Re: [syzbot ci] Re: Virtual Swap Space

[Nhat Pham <nphamcs@gmail.com>]

On Thu, Mar 19, 2026 at 2:36 PM syzbot ci
<syzbot+ci0215525ee2c0ed89@syzkaller.appspotmail.com> wrote:
>
> syzbot ci has tested the following series
>
> [v4] Virtual Swap Space
> https://lore.kernel.org/all/20260318222953.441758-1-nphamcs@gmail.com
> * [PATCH v4 01/21] mm/swap: decouple swap cache from physical swap infrastructure
> * [PATCH v4 02/21] swap: rearrange the swap header file
> * [PATCH v4 03/21] mm: swap: add an abstract API for locking out swapoff
> * [PATCH v4 04/21] zswap: add new helpers for zswap entry operations
> * [PATCH v4 05/21] mm/swap: add a new function to check if a swap entry is in swap cached.
> * [PATCH v4 06/21] mm: swap: add a separate type for physical swap slots
> * [PATCH v4 07/21] mm: create scaffolds for the new virtual swap implementation
> * [PATCH v4 08/21] zswap: prepare zswap for swap virtualization
> * [PATCH v4 09/21] mm: swap: allocate a virtual swap slot for each swapped out page
> * [PATCH v4 10/21] swap: move swap cache to virtual swap descriptor
> * [PATCH v4 11/21] zswap: move zswap entry management to the virtual swap descriptor
> * [PATCH v4 12/21] swap: implement the swap_cgroup API using virtual swap
> * [PATCH v4 13/21] swap: manage swap entry lifecycle at the virtual swap layer
> * [PATCH v4 14/21] mm: swap: decouple virtual swap slot from backing store
> * [PATCH v4 15/21] zswap: do not start zswap shrinker if there is no physical swap slots
> * [PATCH v4 16/21] swap: do not unnecesarily pin readahead swap entries
> * [PATCH v4 17/21] swapfile: remove zeromap bitmap
> * [PATCH v4 18/21] memcg: swap: only charge physical swap slots
> * [PATCH v4 19/21] swap: simplify swapoff using virtual swap
> * [PATCH v4 20/21] swapfile: replace the swap map with bitmaps
> * [PATCH v4 21/21] vswap: batch contiguous vswap free calls
>
> and found the following issue:
> possible deadlock in vswap_iter
>
> Full report is available here:
> https://ci.syzbot.org/series/f8238a2a-370e-404d-b3f7-5945b574bd63
>
> ***
>
> possible deadlock in vswap_iter
>
> tree:      bpf-next
> URL:       https://kernel.googlesource.com/pub/scm/linux/kernel/git/bpf/bpf-next.git
> base:      05f7e89ab9731565d8a62e3b5d1ec206485eeb0b
> arch:      amd64
> compiler:  Debian clang version 21.1.8 (++20251221033036+2078da43e25a-1~exp1~20251221153213.50), Debian LLD 21.1.8
> config:    https://ci.syzbot.org/builds/cf1517a6-d391-46d8-bfbe-98e6be6b93ce/config
> syz repro: https://ci.syzbot.org/findings/b4e84ae7-17d4-4bf8-9c3f-4c13b10a1e52/syz_repro
>
> ============================================
> WARNING: possible recursive locking detected
> syzkaller #0 Not tainted
> --------------------------------------------
> syz.1.18/6001 is trying to acquire lock:
> ffff88811fba0018 (&cluster->lock){+.+.}-{3:3}, at: spin_lock include/linux/spinlock.h:351 [inline]
> ffff88811fba0018 (&cluster->lock){+.+.}-{3:3}, at: vswap_iter+0xfa/0x1b0 mm/vswap.c:274
>
> but task is already holding lock:
> ffff88811fba0018 (&cluster->lock){+.+.}-{3:3}, at: spin_lock_irq include/linux/spinlock.h:376 [inline]
> ffff88811fba0018 (&cluster->lock){+.+.}-{3:3}, at: swap_cache_lock_irq+0xe2/0x190 mm/vswap.c:1529
>
> other info that might help us debug this:
>  Possible unsafe locking scenario:
>
>        CPU0
>        ----
>   lock(&cluster->lock);
>   lock(&cluster->lock);
>
>  *** DEADLOCK ***
>
>  May be due to missing lock nesting notation
>
> 3 locks held by syz.1.18/6001:
>  #0: ffff8881bb523440 (&mm->mmap_lock){++++}-{4:4}, at: mmap_read_lock include/linux/mmap_lock.h:391 [inline]
>  #0: ffff8881bb523440 (&mm->mmap_lock){++++}-{4:4}, at: madvise_lock+0x152/0x2e0 mm/madvise.c:1789
>  #1: ffff88811fba0018 (&cluster->lock){+.+.}-{3:3}, at: spin_lock_irq include/linux/spinlock.h:376 [inline]
>  #1: ffff88811fba0018 (&cluster->lock){+.+.}-{3:3}, at: swap_cache_lock_irq+0xe2/0x190 mm/vswap.c:1529
>  #2: ffffffff8e55a360 (rcu_read_lock){....}-{1:3}, at: rcu_lock_acquire include/linux/rcupdate.h:331 [inline]
>  #2: ffffffff8e55a360 (rcu_read_lock){....}-{1:3}, at: rcu_read_lock include/linux/rcupdate.h:867 [inline]
>  #2: ffffffff8e55a360 (rcu_read_lock){....}-{1:3}, at: vswap_cgroup_record+0x41/0x440 mm/vswap.c:1909
>
> stack backtrace:
> CPU: 0 UID: 0 PID: 6001 Comm: syz.1.18 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
> Call Trace:
>  <TASK>
>  dump_stack_lvl+0xe8/0x150 lib/dump_stack.c:120
>  print_deadlock_bug+0x279/0x290 kernel/locking/lockdep.c:3041
>  check_deadlock kernel/locking/lockdep.c:3093 [inline]
>  validate_chain kernel/locking/lockdep.c:3895 [inline]
>  __lock_acquire+0x253f/0x2cf0 kernel/locking/lockdep.c:5237
>  lock_acquire+0x106/0x330 kernel/locking/lockdep.c:5868
>  __raw_spin_lock include/linux/spinlock_api_smp.h:133 [inline]
>  _raw_spin_lock+0x2e/0x40 kernel/locking/spinlock.c:154
>  spin_lock include/linux/spinlock.h:351 [inline]
>  vswap_iter+0xfa/0x1b0 mm/vswap.c:274
>  vswap_cgroup_record+0xeb/0x440 mm/vswap.c:1910
>  swap_cgroup_record+0xc5/0x130 mm/vswap.c:1933
>  memcg1_swapout+0x358/0x9e0 mm/memcontrol-v1.c:623

Good (syz)bot! We're already holding the cluster lock here - shouldn't
need to reacquire the lock.

Should be an easy-ish fix.