+ mm-memcontrol-fix-null-pointer-deref-in-high_work_func-on-dying-memcg.patch added to mm-unstable branch

+ mm-memcontrol-fix-null-pointer-deref-in-high_work_func-on-dying-memcg.patch added to mm-unstable branch

[Andrew Morton]

The patch titled
     Subject: mm: memcontrol: fix NULL pointer deref in high_work_func() on dying memcg
has been added to the -mm mm-unstable branch.  Its filename is
     mm-memcontrol-fix-null-pointer-deref-in-high_work_func-on-dying-memcg.patch

This patch will shortly appear at
     https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/mm-memcontrol-fix-null-pointer-deref-in-high_work_func-on-dying-memcg.patch

This patch will later appear in the mm-unstable branch at
    git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm

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------------------------------------------------------
From: "T.J. Mercier" <tjmercier@google.com>
Subject: mm: memcontrol: fix NULL pointer deref in high_work_func() on dying memcg
Date: Fri, 23 Oct 2026 15:23:13 -0700

When memory.high is exceeded in non-task context, try_charge_memcg()
schedules memcg->high_work without taking a CSS reference on the memcg,
and high_work_func() invokes reclaim_high() without pinning the memcg.

Before commit f1cf8d2f36dc ("mm: memcontrol: eliminate the problem of
dying memory cgroup for LRU folios"), memory usage above memory.high
kept the memcg's refcount non-zero. With reparenting, folios and objcgs
charged to a child memcg are reparented to its parent in
mem_cgroup_css_offline(), leaving the offlined child's memory usage
permanently above its memory.high even after its last reference is
dropped.

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. This
causes lru_gen_rotate_memcg() to dereference a NULL pprev pointer.

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) <- Taint from lru_gen_rotate_memcg() VM_WARN_ON_ONCE
Tainted: [B]=BAD_PAGE, [W]=WARN
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
Code: fd 7f 09 45 84 ed 0f 85 64 0c 00 00 48 89 cf c6 83 6a 07 00 00 00 48 c1 ef 03 88 83 69 07 00 00 48 b8 00 00 00 00 00 fc ff df <80> 3c 07 00 0f 85 ec 0b 00 00 48 89 31 40 f6 c6 01 0f 85 4e 00
RSP: 0018:ffa00000001ef4c8 EFLAGS: 00010046
RAX: dffffc0000000000 RBX: ff1100000779e440 RCX: 0000000000000000
RDX: 0000000000000001 RSI: ff1100000779b9b0 RDI: 0000000000000000
RBP: ff110000bffd8e40 R08: 0000000000000000 R09: ff1100000779ebb8
R10: ff1100000779ebb0 R11: 0000000000000246 R12: 0000000000000000
R13: 0000000000000000 R14: ffa00000001efb30 R15: 0000000000000002
FS:  0000000000000000(0000) GS:ff1100011c0e7000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 000000000251ac88 CR3: 0000000009d6c000 CR4: 0000000000751ef0
PKRU: 55555554
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>

Link: https://lore.kernel.org/20261023222313.3979455-1-tjmercier@google.com
Fixes: f1cf8d2f36dc ("mm: memcontrol: eliminate the problem of dying memory cgroup for LRU folios")
Signed-off-by: T.J. Mercier <tjmercier@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Roman Gushchin <roman.gushchin@linux.dev>
Cc: Shakeel Butt <shakeel.butt@linux.dev>
Cc: Muchun Song <muchun.song@linux.dev>
Cc: Qi Zheng <qi.zheng@linux.dev>
Cc: <stable@vger.kernel.org>
---

 mm/memcontrol.c |    5 +++++
 1 file changed, 5 insertions(+)

--- a/mm/memcontrol.c~mm-memcontrol-fix-null-pointer-deref-in-high_work_func-on-dying-memcg
+++ a/mm/memcontrol.c
@@ -2457,7 +2457,10 @@ static void high_work_func(struct work_s
 	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);
+	css_put(&memcg->css);
 }
 
 static void high_irq_work_func(struct irq_work *work)
@@ -4463,6 +4466,8 @@ static void mem_cgroup_css_offline(struc
 
 	page_counter_set_min(&memcg->memory, 0);
 	page_counter_set_low(&memcg->memory, 0);
+	page_counter_set_high(&memcg->memory, PAGE_COUNTER_MAX);
+	page_counter_set_high(&memcg->swap, PAGE_COUNTER_MAX);
 
 	zswap_memcg_offline_cleanup(memcg);
 
_

Patches currently in -mm which might be from tjmercier@google.com are

mm-memcontrol-fix-null-pointer-deref-in-high_work_func-on-dying-memcg.patch