Re: [PATCH V15] mm/debug: Add tests validating architecture page table helpers

[Anshuman Khandual <anshuman.khandual@arm.com>]

On 03/07/2020 06:04 AM, Qian Cai wrote:
> 
> 
>> On Mar 6, 2020, at 7:03 PM, Anshuman Khandual <Anshuman.Khandual@arm.com> wrote:
>>
>> Hmm, set_pte_at() function is not preferred here for these tests. The idea
>> is to avoid or atleast minimize TLB/cache flushes triggered from these sort
>> of 'static' tests. set_pte_at() is platform provided and could/might trigger
>> these flushes or some other platform specific synchronization stuff. Just
> 
> Why is that important for this debugging option?

Primarily reason is to avoid TLB/cache flush instructions on the system
during these tests that only involve transforming different page table
level entries through helpers. Unless really necessary, why should it
emit any TLB/cache flush instructions ?

> 
>> wondering is there specific reason with respect to the soft lock up problem
>> making it necessary to use set_pte_at() rather than a simple WRITE_ONCE() ?
> 
> Looks at the s390 version of set_pte_at(), it has this comment,
> vmaddr);
> 
> /*
>  * Certain architectures need to do special things when PTEs
>  * within a page table are directly modified.  Thus, the following
>  * hook is made available.
>  */
> 
> I can only guess that powerpc  could be the same here.

This comment is present in multiple platforms while defining set_pte_at().
Is not 'barrier()' here alone good enough ? Else what exactly set_pte_at()
does as compared to WRITE_ONCE() that avoids the soft lock up, just trying
to understand.