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

[Jianyong Wu]

> -----Original Message-----
> From: Anshuman Khandual <anshuman.khandual@xxxxxxx>
> Sent: Monday, December 13, 2021 2:56 PM
> To: Jianyong Wu <Jianyong.Wu@xxxxxxx>; Catalin Marinas
> <Catalin.Marinas@xxxxxxx>; will@xxxxxxxxxx; akpm@xxxxxxxxxxxxxxxxxxxx
> Cc: ardb@xxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx; linux-arm-
> kernel@xxxxxxxxxxxxxxxxxxx; david@xxxxxxxxxx; 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 12/10/21 3:24 PM, 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>
> > ---
> >  arch/arm64/mm/mmu.c | 7 +++++++
> >  1 file changed, 7 insertions(+)
> >
> > diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c index
> > acfae9b41cc8..98ac09ae9588 100644
> > --- a/arch/arm64/mm/mmu.c
> > +++ b/arch/arm64/mm/mmu.c
> > @@ -63,6 +63,7 @@ static pmd_t bm_pmd[PTRS_PER_PMD]
> __page_aligned_bss
> > __maybe_unused;  static pud_t bm_pud[PTRS_PER_PUD]
> __page_aligned_bss
> > __maybe_unused;
> >
> >  static DEFINE_SPINLOCK(swapper_pgdir_lock);
> > +static DEFINE_SPINLOCK(fixmap_lock);
> >
> >  void set_swapper_pgd(pgd_t *pgdp, pgd_t pgd)  { @@ -329,6 +330,11 @@
> > static void alloc_init_pud(pgd_t *pgdp, unsigned long addr, unsigned long
> end,
> >  	}
> >  	BUG_ON(p4d_bad(p4d));
> >
> > +	/*
> > +	 * fixmap is global resource, thus it needs to be protected by a lock
> > +	 * in case of race condition.
> > +	 */
> > +	spin_lock(&fixmap_lock);
> >  	pudp = pud_set_fixmap_offset(p4dp, addr);
> >  	do {
> >  		pud_t old_pud = READ_ONCE(*pudp);
> > @@ -359,6 +365,7 @@ static void alloc_init_pud(pgd_t *pgdp, unsigned
> long addr, unsigned long end,
> >  	} while (pudp++, addr = next, addr != end);
> >
> >  	pud_clear_fixmap();
> > +	spin_unlock(&fixmap_lock);
> >  }
> >
> >  static void __create_pgd_mapping(pgd_t *pgdir, phys_addr_t phys,
> >
> 
> As the race could only happen with memory hotplug being enabled, could
> not we wrap this around with CONFIG_MEMORY_HOTPLUG, just to narrow
> its scope possibly speed up other non-hotplug cases ?

I think it's better.

Thanks
Jianyong