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Date:   Sat, 7 Mar 2020 08:05:09 +0100
From:   Christophe Leroy <>
To:     Anshuman Khandual <>,
        Qian Cai <>
Cc:, Andrew Morton <>,
        Mike Rapoport <>,
        Vineet Gupta <>,
        Catalin Marinas <>,
        Will Deacon <>,
        Benjamin Herrenschmidt <>,
        Paul Mackerras <>,
        Michael Ellerman <>,
        Heiko Carstens <>,
        Vasily Gorbik <>,
        Christian Borntraeger <>,
        Thomas Gleixner <>,
        Ingo Molnar <>, Borislav Petkov <>,
        "H. Peter Anvin" <>,
        "Kirill A . Shutemov" <>,
        Paul Walmsley <>,
        Palmer Dabbelt <>,,,,,,,,
Subject: Re: [PATCH V15] mm/debug: Add tests validating architecture page
 table helpers

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 

So, the WRITE_ONCE() must definitely become a set_pte_at()


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