgroup_rmdir() uses kernfs_[un]break_active_protection() for
  self-removal.

v2: - Li pointed out in an earlier patch that specifying "name="
      during mount without subsystem specification should succeed if
      there's an existing hierarchy with a matching name although it
      should fail with -EINVAL if a new hierarchy should be created.
      Prior to the conversion, this used by handled by deferring
      failure from NULL return from cgroup_root_from_opts(), which was
      necessary because root was being created before checking for
      existing ones.  Note that cgroup_root_from_opts() returned an
      ERR_PTR() value for error conditions which require immediate
      mount failure.

      As we now have separate search and creation steps, deferring
      failure from cgroup_root_from_opts() is no longer necessary.
      cgroup_root_from_opts() is updated to always return ERR_PTR()
      value on failure.

    - The logic to match existing roots is updated so that a mount
      attempt with a matching name but different subsys_mask are
      rejected.  This was handled by a separate matching loop under
      the comment "Check for name clashes with existing mounts" but
      got lost during conversion.  Merge the check into the main
      search loop.

    - Add __rcu __force casting in RCU_INIT_POINTER() in
      cgroup_destroy_locked() to avoid the sparse address space
      warning reported by kbuild test bot.  Maybe we want an explicit
      interface to use kn->priv as RCU protected pointer?

v3: Make CONFIG_CGROUPS select CONFIG_KERNFS.

v4: Rebased on top of 0ab02ca8 ("cgroup: protect modifications to
    cgroup_idr with cgroup_mutex").
Signed-off-by: NTejun Heo <tj@kernel.org>
Acked-by: NLi Zefan <lizefan@huawei.com>
Cc: kbuild test robot fengguang.wu@intel.com>

Relocate cgroup_init/exit_root_id(), cgroup_free_root(),
cgroup_kill_sb() and cgroup_file_name() in preparation of kernfs
conversion.

These are pure relocations to make kernfs conversion easier to follow.
Signed-off-by: NTejun Heo <tj@kernel.org>
Acked-by: NLi Zefan <lizefan@huawei.com>

* Un-inline seq_css().  After kernfs conversion, the function will
  need to dereference internal data structures.

* Add cgroup_get/put_root() and replace direct super_block->s_active
  manipulatinos with them.  These will be converted to kernfs_root
  refcnting.

* Add cgroup_get/put() and replace dget/put() on cgrp->dentry with
  them.  These will be converted to kernfs refcnting.

* Update current_css_set_cg_links_read() to use cgroup_name() instead
  of reaching into the dentry name.  The end result is the same.

These changes don't make functional differences but will make
transition to kernfs easier.

v2: Rebased on top of 0ab02ca8 ("cgroup: protect modifications to
    cgroup_idr with cgroup_mutex").
Signed-off-by: NTejun Heo <tj@kernel.org>
Acked-by: NLi Zefan <lizefan@huawei.com>

mm/memory-failure.c::hwpoison_filter_task() has been reaching into
cgroup to extract the associated ino to be used as a filtering
criterion.  This is an implementation detail which shouldn't be
depended upon from outside cgroup proper and is about to change with
the scheduled kernfs conversion.

This patch introduces a proper interface to determine the associated
ino, cgroup_ino(), and updates hwpoison_filter_task() to use it
instead of reaching directly into cgroup.
Signed-off-by: NTejun Heo <tj@kernel.org>
Acked-by: NLi Zefan <lizefan@huawei.com>
Cc: Andi Kleen <andi@firstfloor.org>
Cc: Wu Fengguang <fengguang.wu@intel.com>