linux-kernel - Re: [mm/slub] ae107fa919: BUG:unable_to_handle_page_fault_for

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Date:   Sat, 12 Mar 2022 10:21:25 +0100
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Hyeonggon Yoo <42.hyeyoo@...il.com>
Cc:     kernel test robot <oliver.sang@...el.com>,
        Oliver Glitta <glittao@...il.com>, lkp@...ts.01.org,
        lkp@...el.com, LKML <linux-kernel@...r.kernel.org>,
        linux-mm@...ck.org, Mike Rapoport <rppt@...nel.org>
Subject: Re: [mm/slub] ae107fa919: BUG:unable_to_handle_page_fault_for_address

On 3/12/22 02:10, Hyeonggon Yoo wrote:
> On Fri, Mar 11, 2022 at 04:46:00PM +0000, Hyeonggon Yoo wrote:
>> On Fri, Mar 11, 2022 at 04:36:47PM +0100, Vlastimil Babka wrote:
>>> On 3/11/22 15:54, Hyeonggon Yoo wrote:
>>>> On Wed, Mar 09, 2022 at 10:15:31AM +0800, kernel test robot wrote:
>>>>>
>>>>>
>>>>> Greeting,
>>>>>
>>>>> FYI, we noticed the following commit (built with gcc-9):
>>>>>
>>>>> commit: ae107fa91914f098cd54ab77e68f83dd6259e901 ("mm/slub: use stackdepot to save stack trace in objects")
>>>>> https://git.kernel.org/cgit/linux/kernel/git/vbabka/linux.git slub-stackdepot-v3r0
>>>>>
>>>>> in testcase: boot
>>>>>
>>>>> on test machine: qemu-system-x86_64 -enable-kvm -cpu SandyBridge -smp 2 -m 16G
>>>>>
>>>>> caused below changes (please refer to attached dmesg/kmsg for entire log/backtrace):
>>>>>
>>>>
>>>> [+Cc Vlastimil and linux-mm]
>>>
>>> Thanks.
>>> lkp folks: it would be nice if I was CC'd automatically on this, it's a
>>> commit from my git tree and with by s-o-b :)
>>>
>>>> I _strongly_ suspect that this is because we don't initialize
>>>> stack_table[i] = NULL when we allocate it from memblock_alloc().
>>>
>>> No, Mike (CC'd) suggested to drop the array init cycle, because
>>> memblock_alloc would zero the area anyway.
>>
>> Ah, you are right. My mistake.
>>
>>> There has to be a different
>>> reason. Wondering if dmesg contains the stack depot initialization message
>>> at all...
>>
>> I think I found the reason.
>> This is because of CONFIG_SLUB_DEBUG_ON.
>> It can enable debugging without passing boot parameter.
>>
>> if CONFIG_SLUB_DEBUG_ON=y && slub_debug is not passed, we do not call
>> stack_depot_want_early_init(), but the debugging flags are set.
>>
>> And we only call stack_depot_init() later in kmem_cache_create_usercopy().
>>
>> so it crashed while creating boot cache.
> 
> I tested this, and this was the reason.
> It crashed on CONFIG_SLUB_DEBUG_ON=y because stackdepot always assume
> that it was initialized in boot step, or failed
> (stack_depot_disable=true).
> 
> But as it didn't even tried to initialize it, stack_table == NULL &&
> stack_depot_disable == false. So accessing *(NULL + <hash value>)

Thanks for finding the cause!

> Ideas? implementing something like kmem_cache_init_early() again?

I think we could simply make CONFIG_SLUB_DEBUG_ON select/depend on
STACKDEPOT_ALWAYS_INIT?