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Subject: Re: [PATCH v9 15/24] mm: Introduce __vm_normal_page()
To:     Jerome Glisse <jglisse@redhat.com>
Cc:     paulmck@linux.vnet.ibm.com, peterz@infradead.org,
        akpm@linux-foundation.org, kirill@shutemov.name,
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        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, hpa@zytor.com,
        Will Deacon <will.deacon@arm.com>,
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        Alexei Starovoitov <alexei.starovoitov@gmail.com>,
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        Daniel Jordan <daniel.m.jordan@oracle.com>,
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        haren@linux.vnet.ibm.com, khandual@linux.vnet.ibm.com,
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        Tim Chen <tim.c.chen@linux.intel.com>,
        linuxppc-dev@lists.ozlabs.org, x86@kernel.org
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From:   Laurent Dufour <ldufour@linux.vnet.ibm.com>
Date:   Thu, 5 Apr 2018 14:53:19 +0200
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On 04/04/2018 23:59, Jerome Glisse wrote:
> On Wed, Apr 04, 2018 at 06:26:44PM +0200, Laurent Dufour wrote:
>>
>>
>> On 03/04/2018 21:39, Jerome Glisse wrote:
>>> On Tue, Mar 13, 2018 at 06:59:45PM +0100, Laurent Dufour wrote:
>>>> When dealing with the speculative fault path we should use the VMA's field
>>>> cached value stored in the vm_fault structure.
>>>>
>>>> Currently vm_normal_page() is using the pointer to the VMA to fetch the
>>>> vm_flags value. This patch provides a new __vm_normal_page() which is
>>>> receiving the vm_flags flags value as parameter.
>>>>
>>>> Note: The speculative path is turned on for architecture providing support
>>>> for special PTE flag. So only the first block of vm_normal_page is used
>>>> during the speculative path.
>>>
>>> Might be a good idea to explicitly have SPECULATIVE Kconfig option depends
>>> on ARCH_PTE_SPECIAL and a comment for !HAVE_PTE_SPECIAL in the function
>>> explaining that speculative page fault should never reach that point.
>>
>> Unfortunately there is no ARCH_PTE_SPECIAL in the config file, it is defined in
>> the per architecture header files.
>> So I can't do anything in the Kconfig file
> 
> Maybe adding a new Kconfig symbol for ARCH_PTE_SPECIAL very much like
> others ARCH_HAS_
> 
>>
>> However, I can check that at build time, and doing such a check in
>> __vm_normal_page sounds to be a good place, like that:
>>
>> @@ -869,6 +870,14 @@ struct page *__vm_normal_page(struct vm_area_struct *vma,
>> unsigned long addr,
>>
>>         /* !HAVE_PTE_SPECIAL case follows: */
>>
>> +#ifdef CONFIG_SPECULATIVE_PAGE_FAULT
>> +       /* This part should never get called when the speculative page fault
>> +        * handler is turned on. This is mainly because we can't rely on
>> +        * vm_start.
>> +        */
>> +#error CONFIG_SPECULATIVE_PAGE_FAULT requires HAVE_PTE_SPECIAL
>> +#endif
>> +
>>         if (unlikely(vma_flags & (VM_PFNMAP|VM_MIXEDMAP))) {
>>                 if (vma_flags & VM_MIXEDMAP) {
>>                         if (!pfn_valid(pfn))
>>
> 
> I am not a fan of #if/#else/#endif in code. But that's a taste thing.
> I honnestly think that adding a Kconfig for special pte is the cleanest
> solution.

I do agree, but this should be done in a separate series.

I'll see how this could be done but there are some arch (like powerpc) where
this is a bit obfuscated for unknown reason.

For the time being, I'll remove the check and just let the comment in place.