Message-ID: <ccbb3a1f-856c-5879-acbc-fbaff9eb61de@redhat.com>
Date:   Wed, 15 Dec 2021 17:04:18 +0100
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Subject: Re: [PATCH v2] arm64/mm: avoid fixmap race condition when create pud
 mapping
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To:     Catalin Marinas <catalin.marinas@arm.com>,
        Qian Cai <quic_qiancai@quicinc.com>
Cc:     Jianyong Wu <jianyong.wu@arm.com>, will@kernel.org,
        anshuman.khandual@arm.com, akpm@linux-foundation.org,
        ardb@kernel.org, linux-kernel@vger.kernel.org,
        linux-arm-kernel@lists.infradead.org, gshan@redhat.com,
        justin.he@arm.com, nd@arm.com
References: <20211210095432.51798-1-jianyong.wu@arm.com>
 <Ybn4EfweLqKtyW0+@fixkernel.com> <YboRr0BCgjAbYdHe@arm.com>
From:   David Hildenbrand <david@redhat.com>
Organization: Red Hat
In-Reply-To: <YboRr0BCgjAbYdHe@arm.com>
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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