Re: [moderation/CI] Re: memcg: fix dying memcg pinned by swapped out shmem pages

#syz upstream


On Thu, Aug 13, 2026 at 3:24 PM syzbot ci
<[email protected]> wrote:
>
> syzbot ci has tested the following series
>
> [v1] memcg: fix dying memcg pinned by swapped out shmem pages
> https://lore.kernel.org/all/[email protected]
> * [PATCH RFC 1/5] memcg: move memcg private ID refcount to objcg
> * [PATCH RFC 2/5] memcg: get stable memcg first before getting memcgid reference
> * [PATCH RFC 3/5] memcg: remove retry logic in mem_cgroup_private_id_get_online
> * [PATCH RFC 4/5] memcg: add a helper to get online memcg from memcgid
> * [PATCH RFC 5/5] memcg: filter out reparented memcgs got using memcgid
>
> and found the following issue:
> WARNING: refcount bug in __mem_cgroup_uncharge_swap
>
> Full report is available here:
> https://ci.syzbot.org/series/3a8a2a5f-8b6d-4abf-954e-a1a18ee62748
>
> ***
>
> WARNING: refcount bug in __mem_cgroup_uncharge_swap
>
> tree:      linux-next
> URL:       https://kernel.googlesource.com/pub/scm/linux/kernel/git/next/linux-next
> base:      288058d8db5d35623228d84f48d9bea3707d5c85
> arch:      amd64
> compiler:  Debian clang version 22.1.8 (++20260613092233+e80beda6e255-1~exp1~20260613092250.77), Debian LLD 22.1.8
> config:    https://ci.syzbot.org/builds/35dac6f6-b44a-4ea8-9adf-2cdda5584e45/config
> syz repro: https://ci.syzbot.org/findings/6e117c0b-d6e2-44a6-8e35-161cb33f2aff/syz_repro
>
> ------------[ cut here ]------------
> refcount_t: underflow; use-after-free.
> WARNING: lib/refcount.c:28 at refcount_warn_saturate+0xb2/0x110 lib/refcount.c:28, CPU#0: syz.2.19/5863
> Modules linked in:
> CPU: 0 UID: 0 PID: 5863 Comm: syz.2.19 Not tainted syzkaller #0 PREEMPT(full)
> Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.16.2-debian-1.16.2-1 04/01/2014
> RIP: 0010:refcount_warn_saturate+0xb2/0x110 lib/refcount.c:28
> Code: 64 84 8e 0b 67 48 0f b9 3a eb 4a e8 58 fa f2 fc 48 8d 3d 61 84 8e 0b 67 48 0f b9 3a eb 37 e8 45 fa f2 fc 48 8d 3d 5e 84 8e 0b <67> 48 0f b9 3a eb 24 e8 32 fa f2 fc 48 8d 3d 5b 84 8e 0b 67 48 0f
> RSP: 0018:ffffc90003bbf1d0 EFLAGS: 00010293
> RAX: ffffffff84d3ed9b RBX: 0000000000000003 RCX: ffff8881bdf58000
> RDX: 0000000000000000 RSI: ffffffff8f363380 RDI: ffffffff90627200
> RBP: 1ffffffff20ae968 R08: ffff8881bdf58000 R09: 0000000000000005
> R10: 0000000000000004 R11: 0000000000000000 R12: ffff8881026f8000
> R13: ffff88810a914100 R14: ffff88810a914134 R15: 1ffff110204df009
> FS:  00007fe2ee63d6c0(0000) GS:ffff88818d960000(0000) knlGS:0000000000000000
> CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> CR2: 000056066247f0b8 CR3: 000000000e946000 CR4: 00000000000006f0
> Call Trace:
>  <TASK>
>  mem_cgroup_take_from_private_id mm/memcontrol.c:4049 [inline]
>  __mem_cgroup_uncharge_swap+0x204/0x2a0 mm/memcontrol.c:5707
>  mem_cgroup_uncharge_swap include/linux/swap.h:520 [inline]
>  __swap_cluster_free_entries+0x735/0xc50 mm/swapfile.c:1955
>  swap_put_entries_cluster+0x3b1/0x4b0 mm/swapfile.c:1629
>  swap_put_entries_direct+0x137/0x210 mm/swapfile.c:2139
>  zap_nonpresent_ptes mm/memory.c:1838 [inline]
>  do_zap_pte_range mm/memory.c:1905 [inline]
>  zap_pte_range mm/memory.c:2003 [inline]
>  zap_pmd_range mm/memory.c:2089 [inline]
>  zap_pud_range mm/memory.c:2117 [inline]
>  zap_p4d_range mm/memory.c:2138 [inline]
>  __zap_vma_range+0x1d9d/0x4f10 mm/memory.c:2178
>  unmap_vmas+0x390/0x550 mm/memory.c:2247
>  exit_mmap+0x293/0x9f0 mm/mmap.c:1315
>  __mmput+0x118/0x420 kernel/fork.c:1187
>  exit_mm+0x221/0x2d0 kernel/exit.c:615
>  do_exit+0x6cd/0x2360 kernel/exit.c:997
>  do_group_exit+0x22d/0x2f0 kernel/exit.c:1152
>  get_signal+0x121b/0x12c0 kernel/signal.c:3046
>  arch_do_signal_or_restart+0xbb/0x860 arch/x86/kernel/signal.c:337
>  __exit_to_user_mode_loop kernel/entry/common.c:66 [inline]
>  exit_to_user_mode_loop+0x104/0x730 kernel/entry/common.c:101
>  __exit_to_user_mode_prepare include/linux/irq-entry-common.h:207 [inline]
>  syscall_exit_to_user_mode_prepare include/linux/irq-entry-common.h:230 [inline]
>  syscall_exit_to_user_mode include/linux/entry-common.h:318 [inline]
>  do_syscall_64+0x353/0x580 arch/x86/entry/syscall_64.c:100
>  entry_SYSCALL_64_after_hwframe+0x77/0x7f
> RIP: 0033:0x7fe2ed79e0d9
> Code: Unable to access opcode bytes at 0x7fe2ed79e0af.
> RSP: 002b:00007fe2ee63d0e8 EFLAGS: 00000246 ORIG_RAX: 00000000000000ca
> RAX: fffffffffffffe00 RBX: 00007fe2eda25fa8 RCX: 00007fe2ed79e0d9
> RDX: 0000000000000000 RSI: 0000000000000080 RDI: 00007fe2eda25fa8
> RBP: 00007fe2eda25fa0 R08: 0000000000000000 R09: 0000000000000000
> R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000
> R13: 00007fe2eda26038 R14: 00007fff0bed41b0 R15: 00007fff0bed4298
>  </TASK>
> ----------------
> Code disassembly (best guess):
>    0:   64 84 8e 0b 67 48 0f    test   %cl,%fs:0xf48670b(%rsi)
>    7:   b9 3a eb 4a e8          mov    $0xe84aeb3a,%ecx
>    c:   58                      pop    %rax
>    d:   fa                      cli
>    e:   f2 fc                   repnz cld
>   10:   48 8d 3d 61 84 8e 0b    lea    0xb8e8461(%rip),%rdi        # 0xb8e8478
>   17:   67 48 0f b9 3a          ud1    (%edx),%rdi
>   1c:   eb 37                   jmp    0x55
>   1e:   e8 45 fa f2 fc          call   0xfcf2fa68
>   23:   48 8d 3d 5e 84 8e 0b    lea    0xb8e845e(%rip),%rdi        # 0xb8e8488
> * 2a:   67 48 0f b9 3a          ud1    (%edx),%rdi <-- trapping instruction
>   2f:   eb 24                   jmp    0x55
>   31:   e8 32 fa f2 fc          call   0xfcf2fa68
>   36:   48 8d 3d 5b 84 8e 0b    lea    0xb8e845b(%rip),%rdi        # 0xb8e8498
>   3d:   67                      addr32
>   3e:   48                      rex.W
>   3f:   0f                      .byte 0xf
>
>
> ***
>
> If these findings have caused you to resend the series or submit a
> separate fix, please add the following tag to your commit message:
>   Tested-by: [email protected]
>
> ---
> This report is generated by a bot. It may contain errors.
> syzbot ci engineers can be reached at [email protected].
>
> To test a fix for this bug, please reply with `#syz test`
> (on a separate line) and attach the patch to the email.
>
> Notes:
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>   incremental fix).
> - To test a new version of the whole series, please send it directly
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>
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>
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-- 
Ⲡⲟⲓⲙⲏⲛ Ⲅⲉⲱⲣⲅⲓⲟⲥ - Pimyn Girgis
Software Engineer
Kernel Dynamic Analysis
[email protected]