Re: [PATCH 0/5] SLUB debugfs improvements based on stackdepot

From: Vlastimil Babka
Date: Fri Mar 04 2022 - 12:26:49 EST


On 2/26/22 13:18, Hyeonggon Yoo wrote:
> On Fri, Feb 25, 2022 at 07:03:13PM +0100, Vlastimil Babka wrote:
>> Hi,
>>
>> this series combines and revives patches from Oliver's last year
>> bachelor thesis (where I was the advisor) that make SLUB's debugfs
>> files alloc_traces and free_traces more useful.
>> The resubmission was blocked on stackdepot changes that are now merged,
>> as explained in patch 2.
>>
>> Patch 1 is a new preparatory cleanup.
>>
>> Patch 2 originally submitted here [1], was merged to mainline but
>> reverted for stackdepot related issues as explained in the patch.
>>
>> Patches 3-5 originally submitted as RFC here [2]. In this submission I
>> have omitted the new file 'all_objects' (patch 3/3 in [2]) as it might
>> be considered too intrusive so I will postpone it for later. The docs
>> patch is adjusted accordingly.
>>
>
> This problem is not caused by this patch series.
> But I think it's worth mentioning...
>
> It's really weird that some stack traces are not recorded
> when CONFIG_KASAN=y.
>
> I made sure that:
> - Stack Depot did not reach its limit
> - the free path happen on CONFIG_KASAN=y too.
>
> I have no clue why this happen.
>
> # cat dentry/free_traces (CONFIG_KASAN=y)
> 6585 <not-available> age=4294912647 pid=0 cpus=0

I think it's some kind of KASAN quarantining of freed objects, so they
haven't been properly freed through the SLUB layer yet.

> # cat dentry/free_traces (CONFIG_KASAN=n)
> 1246 <not-available> age=4294906877 pid=0 cpus=0
> 379 __d_free+0x20/0x2c age=33/14225/14353 pid=0-122 cpus=0-3
> kmem_cache_free+0x1f4/0x21c
> __d_free+0x20/0x2c
> rcu_core+0x334/0x580
> rcu_core_si+0x14/0x20
> __do_softirq+0x12c/0x2a8
>
> 2 dentry_free+0x58/0xb0 age=14101/14101/14101 pid=158 cpus=0
> kmem_cache_free+0x1f4/0x21c
> dentry_free+0x58/0xb0
> __dentry_kill+0x18c/0x1d0
> dput+0x1c4/0x2fc
> __fput+0xb0/0x230
> ____fput+0x14/0x20
> task_work_run+0x84/0x17c
> do_notify_resume+0x208/0x1330
> el0_svc+0x6c/0x80
> el0t_64_sync_handler+0xa8/0x130
> el0t_64_sync+0x1a0/0x1a4
>
> 1 dentry_free+0x58/0xb0 age=7678 pid=190 cpus=1
> kmem_cache_free+0x1f4/0x21c
> dentry_free+0x58/0xb0
> __dentry_kill+0x18c/0x1d0
> dput+0x1c4/0x2fc
> __fput+0xb0/0x230
> ____fput+0x14/0x20
> task_work_run+0x84/0x17c
> do_exit+0x2dc/0x8e0
> do_group_exit+0x38/0xa4
> __wake_up_parent+0x0/0x34
> invoke_syscall+0x48/0x114
> el0_svc_common.constprop.0+0x44/0xfc
> do_el0_svc+0x2c/0x94
> el0_svc+0x28/0x80
> el0t_64_sync_handler+0xa8/0x130
> el0t_64_sync+0x1a0/0x1a4