Subject: Re: [PATCH] powerpc/mm: bail out early when flushing TLB page

From: Arseny Solokha <[email-masked]>
Date: Mon, 02 Feb 2015 12:07:00 +0700

> On Fri, 2015-01-30 at 19:08 +0700, Arseny Solokha wrote:
>> MMU_NO_CONTEXT is conditionally defined as 0 or (unsigned int)-1. However,
>> in __flush_tlb_page() a corresponding variable is only tested for open
>> coded 0, which can cause NULL pointer dereference if `mm' argument was
>> legitimately passed as such.
>>
>> Bail out early in case the first argument is NULL, thus eliminate confusion
>> between different values of MMU_NO_CONTEXT and avoid disabling and then
>> re-enabling preemption unnecessarily.
> 
> So the comment above isn't quite right... we don't *test* it for open
> coded 0, we test it for MMU_NO_CONTEXT, however we *set* it to 0 for
> NULL mm.
> 
> This is actually correct... on all except 8xx :-) 0 *is* the PID of the
> kernel context, and NULL mm usually means kernel context.

> However, it's correct that this function will not deal properly with a
> NULL mm for other reasons. It must only be called for user contexts.
> 
> Instead of just returning, I would WARN_ON, because if it's ever called
> for a kernel page, then it will not do what's expected and that will
> need fixing. Just a silent return isn't right.

Does this also hold true for __local_flush_tlb_page()? It's called from
local_flush_tlb_page() as follows:

  __local_flush_tlb_page(vma ? vma->vm_mm : NULL, vmaddr,
  			 mmu_get_tsize(mmu_virtual_psize), 0);

However, if MMU_NO_CONTEXT is 0 and __local_flush_tlb_page() is called with mm
set to NULL, it's effectively a no-op. What else am I missing?

Thanks,

Arsény

> This is different from returning on MMU_NO_CONTEXT, in this case, we
> know there's no active TLB entries for the process, and thus nothing to
> flush.
> 
> Cheers,
> Ben.
> 
>> Signed-off-by: Arseny Solokha <[email-masked]>
>> ---
>>  arch/powerpc/mm/tlb_nohash.c | 5 ++++-
>>  1 file changed, 4 insertions(+), 1 deletion(-)
>>
>> diff --git a/arch/powerpc/mm/tlb_nohash.c b/arch/powerpc/mm/tlb_nohash.c
>> index f38ea4d..ab0616b 100644
>> --- a/arch/powerpc/mm/tlb_nohash.c
>> +++ b/arch/powerpc/mm/tlb_nohash.c
>> @@ -284,8 +284,11 @@ void __flush_tlb_page(struct mm_struct *mm, unsigned long vmaddr,
>>  	struct cpumask *cpu_mask;
>>  	unsigned int pid;
>>  
>> +	if (unlikely(!mm))
>> +		return;
>> +
>>  	preempt_disable();
>> -	pid = mm ? mm->context.id : 0
> 
> Here we test mm, if we pass NULL, that means the kernel mm which has PID
> 0, which is not MMU_NO_CONTEXT
>> ;
>> +	pid = mm->context.id;
>>  	if (unlikely(pid == MMU_NO_CONTEXT))
>>  		goto bail;
>>  	cpu_mask = mm_cpumask(mm);