Re: [RFC] arm64: support HAVE_ARCH_RARE_WRITE

[Hoeun Ryu]

> On Mar 4, 2017, at 5:50 AM, Andy Lutomirski <luto@xxxxxxxxxxxxxx> wrote:
> 
>> On Thu, Mar 2, 2017 at 7:00 AM, Hoeun Ryu <hoeun.ryu@xxxxxxxxx> wrote:
>> +unsigned long __rare_write_rw_alias_start = TASK_SIZE_64 / 4;
>> +
>> +__always_inline unsigned long __arch_rare_write_map(void)
>> +{
>> +       struct mm_struct *mm = &rare_write_mm;
>> +
>> +       preempt_disable();
>> +
>> +       __switch_mm(mm);
> 
> ...
> 
>> +__always_inline unsigned long __arch_rare_write_unmap(void)
>> +{
>> +       struct mm_struct *mm = current->active_mm;
>> +
>> +       __switch_mm(mm);
>> +
> 
> This reminds me: this code imposes constraints on the context in which
> it's called.  I'd advise making it very explicit, asserting
> correctness, and putting the onus on the caller to set things up.  For
> example:
> 
> DEBUG_LOCKS_WARN_ON(preemptible() || in_interrupt() || in_nmi());
> 

OK. I will add some onus in the next version.

> in both the map and unmap functions, along with getting rid of the
> preempt_disable().  I don't think we want the preempt-disabledness to
> depend on the arch.  The generic non-arch rare_write helpers can do
> the preempt_disable().
> 

I think I can fix this in the next version when Kees send the next version of RFC.

> This code also won't work if the mm is wacky when called.  On x86, we could do:
> 
> DEBUG_LOCKS_WARN_ON(read_cr3() != current->active_mm->pgd);
> 
> or similar (since that surely doesn't compile as is).
> 
> --Andy

Thank you for the review.