Re: [PATCH v4 0/4] reparent the THP split queue

Zi Yan
2025-10-13T16:37:38Z

On 13 Oct 2025, at 3:23, Qi Zheng wrote:

> Hi Zi,
>
> On 10/11/25 12:25 AM, Zi Yan wrote:
>> On 3 Oct 2025, at 12:53, Qi Zheng wrote:
>
> [snip]
>
>>>
>>
>> Hi Qi,
>>
>> I got CPU soft locks when run "echo 3 | sudo tee /proc/sys/vm/drop_caches"
>> with today's mm-new on a freshly booted system. Reverting Patch 3 (and Patch 4)
>> of your patchset solves the issue.
>>
>> My config file is attached. My kernel relevant kernel parameters are:
>> "cgroup_no_v1=all transparent_hugepage=always thp_shmem=2M:always".
>> The machine is a 8GB 8-core x86_64 VM.
>>
>> The kernel log:
>>
>> [ 36.441539] watchdog: BUG: soft lockup - CPU#0 stuck for 26s! [tee:810]
>> [ 36.441549] Modules linked in:
>> [ 36.441566] CPU: 0 UID: 0 PID: 810 Comm: tee Not tainted 6.17.0-mm-everything-2024-01-29-07-19-no-mglru+ #526 PREEMPT(voluntary)
>> [ 36.441570] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.17.0-debian-1.17.0-1 04/01/2014
>> [ 36.441574] RIP: 0010:_raw_spin_unlock_irqrestore+0x19/0x40
>
> It seems that the softlockup is not caused by lock contention. I have
> also seen the following stack locally:
>
> [14819.914867][ C2] watchdog: BUG: soft lockup - CPU#2 stuck for 49s! [kswapd0:202]
> [14819.914873][ C2] Modules linked in:
> [14819.914877][ C2] CPU: 2 UID: 0 PID: 202 Comm: kswapd0 Tainted: G L 6.1
> [14819.914880][ C2] Tainted: [L]=SOFTLOCKUP
> [14819.914881][ C2] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.12.0-1 04
> [14819.914883][ C2] RIP: 0010:folios_put_refs+0x34/0x210
> [14819.914889][ C2] Code: 57 41 56 49 89 fe 41 55 41 54 55 53 48 83 ec 18 65 48 8b 05 36 51
> [14819.914891][ C2] RSP: 0000:ffffc900039db8e8 EFLAGS: 00000246
> [14819.914893][ C2] RAX: 0000000000000000 RBX: ffff888109e64330 RCX: ffffffff82848bb8
> [14819.914895][ C2] RDX: 0000000000000001 RSI: 0000000000000000 RDI: ffffc900039db960
> [14819.914896][ C2] RBP: ffff888109e642a0 R08: ffff888100403058 R09: ffff888100402fe8
> [14819.914897][ C2] R10: 0000000000000000 R11: ffffffff82711328 R12: ffff888109e64328
> [14819.914898][ C2] R13: ffffc900039dbb30 R14: ffffc900039db960 R15: ffffc900039dbb30
> [14819.914916][ C2] FS: 0000000000000000(0000) GS:ffff888332ad7000(0000) knlGS:00000000000
> [14819.914918][ C2] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> [14819.914919][ C2] CR2: 00007f8d211b3000 CR3: 000000010833c000 CR4: 00000000000006f0
> [14819.914921][ C2] Call Trace:
> [14819.914923][ C2] <TASK>
> [14819.914927][ C2] deferred_split_scan+0x30d/0x420
> [14819.914934][ C2] do_shrink_slab+0x105/0x350
> [14819.914937][ C2] shrink_slab+0x2fa/0x3f0
> [14819.914939][ C2] ? try_to_shrink_lruvec+0x1a6/0x270
> [14819.914944][ C2] shrink_one+0x11d/0x1b0
> [14819.914947][ C2] shrink_node+0x9a4/0xb90
> [14819.914952][ C2] balance_pgdat+0x548/0x910
> [14819.914955][ C2] ? __sched_fork+0x160/0x180
> [14819.914964][ C2] ? __timer_delete_sync+0x2b/0x40
> [14819.914970][ C2] kswapd+0x1b9/0x320
> [14819.914973][ C2] ? __pfx_autoremove_wake_function+0x10/0x10
> [14819.914978][ C2] ? __pfx_kswapd+0x10/0x10
> [14819.914981][ C2] kthread+0xfd/0x250
> [14819.914984][ C2] ? finish_task_switch+0x90/0x270
> [14819.914987][ C2] ? __pfx_kthread+0x10/0x10
> [14819.914989][ C2] ? __pfx_kthread+0x10/0x10
> [14819.914991][ C2] ret_from_fork+0x170/0x190
> [14819.914995][ C2] ? __pfx_kthread+0x10/0x10
> [14819.914996][ C2] ret_from_fork_asm+0x1a/0x30
> [14819.915002][ C2] </TASK>
>
> It seems that the reason is that deferred_split_scan() has been executed
> for too long.
>
> Could you please help test the following changes? I applied them locally
> and no softlockup occurred.
>
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index b5eea2091cdf6..5353c7bd2c9af 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -4286,8 +4286,10 @@ static unsigned long deferred_split_scan(struct shrinker *shrink,
> }
> folios_put(&fbatch);
>
> - if (sc->nr_to_scan)
> + if (sc->nr_to_scan) {
> + cond_resched();
> goto retry;
> + }
>
> /*
> * Stop shrinker if we didn't split any page, but the queue is empty.
>

It does not fix the issue, but only gets rid of the soft lockup warning.
"echo 3 | sudo tee /proc/sys/vm/drop_caches" just runs forever.

Looking at the original code, sc->nr_to_scan was one of the two conditions
on breaking out of split_queue scanning and was never checked again
afterwards. When split_queue size is smaller than nr_to_scan, your code
will retry forever but not the original one. After I added pr_info() to
print sc->nr_to_scan at
1) before retry:,
2) before for (... folio_batch_count();...),
3) before "if (sc->nr_to_scan)",

I see that 1) printed 2, 2) and 3) kept printing 1. It matches my
above guess.

The below patch fixes the issue:

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 43a3c499aec0..d38816a0c117 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -4415,7 +4415,7 @@ static unsigned long deferred_split_scan(struct shrinker *shrink,
}
folios_put(&fbatch);

- if (sc->nr_to_scan)
+ if (sc->nr_to_scan && !list_empty(&ds_queue->split_queue))
goto retry;

/*

>
>> [ 36.441592] Code: 00 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 0f 1f 44 00 00 53 48 89 f3 e8 92 68 fd fe 80 e7 02 74 06 fb 0f 1f 44 00 00 <65> ff 0d d0 5f 7e 01 74 06 5b c3 cc cc cc cc 0f 1f 44 00 00 5b c3
>> [ 36.441594] RSP: 0018:ffffc900029afb60 EFLAGS: 00000202
>> [ 36.441598] RAX: 0000000000000001 RBX: 0000000000000286 RCX: ffff888101168670
>> [ 36.441601] RDX: 0000000000000001 RSI: 0000000000000286 RDI: ffff888101168658
>> [ 36.441602] RBP: 0000000000000001 R08: ffff88813ba44ec0 R09: 0000000000000000
>> [ 36.441603] R10: 00000000000001a8 R11: 0000000000000000 R12: ffff8881011685e0
>> [ 36.441604] R13: 0000000000000000 R14: ffff888101168000 R15: ffffc900029afd60
>> [ 36.441606] FS: 00007f7fe3655740(0000) GS:ffff8881b7e5d000(0000) knlGS:0000000000000000
>> [ 36.441607] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
>> [ 36.441608] CR2: 0000563d4d439bf0 CR3: 000000010873c006 CR4: 0000000000370ef0
>> [ 36.441614] Call Trace:
>> [ 36.441616] <TASK>
>> [ 36.441619] deferred_split_scan+0x1e0/0x480
>> [ 36.441627] ? _raw_spin_unlock_irqrestore+0xe/0x40
>> [ 36.441630] ? kvfree_rcu_queue_batch+0x96/0x1c0
>> [ 36.441634] ? do_raw_spin_unlock+0x46/0xd0
>> [ 36.441639] ? kfree_rcu_monitor+0x1da/0x2c0
>> [ 36.441641] ? list_lru_count_one+0x47/0x90
>> [ 36.441644] do_shrink_slab+0x153/0x360
>> [ 36.441649] shrink_slab+0xd3/0x390
>> [ 36.441652] drop_slab+0x7d/0x130
>> [ 36.441655] drop_caches_sysctl_handler+0x98/0xb0
>> [ 36.441660] proc_sys_call_handler+0x1c7/0x2c0
>> [ 36.441664] vfs_write+0x221/0x450
>> [ 36.441669] ksys_write+0x6c/0xe0
>> [ 36.441672] do_syscall_64+0x50/0x200
>> [ 36.441675] entry_SYSCALL_64_after_hwframe+0x76/0x7e
>> [ 36.441678] RIP: 0033:0x7f7fe36e7687
>> [ 36.441685] Code: 48 89 fa 4c 89 df e8 58 b3 00 00 8b 93 08 03 00 00 59 5e 48 83 f8 fc 74 1a 5b c3 0f 1f 84 00 00 00 00 00 48 8b 44 24 10 0f 05 <5b> c3 0f 1f 80 00 00 00 00 83 e2 39 83 fa 08 75 de e8 23 ff ff ff
>> [ 36.441686] RSP: 002b:00007ffdffcbba10 EFLAGS: 00000202 ORIG_RAX: 0000000000000001
>> [ 36.441688] RAX: ffffffffffffffda RBX: 00007f7fe3655740 RCX: 00007f7fe36e7687
>> [ 36.441689] RDX: 0000000000000002 RSI: 00007ffdffcbbbb0 RDI: 0000000000000003
>> [ 36.441690] RBP: 00007ffdffcbbbb0 R08: 0000000000000000 R09: 0000000000000000
>> [ 36.441691] R10: 0000000000000000 R11: 0000000000000202 R12: 0000000000000002
>> [ 36.441692] R13: 0000558d40be64c0 R14: 00007f7fe383de80 R15: 0000000000000002
>> [ 36.441694] </TASK>
>> [ 64.441531] watchdog: BUG: soft lockup - CPU#0 stuck for 53s! [tee:810]
>> [ 64.441537] Modules linked in:
>> [ 64.441545] CPU: 0 UID: 0 PID: 810 Comm: tee Tainted: G L 6.17.0-mm-everything-2024-01-29-07-19-no-mglru+ #526 PREEMPT(voluntary)
>> [ 64.441548] Tainted: [L]=SOFTLOCKUP
>> [ 64.441552] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.17.0-debian-1.17.0-1 04/01/2014
>> [ 64.441555] RIP: 0010:_raw_spin_unlock_irqrestore+0x19/0x40
>> [ 64.441565] Code: 00 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 0f 1f 44 00 00 53 48 89 f3 e8 92 68 fd fe 80 e7 02 74 06 fb 0f 1f 44 00 00 <65> ff 0d d0 5f 7e 01 74 06 5b c3 cc cc cc cc 0f 1f 44 00 00 5b c3
>> [ 64.441566] RSP: 0018:ffffc900029afb60 EFLAGS: 00000202
>> [ 64.441568] RAX: 0000000000000001 RBX: 0000000000000286 RCX: ffff888101168670
>> [ 64.441570] RDX: 0000000000000001 RSI: 0000000000000286 RDI: ffff888101168658
>> [ 64.441571] RBP: 0000000000000001 R08: ffff88813ba44ec0 R09: 0000000000000000
>> [ 64.441572] R10: 00000000000001a8 R11: 0000000000000000 R12: ffff8881011685e0
>> [ 64.441573] R13: 0000000000000000 R14: ffff888101168000 R15: ffffc900029afd60
>> [ 64.441574] FS: 00007f7fe3655740(0000) GS:ffff8881b7e5d000(0000) knlGS:0000000000000000
>> [ 64.441576] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
>> [ 64.441577] CR2: 0000563d4d439bf0 CR3: 000000010873c006 CR4: 0000000000370ef0
>> [ 64.441581] Call Trace:
>> [ 64.441583] <TASK>
>> [ 64.441591] deferred_split_scan+0x1e0/0x480
>> [ 64.441598] ? _raw_spin_unlock_irqrestore+0xe/0x40
>> [ 64.441599] ? kvfree_rcu_queue_batch+0x96/0x1c0
>> [ 64.441603] ? do_raw_spin_unlock+0x46/0xd0
>> [ 64.441607] ? kfree_rcu_monitor+0x1da/0x2c0
>> [ 64.441610] ? list_lru_count_one+0x47/0x90
>> [ 64.441613] do_shrink_slab+0x153/0x360
>> [ 64.441618] shrink_slab+0xd3/0x390
>> [ 64.441621] drop_slab+0x7d/0x130
>> [ 64.441624] drop_caches_sysctl_handler+0x98/0xb0
>> [ 64.441629] proc_sys_call_handler+0x1c7/0x2c0
>> [ 64.441632] vfs_write+0x221/0x450
>> [ 64.441638] ksys_write+0x6c/0xe0
>> [ 64.441641] do_syscall_64+0x50/0x200
>> [ 64.441645] entry_SYSCALL_64_after_hwframe+0x76/0x7e
>> [ 64.441648] RIP: 0033:0x7f7fe36e7687
>> [ 64.441654] Code: 48 89 fa 4c 89 df e8 58 b3 00 00 8b 93 08 03 00 00 59 5e 48 83 f8 fc 74 1a 5b c3 0f 1f 84 00 00 00 00 00 48 8b 44 24 10 0f 05 <5b> c3 0f 1f 80 00 00 00 00 83 e2 39 83 fa 08 75 de e8 23 ff ff ff
>> [ 64.441656] RSP: 002b:00007ffdffcbba10 EFLAGS: 00000202 ORIG_RAX: 0000000000000001
>> [ 64.441658] RAX: ffffffffffffffda RBX: 00007f7fe3655740 RCX: 00007f7fe36e7687
>> [ 64.441659] RDX: 0000000000000002 RSI: 00007ffdffcbbbb0 RDI: 0000000000000003
>> [ 64.441660] RBP: 00007ffdffcbbbb0 R08: 0000000000000000 R09: 0000000000000000
>> [ 64.441661] R10: 0000000000000000 R11: 0000000000000202 R12: 0000000000000002
>> [ 64.441662] R13: 0000558d40be64c0 R14: 00007f7fe383de80 R15: 0000000000000002
>> [ 64.441663] </TASK>
>>
>>
>>
>> --
>> Best Regards,
>> Yan, Zi

--
Best Regards,
Yan, Zi