Re: Softlockup when test shmem swapout-swapin and compaction

[Baolin Wang <baolin.wang@linux.alibaba.com>]

Add Zi.

On 2025/2/26 15:03, Liu Shixin wrote:
> Hi all,
> 
> I found a softlockup when testing shmem large folio swapout-swapin and compaction:
> 
>   watchdog: BUG: soft lockup - CPU#30 stuck for 179s! [folio_swap:4714]
>   Modules linked in: zram xt_MASQUERADE nf_conntrack_netlink nfnetlink iptable_nat xt_addrtype iptable_filter ip_tantel_rapl_msr intel_rapl_common intel_uncore_frequency_common skx_edac_common nfit libnvdimm kvm_intel kvm rapl cixt4 mbcache jbd2 sr_mod cdrom ata_generic ata_piix virtio_net net_failover ghash_clmulni_intel libata sha512_ssse3
>   CPU: 30 UID: 0 PID: 4714 Comm: folio_swap Kdump: loaded Tainted: G             L     6.14.0-rc4-next-20250225+ #2
>   Tainted: [L]=SOFTLOCKUP
>   Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.13.0-1ubuntu1.1 04/01/2014
>   RIP: 0010:xas_load+0x5d/0xc0
>   Code: 08 48 d3 ea 83 e2 3f 89 d0 48 83 c0 04 48 8b 44 c6 08 48 89 73 18 48 89 c1 83 e1 03 48 83 f9 02 75 08 48 3d
>   RSP: 0000:ffffadf142f1ba60 EFLAGS: 00000293
>   RAX: ffffe524cc4f6700 RBX: ffffadf142f1ba90 RCX: 0000000000000000
>   RDX: 0000000000000011 RSI: ffff9a3e058acb68 RDI: ffffadf142f1ba90
>   RBP: fffffffffffffffe R08: ffffadf142f1bb50 R09: 0000000000000392
>   R10: 0000000000000001 R11: 0000000000000000 R12: 0000000000000011
>   R13: ffffadf142f1bb48 R14: ffff9a3e04e9c588 R15: 0000000000000000
>   FS:  00007fd957666740(0000) GS:ffff9a41ac0e5000(0000) knlGS:0000000000000000
>   CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
>   CR2: 00007fd922860000 CR3: 000000025c360001 CR4: 0000000000772ef0
>   DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
>   DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
>   PKRU: 55555554
>   Call Trace:
>    <IRQ>
>    ? watchdog_timer_fn+0x1c9/0x250
>    ? __pfx_watchdog_timer_fn+0x10/0x10
>    ? __hrtimer_run_queues+0x10e/0x250
>    ? hrtimer_interrupt+0xfb/0x240
>    ? __sysvec_apic_timer_interrupt+0x4e/0xe0
>    ? sysvec_apic_timer_interrupt+0x68/0x90
>    </IRQ>
>    <TASK>
>    ? asm_sysvec_apic_timer_interrupt+0x16/0x20
>    ? xas_load+0x5d/0xc0
>    xas_find+0x153/0x1a0
>    find_get_entries+0x73/0x280
>    shmem_undo_range+0x1fc/0x640
>    shmem_evict_inode+0x109/0x270
>    evict+0x107/0x240
>    ? fsnotify_destroy_marks+0x25/0x180
>    ? _atomic_dec_and_lock+0x35/0x50
>    __dentry_kill+0x71/0x190
>    dput+0xd1/0x190
>    __fput+0x128/0x2a0
>    task_work_run+0x57/0x90
>    syscall_exit_to_user_mode+0x1cb/0x1e0
>    do_syscall_64+0x67/0x170
>    entry_SYSCALL_64_after_hwframe+0x76/0x7e
>   RIP: 0033:0x7fd95776eb8b
> 
> If CONFIG_DEBUG_VM is enabled, we will meet VM_BUG_ON_FOLIO(!folio_test_locked(folio)) in
> shmem_add_to_page_cache() too.  It seems that the problem is related to memory migration or
> compaction which is necessary for reproduction,  although without a clear why.
> 
> To reproduce the problem, we need firstly a zram device as swap backend, and then run the
> reproduction program. The reproduction program consists of three parts:
>   1. A process constantly changes the status of shmem large folio by these interfaces:
>          /sys/kernel/mm/transparent_hugepage/hugepages-<size>/shmem_enabled
>   2. A process constantly echo 1 > /proc/sys/vm/compact_memory
>   3. A process constantly alloc/free/swapout/swapin shmem large folios.
> 
> I'm not sure whether the first process is necessary but the second and third are. In addition,
> I tried hacking to modify compaction_alloc to return NULL, and the problem disappeared,
> so I guess the problem is in migration.
> 
> The problem is different with https://lore.kernel.org/all/1738717785.im3r5g2vxc.none@localhost/
> since I have confirmed this porblem still existed after merge the fixed patch.

Could you check if your version includes Zi's fix[1]? Not sure if it's 
related to the shmem large folio split.

[1] 
https://lore.kernel.org/all/AF487A7A-F685-485D-8D74-756C843D6F0A@nvidia.com/