Re: [PATCH v2] mm: memcontrol: fix NULL pointer deref from high_work_func() on dying memcg

[Muchun Song <muchun.song@linux.dev>]

> On Oct 29, 2026, at 02:35, T.J. Mercier <tjmercier@google.com> 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:
> <TASK>
> ? 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
> </TASK>
> 
> Fixes: e4dde56cd208 ("mm: multi-gen LRU: per-node lru_gen_folio lists")
> Cc: stable@vger.kernel.org
> Signed-off-by: T.J. Mercier <tjmercier@google.com>

Acked-by: Muchun Song <muchun.song@linux.dev>

Thanks.