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

[Anshuman Khandual]

On 03/07/2020 12:35 PM, Christophe Leroy wrote:
> 
> 
> Le 07/03/2020 Ã 01:56, Anshuman Khandual a ÃcritÂ:
>>
>>
>> On 03/07/2020 06:04 AM, Qian Cai wrote:
>>>
>>>
>>>> On Mar 6, 2020, at 7:03 PM, Anshuman Khandual <Anshuman.Khandual@xxxxxxx> 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 ?
> 
> What's the problem with thoses flushes ?
> 
>>
>>>
>>>> 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);
>>>
>>> /*
>>> Â * Certain architectures need to do special things when PTEs
>>>  * within a page table are directly modified. Thus, the following
>>> Â * hook is made available.
>>> Â */
>>>
>>> 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.
>>
> 
> 
> Argh ! I didn't realise that you were writing directly into the page tables. When it works, that's only by chance I guess.
> 
> To properly set the page table entries, set_pte_at() has to be used:
> - On powerpc 8xx, with 16k pages, the page table entry must be copied four times. set_pte_at() does it, WRITE_ONCE() doesn't.
> - On powerpc book3s/32 (hash MMU), the flag _PAGE_HASHPTE must be preserved among writes. set_pte_at() preserves it, WRITE_ONCE() doesn't.
> 
> set_pte_at() also does a few other mandatory things, like calling pte_mkpte()
> 
> So, the WRITE_ONCE() must definitely become a set_pte_at()

Sure, will do. These are part of the clear tests that populates a given
entry with a non zero value before clearing and testing it with pxx_none().
In that context, WRITE_ONCE() seemed sufficient. But pte_clear() might be
closely tied with proper page table entry update and hence a preceding
set_pte_at() will be better.

There are still more WRITE_ONCE() for other page table levels during these
clear tests. set_pmd_at() and set_pud_at() are defined on platforms that
support (and enable) THP and PUD based THP respectively. Hence they could
not be used for clear tests as remaining helpers pmd_clear(), pud_clear(),
p4d_clear() and pgd_clear() still need to be validated with or without
THP support and enablement. We should just leave all other WRITE_ONCE()
instances unchanged. Please correct me if I am missing something here.

> 
> Christophe
>