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Mercier" Cc: hannes@cmpxchg.org, mhocko@kernel.org, roman.gushchin@linux.dev, muchun.song@linux.dev, zhengqi.arch@bytedance.com, akpm@linux-foundation.org, cgroups@vger.kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org, stable@vger.kernel.org Subject: Re: [PATCH v2] mm: memcontrol: fix NULL pointer deref from high_work_func() on dying memcg Message-ID: References: <20261028183537.605854-1-tjmercier@google.com> X-Mailing-List: cgroups@vger.kernel.org List-Id: List-Post: List-Help: List-Subscribe: List-Unsubscribe: List-Owner: Precedence: list MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20261028183537.605854-1-tjmercier@google.com> On Wed, Oct 28, 2026 at 11:35:37AM -0700, T.J. Mercier wrote: > When memory.high is exceeded in non-task context, try_charge_memcg() > schedules memcg->high_work without taking a reference on the memcg, and > high_work_func() invokes reclaim_high() without pinning the memcg. > > cancel_work_sync(&memcg->high_work) only runs at the end of teardown in > mem_cgroup_css_free(), so high_work_func() can start or still be running > reclaim_high() after mem_cgroup_css_released() or shrink_many() has > called lru_gen_release_memcg() and unhashed lruvec->lrugen.list. If > reclaim tries to rotate the memcg, lru_gen_rotate_memcg() dereferences a > NULL pprev pointer. > > With obj_cgroup reparenting, introduced in commit bf4f059954dc ("mm: > memcg/slab: obj_cgroup API"), objects charged to a child memcg are > reparented to its parent in mem_cgroup_css_offline() instead of holding > a reference on the child until uncharged. Subsequent uncharges of those > objects go to the parent, which can leave the child's memory use > permanently above its high limit, so any later charge on the offlined > memcg in non-task context schedules high_work. > > This problem was first exposed when commit e4dde56cd208 ("mm: multi-gen > LRU: per-node lru_gen_folio lists") introduced the memcg LRU, and was > later widened from kernel memory to LRU folios when > commit f1cf8d2f36dc ("mm: memcontrol: eliminate the problem of dying > memory cgroup for LRU folios") converted LRU folios to use obj_cgroup > reparenting. > > Fix this by acquiring a reference with css_tryget_online() in > high_work_func() before calling reclaim_high(), and by resetting > memory.high and swap.high to PAGE_COUNTER_MAX in > mem_cgroup_css_offline() so that charges via references on an offlined > memcg do not trigger high_work on the dead child or skip checking live > ancestors in try_charge_memcg(). > > Oops: general protection fault, probably for non-canonical address 0xdffffc0000000000: 0000 [#1] SMP KASAN NOPTI > KASAN: null-ptr-deref in range [0x0000000000000000-0x0000000000000007] > CPU: 1 UID: 0 PID: 27 Comm: kworker/1:0 Tainted: G B W 7.3.0-rc5-g2bba28e98578 #284 PREEMPT(full) > Tainted: [B]=BAD_PAGE, [W]=WARN <- Taint from lru_gen_rotate_memcg() VM_WARN_ON_ONCE > Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.17.0-debian-1.17.0-1 04/01/2014 > Workqueue: events high_work_func > RIP: 0010:lru_gen_rotate_memcg+0x1a3/0xe90 > Call Trace: > > ? read_ctrl_pos+0x22f/0x3f0 > shrink_lruvec+0x16cb/0x1b80 > ? ret_from_fork_asm+0x1a/0x30 > ? lapic_next_event+0x15/0x20 > ? clockevents_program_event+0x2bb/0x6f0 > ? lapic_next_event+0x15/0x20 > ? __pfx_shrink_lruvec+0x10/0x10 > ? __pfx_clockevents_program_event+0x10/0x10 > ? _raw_spin_lock+0x84/0xe0 > ? __pfx__raw_spin_lock+0x10/0x10 > ? kasan_save_stack+0x30/0x50 > ? psi_group_change+0x381/0x740 > ? __pfx_sched_clock_cpu+0x10/0x10 > ? save_fpregs_to_fpstate+0xc4/0x2c0 > ? finish_task_switch.isra.0+0x1de/0xac0 > ? __schedule+0xf62/0x42c0 > ? __pfx___schedule+0x10/0x10 > ? __pfx___irq_work_queue_local+0x10/0x10 > ? shrink_node+0x626/0x31e0 > shrink_node+0x626/0x31e0 > ? ring_buffer_unlock_commit+0x11d/0x5c0 > ? kswapd_clear_hopeless+0xa0/0x150 > ? __pfx_shrink_node+0x10/0x10 > ? 0xffffffffc0000095 > do_try_to_free_pages+0x255/0x13b0 > ? ring_buffer_unlock_commit+0x11d/0x5c0 > ? reclaim_high+0x10a/0x1b0 > try_to_free_mem_cgroup_pages+0x1f9/0x5a0 > ? __pfx_try_to_free_mem_cgroup_pages+0x10/0x10 > ? irqentry_exit+0xc6/0x670 > reclaim_high+0x10a/0x1b0 > ? __pfx_reclaim_high+0x10/0x10 > ? process_one_work+0x5ff/0x1030 > ? __pfx_high_work_func+0x10/0x10 > process_one_work+0x6e2/0x1030 > ? __pfx___schedule+0x10/0x10 > ? __pfx_process_one_work+0x10/0x10 > worker_thread+0x422/0xbb0 > ? __pfx_worker_thread+0x10/0x10 > kthread+0x2cd/0x3a0 > ? recalc_sigpending+0x15c/0x200 > ? __pfx_kthread+0x10/0x10 > ret_from_fork+0x368/0x590 > ? __pfx_ret_from_fork+0x10/0x10 > ? __switch_to+0x588/0xf50 > ? __pfx_kthread+0x10/0x10 > ret_from_fork_asm+0x1a/0x30 > > > Fixes: e4dde56cd208 ("mm: multi-gen LRU: per-node lru_gen_folio lists") > Cc: stable@vger.kernel.org > Signed-off-by: T.J. Mercier > --- > Changes since V1 (https://lore.kernel.org/all/20261023222313.3979455-1-tjmercier@google.com/) > - Switch to Fixes: e4dde56cd208 ("mm: multi-gen LRU: per-node lru_gen_folio lists") > from Fixes: f1cf8d2f36dc ("mm: memcontrol: eliminate the problem of dying memory cgroup for LRU folios") > per Muchun Song. > - Reorder and update commit message content to describe reparenting > separately from the commits that exposed and then widened the problem. > - Trim the backtrace a bit. > > mm/memcontrol.c | 5 +++++ > 1 file changed, 5 insertions(+) > > diff --git a/mm/memcontrol.c b/mm/memcontrol.c > index a05d9a8baa1b..84f4a8fd4d16 100644 > --- a/mm/memcontrol.c > +++ b/mm/memcontrol.c > @@ -2457,7 +2457,10 @@ static void high_work_func(struct work_struct *work) > struct mem_cgroup *memcg; > > memcg = container_of(work, struct mem_cgroup, high_work); > + if (!css_tryget_online(&memcg->css)) > + return; > reclaim_high(memcg, MEMCG_CHARGE_BATCH, GFP_KERNEL); The patch is correct but I think there are two very specific subtleties which the commit message is not very explicit about. First, I was wondering shouldn't we take reference before schedule_work() and why this is safe. This is safe due to cancel_work_sync() in the mem_cgroup_css_free() which will not free memcg until this there is no high worker. Second, cgroup core calls css_free() after percpu_ref_exit(). So, is it safe to call css_tryget_online() on such css->refcnt. Yes css_tryget_online() is safe but css_tryget() is not safe.