Re: [PATCH v4 12/12] mm: bail out of lazy_mmu_mode_* in interrupt context

[Kevin Brodsky <kevin.brodsky@xxxxxxx>]

On 07/11/2025 15:42, Ryan Roberts wrote:
> On 29/10/2025 10:09, Kevin Brodsky wrote:
>> The lazy MMU mode cannot be used in interrupt context. This is
>> documented in <linux/pgtable.h>, but isn't consistently handled
>> across architectures.
>>
>> arm64 ensures that calls to lazy_mmu_mode_* have no effect in
>> interrupt context, because such calls do occur in certain
>> configurations - see commit b81c688426a9 ("arm64/mm: Disable barrier
>> batching in interrupt contexts"). Other architectures do not check
>> this situation, most likely because it hasn't occurred so far.
>>
>> Both arm64 and x86/Xen also ensure that any lazy MMU optimisation is
>> disabled while in interrupt mode (see queue_pte_barriers() and
>> xen_get_lazy_mode() respectively).
>>
>> Let's handle this in the new generic lazy_mmu layer, in the same
>> fashion as arm64: bail out of lazy_mmu_mode_* if in_interrupt(), and
>> have in_lazy_mmu_mode() return false to disable any optimisation.
>> Also remove the arm64 handling that is now redundant; x86/Xen has
>> its own internal tracking so it is left unchanged.
>>
>> Signed-off-by: Kevin Brodsky <kevin.brodsky@xxxxxxx>
>> ---
>>  arch/arm64/include/asm/pgtable.h | 17 +----------------
>>  include/linux/pgtable.h          | 16 ++++++++++++++--
>>  include/linux/sched.h            |  3 +++
>>  3 files changed, 18 insertions(+), 18 deletions(-)
>>
>> diff --git a/arch/arm64/include/asm/pgtable.h 
>> b/arch/arm64/include/asm/pgtable.h
>> index 61ca88f94551..96987a49e83b 100644
>> --- a/arch/arm64/include/asm/pgtable.h
>> +++ b/arch/arm64/include/asm/pgtable.h
>> @@ -62,37 +62,22 @@ static inline void emit_pte_barriers(void)
>>  
>>  static inline void queue_pte_barriers(void)
>>  {
>> -    if (in_interrupt()) {
>> -            emit_pte_barriers();
>> -            return;
>> -    }
>> -
>>      if (in_lazy_mmu_mode())
>>              test_and_set_thread_flag(TIF_LAZY_MMU_PENDING);
>>      else
>>              emit_pte_barriers();
>>  }
>>  
>> -static inline void arch_enter_lazy_mmu_mode(void)
>> -{
>> -    if (in_interrupt())
>> -            return;
>> -}
>> +static inline void arch_enter_lazy_mmu_mode(void) {}
>>  
>>  static inline void arch_flush_lazy_mmu_mode(void)
>>  {
>> -    if (in_interrupt())
>> -            return;
>> -
>>      if (test_and_clear_thread_flag(TIF_LAZY_MMU_PENDING))
>>              emit_pte_barriers();
>>  }
>>  
>>  static inline void arch_leave_lazy_mmu_mode(void)
>>  {
>> -    if (in_interrupt())
>> -            return;
>> -
>>      arch_flush_lazy_mmu_mode();
>>  }
> Ahh ok, by the time you get to the final state, I think a most of my
> comments/concerns are solved. Certainly this now looks safe for the interrupt
> case, whereas I think the intermediate state when you initially introduce
> nesting is broken. So perhaps you want to look at how to rework it to prevent 
> that.


Agreed, as discussed on patch 7. I might split this patch - first add
the in_interrupt() checks before patch 7, and then remove the
now-redundant checks on arm64.

- Kevin