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

[Jianyong Wu]

Thanks Qian to report this.
I will change to mutex lock and resend it later.

Thanks
Jianyong

> -----Original Message-----
> From: David Hildenbrand <david@xxxxxxxxxx>
> Sent: Thursday, December 16, 2021 12:04 AM
> To: Catalin Marinas <Catalin.Marinas@xxxxxxx>; Qian Cai
> <quic_qiancai@xxxxxxxxxxx>
> Cc: Jianyong Wu <Jianyong.Wu@xxxxxxx>; will@xxxxxxxxxx; Anshuman
> Khandual <Anshuman.Khandual@xxxxxxx>; akpm@xxxxxxxxxxxxxxxxxxxx;
> ardb@xxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx; linux-arm-
> kernel@xxxxxxxxxxxxxxxxxxx; gshan@xxxxxxxxxx; Justin He
> <Justin.He@xxxxxxx>; nd <nd@xxxxxxx>
> Subject: Re: [PATCH v2] arm64/mm: avoid fixmap race condition when create
> pud mapping
> 
> 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@xxxxxxx>
> >>
> >> 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