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 à 01:56, Anshuman Khandual a écrit :
>>
>>
>> On 03/07/2020 06:04 AM, Qian Cai wrote:
>>>
>>>
 On Mar 6, 2020, at 7:03 PM, Anshuman Khandual  
 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);
>>>
>>> /*
>>>   * 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.
>>
> 
> 
> 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
> 

___
linux-snps-arc mailing list
linux-snps-arc@lists.infradead.org
http://lists.infradead.org/mailman/listinfo/linux-snps-arc

Re: [PATCH v3 01/17] gcc PR 88409: miscompilation due to missing cc clobber in longlong.h macros

[Vineet Gupta]

On 3/9/20 8:23 AM, Alexander Cherepanov wrote:
>>
>> PR 89877
>>
>> * longlong.h [__arc__] (add_ss): Add cc clobber
>> (sub_ddmmss): Likewise.
>
> Is PR number in subject (88409) wrong? The one in Changelog (89877) seems to 
> be
> right.

Yes you are right. I'll fix this up.

Thx,
-Vineet

___
linux-snps-arc mailing list
linux-snps-arc@lists.infradead.org
http://lists.infradead.org/mailman/listinfo/linux-snps-arc

Re: [PATCH v3 12/17] ARC: ABI lists

[Florian Weimer]

* Vineet Gupta:

> On 3/7/20 7:20 AM, Florian Weimer wrote:
>> * Vineet Gupta:
>> 
>>> +GLIBC_2.32 sysctl F
>> 
>> This looks like a mistake, given that ARC doesn't even have the sysctl
>> system call.
>> 
>> I think the current way of suppressing that involves creating an empty
>> sysctl.mk file, as in sysdeps/unix/sysv/linux/x86_64/x32/sysctl.mk.
>> (Unfortunately, the ports added after x32 forgot to do that because
>> the in-tree default is wrong.)
>
> For now I've added an empty file and in a later sweep we can remove all the 
> empty
> ones as you suggest below. Thx for taking a look.

Thanks.  I posted my removal patch as well:

  

___
linux-snps-arc mailing list
linux-snps-arc@lists.infradead.org
http://lists.infradead.org/mailman/listinfo/linux-snps-arc