Before 2e91fa7f ("cgroup: keep zombies associated with their
original cgroups"), all dead tasks were associated with init_css_set.
If a zombie task is requested for migration, while migration prep
operations would still be performed on init_css_set, the actual
migration would ignore zombie tasks.  As init_css_set is always valid,
this worked fine.

However, after 2e91fa7f, zombie tasks stay with the css_set it was
associated with at the time of death.  Let's say a task T associated
with cgroup A on hierarchy H-1 and cgroup B on hiearchy H-2.  After T
becomes a zombie, it would still remain associated with A and B.  If A
only contains zombie tasks, it can be removed.  On removal, A gets
marked offline but stays pinned until all zombies are drained.  At
this point, if migration is initiated on T to a cgroup C on hierarchy
H-2, migration path would try to prepare T's css_set for migration and
trigger the following.

 WARNING: CPU: 0 PID: 1576 at kernel/cgroup.c:474 cgroup_get+0x121/0x160()
 CPU: 0 PID: 1576 Comm: bash Not tainted 4.4.0-work+ #289
 ...
 Call Trace:
  [<ffffffff8127e63c>] dump_stack+0x4e/0x82
  [<ffffffff810445e8>] warn_slowpath_common+0x78/0xb0
  [<ffffffff810446d5>] warn_slowpath_null+0x15/0x20
  [<ffffffff810c33e1>] cgroup_get+0x121/0x160
  [<ffffffff810c349b>] link_css_set+0x7b/0x90
  [<ffffffff810c4fbc>] find_css_set+0x3bc/0x5e0
  [<ffffffff810c5269>] cgroup_migrate_prepare_dst+0x89/0x1f0
  [<ffffffff810c7547>] cgroup_attach_task+0x157/0x230
  [<ffffffff810c7a17>] __cgroup_procs_write+0x2b7/0x470
  [<ffffffff810c7bdc>] cgroup_tasks_write+0xc/0x10
  [<ffffffff810c4790>] cgroup_file_write+0x30/0x1b0
  [<ffffffff811c68fc>] kernfs_fop_write+0x13c/0x180
  [<ffffffff81151673>] __vfs_write+0x23/0xe0
  [<ffffffff81152494>] vfs_write+0xa4/0x1a0
  [<ffffffff811532d4>] SyS_write+0x44/0xa0
  [<ffffffff814af2d7>] entry_SYSCALL_64_fastpath+0x12/0x6f

It doesn't make sense to prepare migration for css_sets pointing to
dead cgroups as they are guaranteed to contain only zombies which are
ignored later during migration.  This patch makes cgroup destruction
path mark all affected css_sets as dead and updates the migration path
to ignore them during preparation.
Signed-off-by: Tejun Heo <tj@kernel.org>
Fixes: 2e91fa7f ("cgroup: keep zombies associated with their original cgroups")
Cc: stable@vger.kernel.org # v4.4+

Corrected the heading to match with index.
Signed-off-by: Parav Pandit <pandit.parav@gmail.com>
Signed-off-by: Tejun Heo <tj@kernel.org>

Some controllers, perf_event for now and possibly freezer in the
future, don't really make sense to control explicitly through
"cgroup.subtree_control".  For example, the primary role of perf_event
is identifying the cgroups of tasks; however, because the controller
also keeps a small amount of state per cgroup, it can't be replaced
with simple cgroup membership tests.

This patch implements cgroup_subsys->implicit_on_dfl flag.  When set,
the controller is implicitly enabled on all cgroups on the v2
hierarchy so that utility type controllers such as perf_event can be
enabled and function transparently.

An implicit controller doesn't show up in "cgroup.controllers" or
"cgroup.subtree_control", is exempt from no internal process rule and
can be stolen from the default hierarchy even if there are non-root
csses.

v2: Reimplemented on top of the recent updates to css handling and
    subsystem rebinding.  Rebinding implicit subsystems is now a
    simple matter of exempting it from the busy subsystem check.
Signed-off-by: Tejun Heo <tj@kernel.org>

Migration can be multi-target on the default hierarchy when a
controller is enabled - processes belonging to each child cgroup have
to be moved to the child cgroup itself to refresh css association.

This isn't a problem for cgroup_migrate_add_src() as each source
css_set still maps to single source and target cgroups; however,
cgroup_migrate_prepare_dst() is called once after all source css_sets
are added and thus might not have a single destination cgroup.  This
is currently worked around by specifying NULL for @dst_cgrp and using
the source's default cgroup as destination as the only multi-target
migration in use is self-targetting.  While this works, it's subtle
and clunky.

As all taget cgroups are already specified while preparing the source
css_sets, this clunkiness can easily be removed by recording the
target cgroup in each source css_set.  This patch adds
css_set->mg_dst_cgrp which is recorded on cgroup_migrate_src() and
used by cgroup_migrate_prepare_dst().  This also makes migration code
ready for arbitrary multi-target migration.
Signed-off-by: Tejun Heo <tj@kernel.org>

On the default hierarchy, a migration can be multi-source and/or
multi-destination.  cgroup_taskest_migrate() used to incorrectly
assume single destination cgroup but the bug has been fixed by
1f7dd3e5 ("cgroup: fix handling of multi-destination migration
from subtree_control enabling").

Since the commit, @dst_cgrp to cgroup[_taskset]_migrate() is only used
to determine which subsystems are affected or which cgroup_root the
migration is taking place in.  As such, @dst_cgrp is misleading.  This
patch replaces @dst_cgrp with @root.
Signed-off-by: Tejun Heo <tj@kernel.org>

cgroup_migrate_prepare_dst() verifies whether the destination cgroup
is allowable; however, the test doesn't really belong there.  It's too
deep and common in the stack and as a result the test itself is gated
by another test.

Separate the test out into cgroup_may_migrate_to() and update
cgroup_attach_task() and cgroup_transfer_tasks() to perform the test
directly.  This doesn't cause any behavior differences.
Signed-off-by: Tejun Heo <tj@kernel.org>

cgroup_update_dfl_csses() should move each task in the subtree to
self; however, it was incorrectly calling cgroup_migrate_add_src()
with the root of the subtree as @dst_cgrp.  Fortunately,
cgroup_migrate_add_src() currently uses @dst_cgrp only to determine
the hierarchy and the bug doesn't cause any actual breakages.  Fix it.
Signed-off-by: Tejun Heo <tj@kernel.org>

Trivial correction in menuconfig help to reflect PIDs as
controller instead of subsystem to align to rest of the text
and documentation.
Signed-off-by: Parav Pandit <pandit.parav@gmail.com>
Signed-off-by: Tejun Heo <tj@kernel.org>

Signed-off-by: Haosdent Huang <haosdent@gmail.com>
Signed-off-by: Tejun Heo <tj@kernel.org>

The existing sequences of operations ensure that the offlining csses
are drained before cgroup_update_dfl_csses(), so even though
cgroup_update_dfl_csses() uses css_for_each_descendant_pre() to walk
the target cgroups, it doesn't end up operating on dead cgroups.
Also, the function explicitly excludes the subtree root from
operation.

This is fragile and inconsistent with the rest of css update
operations.  This patch updates cgroup_update_dfl_csses() to use
cgroup_for_each_live_descendant_pre() instead and include the subtree
root.
Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Zefan Li <lizefan@huawei.com>

During prep, cgroup_setup_root() allocates cgrp_cset_links matching
the number of existing css_sets to later link the new root.  This is
fine for now as the only operation which can happen inbetween is
rebind_subsystems() and rebinding of empty subsystems doesn't create
new css_sets.

However, while not yet allowed, with the recent reimplementation,
rebind_subsystems() can rebind subsystems with descendant csses and
thus can create new css_sets.  This patch makes cgroup_setup_root()
allocate 2x of the existing css_sets so that later use of live
subsystem rebinding doesn't blow up.
Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Zefan Li <lizefan@huawei.com>

cgroup_calc_subtree_ss_mask() currently takes @cgrp and
@subtree_control.  @cgrp is used for two purposes - to decide whether
it's for default hierarchy and the mask of available subsystems.  The
former doesn't matter as the results are the same regardless.  The
latter can be specified directly through a subsystem mask.

This patch makes cgroup_calc_subtree_ss_mask() perform the same
calculations for both default and legacy hierarchies and take
@this_ss_mask for available subsystems.  @cgrp is no longer used and
dropped.  This is to allow using the function in contexts where
available controllers can't be decided from the cgroup.

v2: cgroup_refres_subtree_ss_mask() is removed by a previous patch.
    Updated accordingly.
Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Zefan Li <lizefan@huawei.com>

rebind_subsystem() open codes quite a bit of css and interface file
manipulations.  It tries to be fail-safe but doesn't quite achieve it.
It can be greatly simplified by using the new css management helpers.
This patch reimplements rebind_subsytsems() using
cgroup_apply_control() and friends.

* The half-baked rollback on file creation failure is dropped.  It is
  an extremely cold path, failure isn't critical, and, aside from
  kernel bugs, the only reason it can fail is memory allocation
  failure which pretty much doesn't happen for small allocations.

* As cgroup_apply_control_disable() is now used to clean up root
  cgroup on rebind, make sure that it doesn't end up killing root
  csses.

* All callers of rebind_subsystems() are updated to use
  cgroup_lock_and_drain_offline() as the apply_control functions
  require drained subtree.

* This leaves cgroup_refresh_subtree_ss_mask() without any user.
  Removed.

* css_populate_dir() and css_clear_dir() no longer needs
  @cgrp_override parameter.  Dropped.

* While at it, add WARN_ON() to rebind_subsystem() calls which are
  expected to always succeed just in case.

While the rules visible to userland aren't changed, this
reimplementation not only simplifies rebind_subsystems() but also
allows it to disable and enable csses recursively.  This can be used
to implement more flexible rebinding.
Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Zefan Li <lizefan@huawei.com>

cgroup_create() manually updates control masks and creates child csses
which cgroup_mkdir() then manually populates.  Both can be simplified
by using cgroup_apply_enable_control() and friends.  The only catch is
that it calls css_populate_dir() with NULL cgroup->kn during
cgroup_create().  This is worked around by making the function noop on
NULL kn.
Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Zefan Li <lizefan@huawei.com>

cgroup_drain_offline() is used to wait for csses being offlined to
uninstall itself from cgroup->subsys[] array so that new csses can be
installed.  The function's only user, cgroup_subtree_control_write(),
calls it after performing some checks and restarts the whole process
via restart_syscall() if draining has to release cgroup_mutex to wait.

This can be simplified by draining before other synchronized
operations so that there's nothing to restart.  This patch converts
cgroup_drain_offline() to cgroup_lock_and_drain_offline() which
performs both locking and draining and updates cgroup_kn_lock_live()
use it instead of cgroup_mutex() if requested.  This combined locking
and draining operations are easier to use and less error-prone.

While at it, add WARNs in control_apply functions which triggers if
the subtree isn't properly drained.
Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Zefan Li <lizefan@huawei.com>

Factor out cgroup_{apply|finalize}_control() so that control mask
update can be done in several simple steps.  This patch doesn't
introduce behavior changes.
Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Zefan Li <lizefan@huawei.com>

While controllers are being enabled and disabled in
cgroup_subtree_control_write(), the original subsystem masks are
stashed in local variables so that they can be restored if the
operation fails in the middle.

This patch adds dedicated fields to struct cgroup to be used instead
of the local variables and implements functions to stash the current
values, propagate the changes and restore them recursively.  Combined
with the previous changes, this makes subsystem management operations
fully recursive and modularlized.  This will be used to expand cgroup
core functionalities.

While at it, remove now unused @css_enable and @css_disable from
cgroup_subtree_control_write().
Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Zefan Li <lizefan@huawei.com>

The three factored out css management operations -
cgroup_drain_offline() and cgroup_apply_control_{disable|enable}() -
only depend on the current state of the target cgroups and idempotent
and thus can be easily made to operate on the subtree instead of the
immediate children.

This patch introduces the iterators which walk live subtree and
converts the three functions to operate on the subtree including self
instead of the children.  While this leads to spurious walking and be
slightly more expensive, it will allow them to be used for wider scope
of operations.

Note that cgroup_drain_offline() now tests for whether a css is dying
before trying to drain it.  This is to avoid trying to drain live
csses as there can be mix of live and dying csses in a subtree unlike
children of the same parent.
Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Zefan Li <lizefan@huawei.com>

Factor out css enabling and showing into cgroup_apply_control_enable().

* Nest subsystem walk inside child walk.  The child walk will later be
  converted to subtree walk which is a bit more expensive.

* Instead of operating on the differential masks @css_enable, simply
  enable or show csses according to the current cgroup_control() and
  cgroup_ss_mask().  This leads to the same result and is simpler and
  more robust.
Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Zefan Li <lizefan@huawei.com>

Factor out css disabling and hiding into cgroup_apply_control_disable().

* Nest subsystem walk inside child walk.  The child walk will later be
  converted to subtree walk which is a bit more expensive.

* Instead of operating on the differential masks @css_enable and
  @css_disable, simply disable or hide csses according to the current
  cgroup_control() and cgroup_ss_mask().  This leads to the same
  result and is simpler and more robust.

* This allows error handling path to share the same code.
Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Zefan Li <lizefan@huawei.com>

Factor out async css offline draining into cgroup_drain_offline().

* Nest subsystem walk inside child walk.  The child walk will later be
  converted to subtree walk which is a bit more expensive.

* Relocate the draining above subsystem mask preparation, which
  doesn't create any behavior differences but helps further
  refactoring.
Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Zefan Li <lizefan@huawei.com>

When a controller is enabled and visible on a non-root cgroup is
determined by subtree_control and subtree_ss_mask of the parent
cgroup.  For a root cgroup, by the type of the hierarchy and which
controllers are attached to it.  Deciding the above on each usage is
fragile and unnecessarily complicates the users.

This patch introduces cgroup_control() and cgroup_ss_mask() which
calculate and return the [visibly] enabled subsyste mask for the
specified cgroup and conver the existing usages.

* cgroup_e_css() is restructured for simplicity.

* cgroup_calc_subtree_ss_mask() and cgroup_subtree_control_write() no
  longer need to distinguish root and non-root cases.

* With cgroup_control(), cgroup_controllers_show() can now handle both
  root and non-root cases.  cgroup_root_controllers_show() is removed.

v2: cgroup_control() updated to yield the correct result on v1
    hierarchies too.  cgroup_subtree_control_write() converted.
Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Zefan Li <lizefan@huawei.com>

We're in the process of refactoring cgroup and css management paths to
separate them out to eventually allow cgroups which aren't visible
through cgroup fs.  This patch factors out cgroup_create() out of
cgroup_mkdir().  cgroup_create() contains all internal object creation
and initialization.  cgroup_mkdir() uses cgroup_create() to create the
internal cgroup and adds interface directory and file creation.

This patch doesn't cause any behavior differences.
Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Zefan Li <lizefan@huawei.com>

Currently, operations to initialize internal objects and create
interface directory and files are intermixed in cgroup_mkdir().  We're
in the process of refactoring cgroup and css management paths to
separate them out to eventually allow cgroups which aren't visible
through cgroup fs.

This patch reorders operations inside cgroup_mkdir() so that interface
directory and file handling comes after internal object
initialization.  This will enable further refactoring.
Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Zefan Li <lizefan@huawei.com>

Currently, whether a css (cgroup_subsys_state) has its interface files
created is not tracked and assumed to change together with the owning
cgroup's lifecycle.  cgroup directory and interface creation is being
separated out from internal object creation to help refactoring and
eventually allow cgroups which are not visible through cgroupfs.

This patch adds CSS_VISIBLE to track whether a css has its interface
files created and perform management operations only when necessary
which helps decoupling interface file handling from internal object
lifecycle.  After this patch, all css interface file management
functions can be called regardless of the current state and will
achieve the expected result.
Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Zefan Li <lizefan@huawei.com>

Currently, interface files are created when a css is created depending
on whether @visible is set.  This patch separates out the two into
separate steps to help code refactoring and eventually allow cgroups
which aren't visible through cgroup fs.

Move css_populate_dir() out of create_css() and drop @visible.  While
at it, rename the function to css_create() for consistency.
Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Zefan Li <lizefan@huawei.com>

During task migration, tasks may transfer between two css_sets which
are associated with the same cgroup.  If those tasks are the only
tasks in the cgroup, this currently triggers a spurious de-populated
event on the cgroup.

Fix it by bumping up populated count before bumping it down during
migration to ensure that it doesn't reach zero spuriously.
Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Zefan Li <lizefan@huawei.com>

css_sets are hashed by their subsys[] contents and in cgroup_init()
init_css_set is hashed early, before subsystem inits, when all entries
in its subsys[] are NULL, so that cgroup_dfl_root initialization can
find and link to it.  As subsystems are initialized,
init_css_set.subsys[] is filled up but the hashing is never updated
making init_css_set hashed in the wrong place.  While incorrect, this
doesn't cause a critical failure as css_set management code would
create an identical css_set dynamically.

Fix it by rehashing init_css_set after subsystems are initialized.
While at it, drop unnecessary @key local variable.
Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Zefan Li <lizefan@huawei.com>

An associated css can be around for quite a while after a cgroup
directory has been removed. In general, it makes sense to reset it to
defaults so as not to worry about any remnants. For instance, memory
cgroup needs to reset memory.low, otherwise pages charged to a dead
cgroup might never get reclaimed. There's ->css_reset callback, which
would fit perfectly for the purpose. Currently, it's only called when a
subsystem is disabled in the unified hierarchy and there are other
subsystems dependant on it. Let's call it on css destruction as well.
Suggested-by: Johannes Weiner <hannes@cmpxchg.org>
Signed-off-by: Vladimir Davydov <vdavydov@virtuozzo.com>
Signed-off-by: Tejun Heo <tj@kernel.org>

There is a mistake about the print format name:id <--> %d:%s, which
the name is 'char *' type and id is 'int' type.  Change "name:id" to
"id:name" instead to be consistent with "cgroup_subsys %d:%s".
Signed-off-by: Xiubo Li <lixiubo@cmss.chinamobile.com>
Acked-by: Zefan Li <lizefan@huawei.com>
Signed-off-by: Tejun Heo <tj@kernel.org>

No internal process rule is enforced by cgroup_migrate_prepare_dst()
during process migration.  It tests whether the target cgroup's
->child_subsys_mask is zero which is different from "subtree_control"
write path which tests ->subtree_control.  This hasn't mattered
because up until now, both ->child_subsys_mask and ->subtree_control
are zero or non-zero at the same time.  However, with the planned
addition of implicit controllers, this will no longer be true.

This patch prepares for the change by making
cgorup_migrate_prepare_dst() test ->subtree_control instead.
Signed-off-by: Tejun Heo <tj@kernel.org>

The function currently returns -EBADF for a directory on the default
hierarchy.  Make it also recognize cgroup2_fs_type.  This will be used
for perf_event cgroup2 support.
Signed-off-by: Tejun Heo <tj@kernel.org>

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Li Zefan <lizefan@huawei.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>

These var names are unnecessarily unwiedly and another similar
variable will be added.  Let's shorten them.
Signed-off-by: Tejun Heo <tj@kernel.org>

After the recent do_each_subsys_mask() conversion, there's no reason
to use ulong for subsystem masks.  We'll be adding more subsystem
masks to persistent data structures, let's reduce its size to u16
which should be enough for now and the foreseeable future.

This doesn't create any noticeable behavior differences.

v2: Johannes spotted that the initial patch missed cgroup_no_v1_mask.
    Converted.
Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>

There are several places in cgroup_subtree_control_write() which can
use do_each_subsys_mask() instead of manual mask testing.  Use it.

No functional changes.
Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>

for_each_subsys_which() allows iterating subsystems specified in a
subsystem bitmask; unfortunately, it requires the mask to be an
unsigned long l-value which can be inconvenient and makes it awkward
to use a smaller type for subsystem masks.

This patch converts for_each_subsy_which() to do-while style which
allows it to drop the l-value requirement.  The new iterator is named
do_each_subsys_mask() / while_each_subsys_mask().
Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Aleksa Sarai <cyphar@cyphar.com>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>

For consistency with cgroup->subtree_control.

* cgroup->child_subsys_mask -> cgroup->subtree_ss_mask
* cgroup_calc_child_subsys_mask() -> cgroup_calc_subtree_ss_mask()
* cgroup_refresh_child_subsys_mask() -> cgroup_refresh_subtree_ss_mask()

No functional changes.
Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>

This reverts commit 56c807ba.

cgroup_subsys->css_e_css_changed() was supposed to be used by cgroup
writeback support; however, the change to per-inode cgroup association
made it unnecessary and the callback doesn't have any user.  Remove
it.
Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>

cgroup_addrm_files() incorrectly returned 0 after add failure.  Fix
it.
Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>