Re: [PATCH v2] arm64/mm: avoid fixmap race condition when create pud mapping

David Hildenbrand
2021-12-15T16:04:31Z

On 15.12.21 17:02, Catalin Marinas wrote:
> On Wed, Dec 15, 2021 at 09:13:37AM -0500, Qian Cai wrote:
>> On Fri, Dec 10, 2021 at 05:54:32PM +0800, Jianyong Wu wrote:
>>> fixmap is a global resource and is used recursively in create pud mapping.
>>> It may lead to race condition when alloc_init_pud is called concurrently.
>>>
>>> Fox example:
>>> alloc_init_pud is called when kernel_init. If memory hotplug
>>> thread, which will also call alloc_init_pud, happens during
>>> kernel_init, the race for fixmap occurs.
>>>
>>> The race condition flow can be:
>>>
>>> *************** begin **************
>>>
>>> kerenl_init thread virtio-mem workqueue thread
>>> ================== ======== ==================
>>> alloc_init_pud(...)
>>> pudp = pud_set_fixmap_offset(..) alloc_init_pud(...)
>>> ... ...
>>> READ_ONCE(*pudp) //OK! pudp = pud_set_fixmap_offset(
>>> ... ...
>>> pud_clear_fixmap() //fixmap break
>>> READ_ONCE(*pudp) //CRASH!
>>>
>>> **************** end ***************
>>>
>>> Hence, a spin lock is introduced to protect the fixmap during create pdg
>>> mapping.
>>>
>>> Signed-off-by: Jianyong Wu <jianyong.wu@arm.com>
>>
>> I am afraid there is a problem to take a spinlock there.
>>
>> node 0 deferred pages initialised in 2740ms
>> pgdatinit0 (176) used greatest stack depth: 59184 bytes left
>> devtmpfs: initialized
>> KASLR disabled due to lack of seed
>> BUG: sleeping function called from invalid context at mm/page_alloc.c:5151
>
> Thanks for the report. Definitely a bug, GFP_KERNEL_ACCOUNT allows for
> the page allocator to sleep. I'll drop the patch from the arm64
> for-next/fixes branch for now and sort it out later.
>

Sounds like we need a mutex then.

--
Thanks,

David / dhildenb