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

From: Catalin Marinas <[email-masked]>
Date: Wed, 15 Dec 2021 16:02:55 +0000

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 <[email-masked]>
> 
> 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.

-- 
Catalin

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