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

Hi Catalin,

-----Original Message-----
From: Catalin Marinas <catalin.marinas@arm.com>
Sent: Friday, December 10, 2021 7:22 PM
To: Jianyong Wu <redacted>
Cc: will@kernel.org; Anshuman Khandual <redacted>;
akpm@linux-foundation.org; ardb@kernel.org; linux-
kernel@vger.kernel.org; linux-arm-kernel@lists.infradead.org;
david@redhat.com; gshan@redhat.com; Justin He [off-list ref];
nd [off-list ref]; Mark Rutland [off-list ref]
Subject: Re: [PATCH v2] arm64/mm: avoid fixmap race condition when create
pud mapping

On Fri, Dec 10, 2021 at 05:54:32PM +0800, Jianyong Wu wrote:

quoted

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 <redacted>

It looks fine to me:

Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>

Do we need a cc stable? Fixmap was introduced in 4.6, so usually:

Fixes: f4710445458c ("arm64: mm: use fixmap when creating page tables")
Cc: <redacted> # 4.6.x

but I haven't checked when memory hotplug was added to be able to trigger
the race. It may not need to go back that far.

For now, I only find the memory hotplug can conflict with it. I see that memory hotplug is enabled on arm64 in 4.20, thus what about changing "4.6" to "4.20".
 
Thanks
Jianyong

--
Catalin

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