From 13e786b64bd3fd81c7eb22aa32bf8305c32f2ccf Mon Sep 17 00:00:00 2001 From: Petr Malat Date: Thu, 23 Apr 2026 04:48:26 -0500 Subject: [PATCH 1/9] cgroup: Increment nr_dying_subsys_* from rmdir context Incrementing nr_dying_subsys_* in offline_css(), which is executed by cgroup_offline_wq worker, leads to a race where user can see the value to be 0 if he reads cgroup.stat after calling rmdir and before the worker executes. This makes the user wrongly expect resources released by the removed cgroup to be available for a new assignment. Increment nr_dying_subsys_* from kill_css(), which is called from the cgroup_rmdir() context. Fixes: ab0312526867 ("cgroup: Show # of subsystem CSSes in cgroup.stat") Signed-off-by: Petr Malat Signed-off-by: Tejun Heo --- kernel/cgroup/cgroup.c | 22 ++++++++++++---------- 1 file changed, 12 insertions(+), 10 deletions(-) diff --git a/kernel/cgroup/cgroup.c b/kernel/cgroup/cgroup.c index 3243c2087ee3..c928dea9dea6 100644 --- a/kernel/cgroup/cgroup.c +++ b/kernel/cgroup/cgroup.c @@ -5724,16 +5724,6 @@ static void offline_css(struct cgroup_subsys_state *css) RCU_INIT_POINTER(css->cgroup->subsys[ss->id], NULL); wake_up_all(&css->cgroup->offline_waitq); - - css->cgroup->nr_dying_subsys[ss->id]++; - /* - * Parent css and cgroup cannot be freed until after the freeing - * of child css, see css_free_rwork_fn(). - */ - while ((css = css->parent)) { - css->nr_descendants--; - css->cgroup->nr_dying_subsys[ss->id]++; - } } /** @@ -6045,6 +6035,8 @@ static void css_killed_ref_fn(struct percpu_ref *ref) */ static void kill_css(struct cgroup_subsys_state *css) { + struct cgroup_subsys *ss = css->ss; + lockdep_assert_held(&cgroup_mutex); if (css->flags & CSS_DYING) @@ -6081,6 +6073,16 @@ static void kill_css(struct cgroup_subsys_state *css) * css is confirmed to be seen as killed on all CPUs. */ percpu_ref_kill_and_confirm(&css->refcnt, css_killed_ref_fn); + + css->cgroup->nr_dying_subsys[ss->id]++; + /* + * Parent css and cgroup cannot be freed until after the freeing + * of child css, see css_free_rwork_fn(). + */ + while ((css = css->parent)) { + css->nr_descendants--; + css->cgroup->nr_dying_subsys[ss->id]++; + } } /** From 981cd338614c96070cf9854679014fd027c1fb1d Mon Sep 17 00:00:00 2001 From: =?UTF-8?q?Petr=20Van=C4=9Bk?= Date: Sat, 25 Apr 2026 10:03:54 +0200 Subject: [PATCH 2/9] docs: cgroup: fix typo 'protetion' -> 'protection' MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit Fix a small typo in the description of the memory_hugetlb_accounting mount option. Signed-off-by: Petr Vaněk Signed-off-by: Tejun Heo --- Documentation/admin-guide/cgroup-v2.rst | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) diff --git a/Documentation/admin-guide/cgroup-v2.rst b/Documentation/admin-guide/cgroup-v2.rst index 8ad0b2781317..6efd0095ed99 100644 --- a/Documentation/admin-guide/cgroup-v2.rst +++ b/Documentation/admin-guide/cgroup-v2.rst @@ -220,7 +220,7 @@ cgroup v2 currently supports the following mount options. memory_hugetlb_accounting Count HugeTLB memory usage towards the cgroup's overall memory usage for the memory controller (for the purpose of - statistics reporting and memory protetion). This is a new + statistics reporting and memory protection). This is a new behavior that could regress existing setups, so it must be explicitly opted in with this mount option. From 93618edf753838a727dbff63c7c291dee22d656b Mon Sep 17 00:00:00 2001 From: Tejun Heo Date: Fri, 1 May 2026 08:31:22 -1000 Subject: [PATCH 3/9] cgroup: Defer css percpu_ref kill on rmdir until cgroup is depopulated A chain of commits going back to v7.0 reworked rmdir to satisfy the controller invariant that a subsystem's ->css_offline() must not run while tasks are still doing kernel-side work in the cgroup. [1] d245698d727a ("cgroup: Defer task cgroup unlink until after the task is done switching out") [2] a72f73c4dd9b ("cgroup: Don't expose dead tasks in cgroup") [3] 1b164b876c36 ("cgroup: Wait for dying tasks to leave on rmdir") [4] 4c56a8ac6869 ("cgroup: Fix cgroup_drain_dying() testing the wrong condition") [5] 13e786b64bd3 ("cgroup: Increment nr_dying_subsys_* from rmdir context") [1] moved task cset unlink from do_exit() to finish_task_switch() so a task's cset link drops only after the task has fully stopped scheduling. That made tasks past exit_signals() linger on cset->tasks until their final context switch, which led to a series of problems as what userspace expected to see after rmdir diverged from what the kernel needs to wait for. [2]-[5] tried to bridge that divergence: [2] filtered the exiting tasks from cgroup.procs; [3] had rmdir(2) sleep in TASK_UNINTERRUPTIBLE for them; [4] fixed the wait's condition; [5] made nr_dying_subsys_* visible synchronously. The cgroup_drain_dying() wait in [3] turned out to be a dead end. When the rmdir caller is also the reaper of a zombie that pins a pidns teardown (e.g. host PID 1 systemd reaping orphan pids that were re-parented to it during the same teardown), rmdir blocks in TASK_UNINTERRUPTIBLE waiting for those pids to free, the pids can't free because PID 1 is the reaper and it's stuck in rmdir, and the system A-A deadlocks. No internal lock ordering breaks this; the wait itself is the bug. The css killing side that drove the original reorder, however, can be made cleanly asynchronous: ->css_offline() is already async, run from css_killed_work_fn() driven by percpu_ref_kill_and_confirm(). The fix is to make that chain start only after all tasks have left the cgroup. rmdir's user-visible side then returns as soon as cgroup.procs and friends are empty, while ->css_offline() still runs only after the cgroup is fully drained. Verified by the original reproducer (pidns teardown + zombie reaper, runs under vng) which hangs vanilla and succeeds here, and by per-commit deterministic repros for [2], [3], [4], [5] with a boot parameter that widens the post-exit_signals() window so each state is reliably reachable. Some stress tests on top of that. cgroup_apply_control_disable() has the same shape of pre-existing race: when a controller is disabled via subtree_control, kill_css() ran synchronously while tasks past exit_signals() could still be linked to the cgroup's csets, and ->css_offline() could fire before they drained. This patch preserves the existing synchronous behavior at that call site (kill_css_sync() + kill_css_finish() back-to-back) and a follow-up patch will defer kill_css_finish() there using a per-css trigger. This seems like the right approach and I don't see problems with it. The changes are somewhat invasive but not excessively so, so backporting to -stable should be okay. If something does turn out to be wrong, the fallback is to revert the entire chain ([1]-[5]) and rework in the development branch instead. v2: Pin cgrp across the deferred destroy work with explicit cgroup_get()/cgroup_put() around queue_work() and the work_fn. v1 wasn't actually broken (ordered cgroup_offline_wq + queue_work order in cgroup_task_dead() saved it) but the explicit ref removes the dependency on those non-obvious invariants. Also note the pre-existing cgroup_apply_control_disable() race in the description; a follow-up will defer kill_css_finish() there. Fixes: 1b164b876c36 ("cgroup: Wait for dying tasks to leave on rmdir") Cc: stable@vger.kernel.org # v7.0+ Reported-and-tested-by: Martin Pitt Link: https://lore.kernel.org/all/afHNg2VX2jy9bW7y@piware.de/ Link: https://lore.kernel.org/all/35e0670adb4abeab13da2c321582af9f@kernel.org/ Signed-off-by: Tejun Heo Acked-by: Sebastian Andrzej Siewior --- include/linux/cgroup-defs.h | 4 +- kernel/cgroup/cgroup.c | 244 +++++++++++++++++------------------- 2 files changed, 116 insertions(+), 132 deletions(-) diff --git a/include/linux/cgroup-defs.h b/include/linux/cgroup-defs.h index f42563739d2e..50a784da7a81 100644 --- a/include/linux/cgroup-defs.h +++ b/include/linux/cgroup-defs.h @@ -611,8 +611,8 @@ struct cgroup { /* used to wait for offlining of csses */ wait_queue_head_t offline_waitq; - /* used by cgroup_rmdir() to wait for dying tasks to leave */ - wait_queue_head_t dying_populated_waitq; + /* defers killing csses after removal until cgroup is depopulated */ + struct work_struct finish_destroy_work; /* used to schedule release agent */ struct work_struct release_agent_work; diff --git a/kernel/cgroup/cgroup.c b/kernel/cgroup/cgroup.c index c928dea9dea6..bd10a7e2f9c5 100644 --- a/kernel/cgroup/cgroup.c +++ b/kernel/cgroup/cgroup.c @@ -264,10 +264,12 @@ static void cgroup_finalize_control(struct cgroup *cgrp, int ret); static void css_task_iter_skip(struct css_task_iter *it, struct task_struct *task); static int cgroup_destroy_locked(struct cgroup *cgrp); +static void cgroup_finish_destroy(struct cgroup *cgrp); +static void kill_css_sync(struct cgroup_subsys_state *css); +static void kill_css_finish(struct cgroup_subsys_state *css); static struct cgroup_subsys_state *css_create(struct cgroup *cgrp, struct cgroup_subsys *ss); static void css_release(struct percpu_ref *ref); -static void kill_css(struct cgroup_subsys_state *css); static int cgroup_addrm_files(struct cgroup_subsys_state *css, struct cgroup *cgrp, struct cftype cfts[], bool is_add); @@ -797,6 +799,16 @@ static void cgroup_update_populated(struct cgroup *cgrp, bool populated) if (was_populated == cgroup_is_populated(cgrp)) break; + /* + * Subtree just emptied below an offlined cgrp. Fire deferred + * destroy. The transition is one-shot. + */ + if (was_populated && !css_is_online(&cgrp->self)) { + cgroup_get(cgrp); + WARN_ON_ONCE(!queue_work(cgroup_offline_wq, + &cgrp->finish_destroy_work)); + } + cgroup1_check_for_release(cgrp); TRACE_CGROUP_PATH(notify_populated, cgrp, cgroup_is_populated(cgrp)); @@ -2039,6 +2051,16 @@ static int cgroup_reconfigure(struct fs_context *fc) return 0; } +static void cgroup_finish_destroy_work_fn(struct work_struct *work) +{ + struct cgroup *cgrp = container_of(work, struct cgroup, finish_destroy_work); + + cgroup_lock(); + cgroup_finish_destroy(cgrp); + cgroup_unlock(); + cgroup_put(cgrp); +} + static void init_cgroup_housekeeping(struct cgroup *cgrp) { struct cgroup_subsys *ss; @@ -2065,7 +2087,7 @@ static void init_cgroup_housekeeping(struct cgroup *cgrp) #endif init_waitqueue_head(&cgrp->offline_waitq); - init_waitqueue_head(&cgrp->dying_populated_waitq); + INIT_WORK(&cgrp->finish_destroy_work, cgroup_finish_destroy_work_fn); INIT_WORK(&cgrp->release_agent_work, cgroup1_release_agent); } @@ -3375,7 +3397,8 @@ static void cgroup_apply_control_disable(struct cgroup *cgrp) if (css->parent && !(cgroup_ss_mask(dsct) & (1 << ss->id))) { - kill_css(css); + kill_css_sync(css); + kill_css_finish(css); } else if (!css_visible(css)) { css_clear_dir(css); if (ss->css_reset) @@ -5514,7 +5537,7 @@ static struct cftype cgroup_psi_files[] = { * css destruction is four-stage process. * * 1. Destruction starts. Killing of the percpu_ref is initiated. - * Implemented in kill_css(). + * Implemented in kill_css_finish(). * * 2. When the percpu_ref is confirmed to be visible as killed on all CPUs * and thus css_tryget_online() is guaranteed to fail, the css can be @@ -5993,7 +6016,7 @@ int cgroup_mkdir(struct kernfs_node *parent_kn, const char *name, umode_t mode) /* * This is called when the refcnt of a css is confirmed to be killed. * css_tryget_online() is now guaranteed to fail. Tell the subsystem to - * initiate destruction and put the css ref from kill_css(). + * initiate destruction and put the css ref from kill_css_finish(). */ static void css_killed_work_fn(struct work_struct *work) { @@ -6025,15 +6048,12 @@ static void css_killed_ref_fn(struct percpu_ref *ref) } /** - * kill_css - destroy a css - * @css: css to destroy + * kill_css_sync - synchronous half of css teardown + * @css: css being killed * - * This function initiates destruction of @css by removing cgroup interface - * files and putting its base reference. ->css_offline() will be invoked - * asynchronously once css_tryget_online() is guaranteed to fail and when - * the reference count reaches zero, @css will be released. + * See cgroup_destroy_locked(). */ -static void kill_css(struct cgroup_subsys_state *css) +static void kill_css_sync(struct cgroup_subsys_state *css) { struct cgroup_subsys *ss = css->ss; @@ -6056,24 +6076,6 @@ static void kill_css(struct cgroup_subsys_state *css) */ css_clear_dir(css); - /* - * Killing would put the base ref, but we need to keep it alive - * until after ->css_offline(). - */ - css_get(css); - - /* - * cgroup core guarantees that, by the time ->css_offline() is - * invoked, no new css reference will be given out via - * css_tryget_online(). We can't simply call percpu_ref_kill() and - * proceed to offlining css's because percpu_ref_kill() doesn't - * guarantee that the ref is seen as killed on all CPUs on return. - * - * Use percpu_ref_kill_and_confirm() to get notifications as each - * css is confirmed to be seen as killed on all CPUs. - */ - percpu_ref_kill_and_confirm(&css->refcnt, css_killed_ref_fn); - css->cgroup->nr_dying_subsys[ss->id]++; /* * Parent css and cgroup cannot be freed until after the freeing @@ -6086,44 +6088,88 @@ static void kill_css(struct cgroup_subsys_state *css) } /** - * cgroup_destroy_locked - the first stage of cgroup destruction + * kill_css_finish - deferred half of css teardown + * @css: css being killed + * + * See cgroup_destroy_locked(). + */ +static void kill_css_finish(struct cgroup_subsys_state *css) +{ + lockdep_assert_held(&cgroup_mutex); + + /* + * Skip on re-entry: cgroup_apply_control_disable() may have killed @css + * earlier. cgroup_destroy_locked() can still walk it because + * offline_css() (which NULLs cgrp->subsys[ssid]) runs async. + */ + if (percpu_ref_is_dying(&css->refcnt)) + return; + + /* + * Killing would put the base ref, but we need to keep it alive until + * after ->css_offline(). + */ + css_get(css); + + /* + * cgroup core guarantees that, by the time ->css_offline() is invoked, + * no new css reference will be given out via css_tryget_online(). We + * can't simply call percpu_ref_kill() and proceed to offlining css's + * because percpu_ref_kill() doesn't guarantee that the ref is seen as + * killed on all CPUs on return. + * + * Use percpu_ref_kill_and_confirm() to get notifications as each css is + * confirmed to be seen as killed on all CPUs. + */ + percpu_ref_kill_and_confirm(&css->refcnt, css_killed_ref_fn); +} + +/** + * cgroup_destroy_locked - destroy @cgrp (called on rmdir) * @cgrp: cgroup to be destroyed * - * css's make use of percpu refcnts whose killing latency shouldn't be - * exposed to userland and are RCU protected. Also, cgroup core needs to - * guarantee that css_tryget_online() won't succeed by the time - * ->css_offline() is invoked. To satisfy all the requirements, - * destruction is implemented in the following two steps. + * Tear down @cgrp on behalf of rmdir. Constraints: * - * s1. Verify @cgrp can be destroyed and mark it dying. Remove all - * userland visible parts and start killing the percpu refcnts of - * css's. Set up so that the next stage will be kicked off once all - * the percpu refcnts are confirmed to be killed. + * - Userspace: rmdir must succeed when cgroup.procs and friends are empty. * - * s2. Invoke ->css_offline(), mark the cgroup dead and proceed with the - * rest of destruction. Once all cgroup references are gone, the - * cgroup is RCU-freed. + * - Kernel: subsystem ->css_offline() must not run while any task in @cgrp's + * subtree is still doing kernel work. A task hidden from cgroup.procs (past + * exit_signals() with signal->live cleared) can still schedule, allocate, and + * consume resources until its final context switch. Dying descendants in the + * subtree can host such tasks too. * - * This function implements s1. After this step, @cgrp is gone as far as - * the userland is concerned and a new cgroup with the same name may be - * created. As cgroup doesn't care about the names internally, this - * doesn't cause any problem. + * - Kernel: css_tryget_online() must fail by the time ->css_offline() runs. + * + * The destruction runs in three parts: + * + * - This function: synchronous user-visible state teardown plus kill_css_sync() + * on each subsystem css. + * + * - cgroup_finish_destroy(): kicks the percpu_ref kill via kill_css_finish() on + * each subsystem css. Fires once @cgrp's subtree is fully drained, either + * inline here or from cgroup_update_populated(). + * + * - The percpu_ref kill chain: css_killed_ref_fn -> css_killed_work_fn -> + * ->css_offline() -> release/free. + * + * Return 0 on success, -EBUSY if a userspace-visible task or an online child + * remains. */ static int cgroup_destroy_locked(struct cgroup *cgrp) - __releases(&cgroup_mutex) __acquires(&cgroup_mutex) { struct cgroup *tcgrp, *parent = cgroup_parent(cgrp); struct cgroup_subsys_state *css; struct cgrp_cset_link *link; + struct css_task_iter it; + struct task_struct *task; int ssid, ret; lockdep_assert_held(&cgroup_mutex); - /* - * Only migration can raise populated from zero and we're already - * holding cgroup_mutex. - */ - if (cgroup_is_populated(cgrp)) + css_task_iter_start(&cgrp->self, 0, &it); + task = css_task_iter_next(&it); + css_task_iter_end(&it); + if (task) return -EBUSY; /* @@ -6147,9 +6193,8 @@ static int cgroup_destroy_locked(struct cgroup *cgrp) link->cset->dead = true; spin_unlock_irq(&css_set_lock); - /* initiate massacre of all css's */ for_each_css(css, ssid, cgrp) - kill_css(css); + kill_css_sync(css); /* clear and remove @cgrp dir, @cgrp has an extra ref on its kn */ css_clear_dir(&cgrp->self); @@ -6180,79 +6225,27 @@ static int cgroup_destroy_locked(struct cgroup *cgrp) /* put the base reference */ percpu_ref_kill(&cgrp->self.refcnt); + if (!cgroup_is_populated(cgrp)) + cgroup_finish_destroy(cgrp); + return 0; }; /** - * cgroup_drain_dying - wait for dying tasks to leave before rmdir - * @cgrp: the cgroup being removed + * cgroup_finish_destroy - deferred half of @cgrp destruction + * @cgrp: cgroup whose subtree just became empty * - * cgroup.procs and cgroup.threads use css_task_iter which filters out - * PF_EXITING tasks so that userspace doesn't see tasks that have already been - * reaped via waitpid(). However, cgroup_has_tasks() - which tests whether the - * cgroup has non-empty css_sets - is only updated when dying tasks pass through - * cgroup_task_dead() in finish_task_switch(). This creates a window where - * cgroup.procs reads empty but cgroup_has_tasks() is still true, making rmdir - * fail with -EBUSY from cgroup_destroy_locked() even though userspace sees no - * tasks. - * - * This function aligns cgroup_has_tasks() with what userspace can observe. If - * cgroup_has_tasks() but the task iterator sees nothing (all remaining tasks are - * PF_EXITING), we wait for cgroup_task_dead() to finish processing them. As the - * window between PF_EXITING and cgroup_task_dead() is short, the wait is brief. - * - * This function only concerns itself with this cgroup's own dying tasks. - * Whether the cgroup has children is cgroup_destroy_locked()'s problem. - * - * Each cgroup_task_dead() kicks the waitqueue via cset->cgrp_links, and we - * retry the full check from scratch. - * - * Must be called with cgroup_mutex held. + * See cgroup_destroy_locked() for the rationale. */ -static int cgroup_drain_dying(struct cgroup *cgrp) - __releases(&cgroup_mutex) __acquires(&cgroup_mutex) +static void cgroup_finish_destroy(struct cgroup *cgrp) { - struct css_task_iter it; - struct task_struct *task; - DEFINE_WAIT(wait); + struct cgroup_subsys_state *css; + int ssid; lockdep_assert_held(&cgroup_mutex); -retry: - if (!cgroup_has_tasks(cgrp)) - return 0; - /* Same iterator as cgroup.threads - if any task is visible, it's busy */ - css_task_iter_start(&cgrp->self, 0, &it); - task = css_task_iter_next(&it); - css_task_iter_end(&it); - - if (task) - return -EBUSY; - - /* - * All remaining tasks are PF_EXITING and will pass through - * cgroup_task_dead() shortly. Wait for a kick and retry. - * - * cgroup_has_tasks() can't transition from false to true while we're - * holding cgroup_mutex, but the true to false transition happens - * under css_set_lock (via cgroup_task_dead()). We must retest and - * prepare_to_wait() under css_set_lock. Otherwise, the transition - * can happen between our first test and prepare_to_wait(), and we - * sleep with no one to wake us. - */ - spin_lock_irq(&css_set_lock); - if (!cgroup_has_tasks(cgrp)) { - spin_unlock_irq(&css_set_lock); - return 0; - } - prepare_to_wait(&cgrp->dying_populated_waitq, &wait, - TASK_UNINTERRUPTIBLE); - spin_unlock_irq(&css_set_lock); - mutex_unlock(&cgroup_mutex); - schedule(); - finish_wait(&cgrp->dying_populated_waitq, &wait); - mutex_lock(&cgroup_mutex); - goto retry; + for_each_css(css, ssid, cgrp) + kill_css_finish(css); } int cgroup_rmdir(struct kernfs_node *kn) @@ -6264,12 +6257,9 @@ int cgroup_rmdir(struct kernfs_node *kn) if (!cgrp) return 0; - ret = cgroup_drain_dying(cgrp); - if (!ret) { - ret = cgroup_destroy_locked(cgrp); - if (!ret) - TRACE_CGROUP_PATH(rmdir, cgrp); - } + ret = cgroup_destroy_locked(cgrp); + if (!ret) + TRACE_CGROUP_PATH(rmdir, cgrp); cgroup_kn_unlock(kn); return ret; @@ -7029,7 +7019,6 @@ void cgroup_task_exit(struct task_struct *tsk) static void do_cgroup_task_dead(struct task_struct *tsk) { - struct cgrp_cset_link *link; struct css_set *cset; unsigned long flags; @@ -7043,11 +7032,6 @@ static void do_cgroup_task_dead(struct task_struct *tsk) if (thread_group_leader(tsk) && atomic_read(&tsk->signal->live)) list_add_tail(&tsk->cg_list, &cset->dying_tasks); - /* kick cgroup_drain_dying() waiters, see cgroup_rmdir() */ - list_for_each_entry(link, &cset->cgrp_links, cgrp_link) - if (waitqueue_active(&link->cgrp->dying_populated_waitq)) - wake_up(&link->cgrp->dying_populated_waitq); - if (dl_task(tsk)) dec_dl_tasks_cs(tsk); From d8769544bde51b0ac980d10f8fe9f9fed6c95995 Mon Sep 17 00:00:00 2001 From: "T.J. Mercier" Date: Thu, 30 Apr 2026 13:11:42 -0700 Subject: [PATCH 4/9] docs: cgroup-v1: Update charge-commit section Commit 1d8f136a421f ("memcg/hugetlb: remove memcg hugetlb try-commit-cancel protocol") removed mem_cgroup_commit_charge() and mem_cgroup_cancel_charge(), but the docs still refer to those functions. There is no longer any charge cancellation. Update the docs to match the code. Signed-off-by: T.J. Mercier Signed-off-by: Tejun Heo --- Documentation/admin-guide/cgroup-v1/memcg_test.rst | 6 ++---- 1 file changed, 2 insertions(+), 4 deletions(-) diff --git a/Documentation/admin-guide/cgroup-v1/memcg_test.rst b/Documentation/admin-guide/cgroup-v1/memcg_test.rst index 9f8e27355cba..7c7cd457cf69 100644 --- a/Documentation/admin-guide/cgroup-v1/memcg_test.rst +++ b/Documentation/admin-guide/cgroup-v1/memcg_test.rst @@ -47,21 +47,19 @@ Please note that implementation details can be changed. Called when swp_entry's refcnt goes down to 0. A charge against swap disappears. -3. charge-commit-cancel +3. charge-commit ======================= Memcg pages are charged in two steps: - mem_cgroup_try_charge() - - mem_cgroup_commit_charge() or mem_cgroup_cancel_charge() + - commit_charge() At try_charge(), there are no flags to say "this page is charged". at this point, usage += PAGE_SIZE. At commit(), the page is associated with the memcg. - At cancel(), simply usage -= PAGE_SIZE. - Under below explanation, we assume CONFIG_SWAP=y. 4. Anonymous From dde2f938d02f2c740d49bb5113dea941f941026a Mon Sep 17 00:00:00 2001 From: Chen Wandun Date: Thu, 7 May 2026 18:54:34 +0800 Subject: [PATCH 5/9] cgroup/cpuset: move PF_EXITING check before __GFP_HARDWALL in cpuset_current_node_allowed() MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit Since prepare_alloc_pages() unconditionally adds __GFP_HARDWALL for the fast path when cpusets are enabled, the __GFP_HARDWALL check in cpuset_current_node_allowed() causes the PF_EXITING escape path to be skipped on the first allocation attempt. This makes it unreachable in the common case, so dying tasks can get stuck in direct reclaim or even trigger OOM while trying to exit, despite being allowed to allocate from any node. Move the PF_EXITING check before __GFP_HARDWALL so that dying tasks can allocate memory from any node to exit quickly, even when cpusets are enabled. Also update the function comment to reflect the actual behavior of prepare_alloc_pages() and the corrected check ordering. Signed-off-by: Chen Wandun Acked-by: Michal Koutný Acked-by: Waiman Long Signed-off-by: Tejun Heo --- kernel/cgroup/cpuset.c | 14 ++++++++------ 1 file changed, 8 insertions(+), 6 deletions(-) diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c index e3a081a07c6d..a48901a0416a 100644 --- a/kernel/cgroup/cpuset.c +++ b/kernel/cgroup/cpuset.c @@ -4176,11 +4176,11 @@ static struct cpuset *nearest_hardwall_ancestor(struct cpuset *cs) * current's mems_allowed, yes. If it's not a __GFP_HARDWALL request and this * node is set in the nearest hardwalled cpuset ancestor to current's cpuset, * yes. If current has access to memory reserves as an oom victim, yes. - * Otherwise, no. + * If the current task is PF_EXITING, yes. Otherwise, no. * * GFP_USER allocations are marked with the __GFP_HARDWALL bit, * and do not allow allocations outside the current tasks cpuset - * unless the task has been OOM killed. + * unless the task has been OOM killed or is exiting. * GFP_KERNEL allocations are not so marked, so can escape to the * nearest enclosing hardwalled ancestor cpuset. * @@ -4194,7 +4194,9 @@ static struct cpuset *nearest_hardwall_ancestor(struct cpuset *cs) * The first call here from mm/page_alloc:get_page_from_freelist() * has __GFP_HARDWALL set in gfp_mask, enforcing hardwall cpusets, * so no allocation on a node outside the cpuset is allowed (unless - * in interrupt, of course). + * in interrupt, of course). The PF_EXITING check must therefore + * come before the __GFP_HARDWALL check, otherwise a dying task + * would be blocked on the fast path. * * The second pass through get_page_from_freelist() doesn't even call * here for GFP_ATOMIC calls. For those calls, the __alloc_pages() @@ -4204,6 +4206,7 @@ static struct cpuset *nearest_hardwall_ancestor(struct cpuset *cs) * in_interrupt - any node ok (current task context irrelevant) * GFP_ATOMIC - any node ok * tsk_is_oom_victim - any node ok + * PF_EXITING - any node ok (let dying task exit quickly) * GFP_KERNEL - any node in enclosing hardwalled cpuset ok * GFP_USER - only nodes in current tasks mems allowed ok. */ @@ -4223,11 +4226,10 @@ bool cpuset_current_node_allowed(int node, gfp_t gfp_mask) */ if (unlikely(tsk_is_oom_victim(current))) return true; - if (gfp_mask & __GFP_HARDWALL) /* If hardwall request, stop here */ - return false; - if (current->flags & PF_EXITING) /* Let dying task have memory */ return true; + if (gfp_mask & __GFP_HARDWALL) /* If hardwall request, stop here */ + return false; /* Not hardwall and node outside mems_allowed: scan up cpusets */ spin_lock_irqsave(&callback_lock, flags); From 796ad622040f7f955ccc3973085e953415920496 Mon Sep 17 00:00:00 2001 From: Guopeng Zhang Date: Mon, 11 May 2026 09:31:50 +0800 Subject: [PATCH 6/9] cgroup/dmem: Return -ENOMEM on failed pool preallocation get_cg_pool_unlocked() handles allocation failures under dmemcg_lock by dropping the lock, preallocating a pool with GFP_KERNEL, and retrying the locked lookup and creation path. If the fallback allocation fails too, pool remains NULL. Since the loop condition is while (!pool), the function can keep retrying instead of propagating the allocation failure to the caller. Set pool to ERR_PTR(-ENOMEM) when the fallback allocation fails so the loop exits through the existing common return path. The callers already handle ERR_PTR() from get_cg_pool_unlocked(), so this restores the expected error path. Fixes: b168ed458dde ("kernel/cgroup: Add "dmem" memory accounting cgroup") Cc: stable@vger.kernel.org # v6.14+ Signed-off-by: Guopeng Zhang Signed-off-by: Tejun Heo --- kernel/cgroup/dmem.c | 1 + 1 file changed, 1 insertion(+) diff --git a/kernel/cgroup/dmem.c b/kernel/cgroup/dmem.c index 1ab1fb47f271..4753a67d0f0f 100644 --- a/kernel/cgroup/dmem.c +++ b/kernel/cgroup/dmem.c @@ -602,6 +602,7 @@ get_cg_pool_unlocked(struct dmemcg_state *cg, struct dmem_cgroup_region *region) pool = NULL; continue; } + pool = ERR_PTR(-ENOMEM); } } From e32e6f02168f2ad7991eb5d160d312d2001520c8 Mon Sep 17 00:00:00 2001 From: Hongfu Li Date: Sat, 9 May 2026 16:03:28 +0800 Subject: [PATCH 7/9] selftests/cgroup: Fix cg_read_strcmp() empty string comparison cg_read_strcmp() allocated a buffer sized to strlen(expected) + 1, then passed it to read_text() which calls read(fd, buf, size-1). When comparing against an empty string (""), strlen("") = 0 gives a 1-byte buffer, and read() is asked to read 0 bytes. The file content is never actually read, so strcmp("", buf) always returns 0 regardless of the real content. This caused cg_test_proc_killed() to always report the cgroup as empty immediately, making OOM tests pass without verifying that processes were killed. Signed-off-by: Hongfu Li Signed-off-by: Tejun Heo --- tools/testing/selftests/cgroup/lib/cgroup_util.c | 5 +++-- 1 file changed, 3 insertions(+), 2 deletions(-) diff --git a/tools/testing/selftests/cgroup/lib/cgroup_util.c b/tools/testing/selftests/cgroup/lib/cgroup_util.c index 6a7295347e90..42f54936f4bb 100644 --- a/tools/testing/selftests/cgroup/lib/cgroup_util.c +++ b/tools/testing/selftests/cgroup/lib/cgroup_util.c @@ -106,8 +106,9 @@ int cg_read_strcmp(const char *cgroup, const char *control, /* Handle the case of comparing against empty string */ if (!expected) return -1; - else - size = strlen(expected) + 1; + + /* needs size > 1, otherwise cg_read() reads 0 bytes */ + size = (expected[0] == '\0') ? 2 : strlen(expected) + 1; buf = malloc(size); if (!buf) From 2a3d7256faf06d1a15bb5b07e851ac4e1680c26d Mon Sep 17 00:00:00 2001 From: Hongfu Li Date: Mon, 11 May 2026 09:39:57 +0800 Subject: [PATCH 8/9] selftests/cgroup: Fix string comparison in write_test Use string comparison (!=) instead of numeric comparison (-ne) for cpuset values like "0-1". For example: $ [[ "0-1" != "2-3" ]] && echo "true" || echo "false" true $ [[ "0-1" -ne "2-3" ]] && echo "true" || echo "false" false Signed-off-by: Hongfu Li Signed-off-by: Tejun Heo --- tools/testing/selftests/cgroup/test_cpuset_v1_base.sh | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) diff --git a/tools/testing/selftests/cgroup/test_cpuset_v1_base.sh b/tools/testing/selftests/cgroup/test_cpuset_v1_base.sh index 42a6628fb8bc..1c0444729e70 100755 --- a/tools/testing/selftests/cgroup/test_cpuset_v1_base.sh +++ b/tools/testing/selftests/cgroup/test_cpuset_v1_base.sh @@ -18,7 +18,7 @@ write_test() { echo "testing $interface $value" echo $value > $dir/$interface new=$(cat $dir/$interface) - [[ $value -ne $(cat $dir/$interface) ]] && { + [[ "$value" != "$new" ]] && { echo "$interface write $value failed: new:$new" exit 1 } From 4a39eda5fdd867fc39f3c039714dd432cee00268 Mon Sep 17 00:00:00 2001 From: Guopeng Zhang Date: Sat, 9 May 2026 18:20:30 +0800 Subject: [PATCH 9/9] cgroup/cpuset: Reset DL migration state on can_attach() failure cpuset_can_attach() accumulates temporary SCHED_DEADLINE migration state in the destination cpuset while walking the taskset. If a later task_can_attach() or security_task_setscheduler() check fails, cgroup_migrate_execute() treats cpuset as the failing subsystem and does not call cpuset_cancel_attach() for it. The partially accumulated state is then left behind and can be consumed by a later attach, corrupting cpuset DL task accounting and pending DL bandwidth accounting. Reset the pending DL migration state from the common error exit when ret is non-zero. Successful can_attach() keeps the state for cpuset_attach() or cpuset_cancel_attach(). Fixes: 2ef269ef1ac0 ("cgroup/cpuset: Free DL BW in case can_attach() fails") Cc: stable@vger.kernel.org # v6.10+ Signed-off-by: Guopeng Zhang Signed-off-by: Tejun Heo Reviewed-by: Chen Ridong Reviewed-by: Waiman Long --- kernel/cgroup/cpuset.c | 8 ++++---- 1 file changed, 4 insertions(+), 4 deletions(-) diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c index a48901a0416a..3fbf6e7f68c3 100644 --- a/kernel/cgroup/cpuset.c +++ b/kernel/cgroup/cpuset.c @@ -3050,16 +3050,13 @@ static int cpuset_can_attach(struct cgroup_taskset *tset) int cpu = cpumask_any_and(cpu_active_mask, cs->effective_cpus); if (unlikely(cpu >= nr_cpu_ids)) { - reset_migrate_dl_data(cs); ret = -EINVAL; goto out_unlock; } ret = dl_bw_alloc(cpu, cs->sum_migrate_dl_bw); - if (ret) { - reset_migrate_dl_data(cs); + if (ret) goto out_unlock; - } cs->dl_bw_cpu = cpu; } @@ -3070,7 +3067,10 @@ static int cpuset_can_attach(struct cgroup_taskset *tset) * changes which zero cpus/mems_allowed. */ cs->attach_in_progress++; + out_unlock: + if (ret) + reset_migrate_dl_data(cs); mutex_unlock(&cpuset_mutex); return ret; }