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

[Qian Cai]

> On Mar 6, 2020, at 7:56 PM, Anshuman Khandual <anshuman.khandual at arm.com> wrote:
> 
> 
> 
> On 03/07/2020 06:04 AM, Qian Cai wrote:
>> 
>> 
>>> On Mar 6, 2020, at 7:03 PM, Anshuman Khandual <Anshuman.Khandual at 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()

No, barrier() is not enough.

> does as compared to WRITE_ONCE() that avoids the soft lock up, just trying
> to understand.

I surely can spend hours to figure which exact things in set_pte_at() is necessary for
pte_clear() not to stuck, and then propose a solution and possible need to retest on
multiple arches. I am not sure if that is a good use of my time just to saving
a few TLB/cache flush on a debug kernel?