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Mercier" , 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: On Wed, Oct 28, 2026 at 06:12:45PM -0400, Johannes Weiner wrote: > 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 > > Hrm, yes I suppose the work itself keeps the memcg allocated, but not > from going deep into the destruction sequence. I was briefly wondering > whether mglru shouldn't move its callback from css_released into > css_free, but I think your fix is more robust after all: don't send a > barely allocated memcg down the reclaim stack. > > The _online variant of the tryget seems somewhat unnecessary. There is > no real harm in reclaiming an offline group - and it was the standard > way, before reparenting, to garbage collect an offline group with > lingering pages. With the updated Fixes, this would look odd in > backports to kernels without reparenting. The PAGE_COUNTER_MAX reset > would also cause reclaim_high to bail. > > So I wonder if css_tryget() would be simpler and just as correct. But > no strong feelings either way: css_tryget() is not safe after percpu_ref_exit() which can happen here. Only css_tryget_online() is safe.