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SubjectRe: [PATCH RFC 01/10] mm: Add pmd support for _PAGE_SPECIAL
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On 2/3/20 9:34 PM, Matthew Wilcox wrote:
> On Fri, Jan 10, 2020 at 07:03:04PM +0000, Joao Martins wrote:
>> +++ b/arch/x86/include/asm/pgtable.h
>> @@ -293,6 +293,15 @@ static inline int pgd_devmap(pgd_t pgd)
>> {
>> return 0;
>> }
>> +#endif
>> +
>> +#ifdef CONFIG_ARCH_HAS_PTE_SPECIAL
>> +static inline int pmd_special(pmd_t pmd)
>> +{
>> + return !!(pmd_flags(pmd) & _PAGE_SPECIAL);
>> +}
>> +#endif
>
> The ifdef/endif don't make much sense here; x86 does have PTE_SPECIAL,
> and this is an x86 header file, so that can be assumed.
>
Gotcha.

>> +++ b/mm/gup.c
>> @@ -2079,6 +2079,9 @@ static int gup_huge_pmd(pmd_t orig, pmd_t *pmdp, unsigned long addr,
>> return __gup_device_huge_pmd(orig, pmdp, addr, end, pages, nr);
>> }
>>
>> + if (pmd_special(orig))
>> + return 0;
>
> Here, you're calling it unconditionally. I think you need a pmd_special()
> conditionally defined in include/asm-generic/pgtable.h
>
> +#ifndef CONFIG_ARCH_HAS_PTE_SPECIAL
> +static inline bool pmd_special(pmd_t pmd)
> +{
> + return false;
> +}
> +#endif
>
> (oh, and plese use bool instead of int; I know that's different from
> pte_special(), but pte_special() predates bool and nobody's done the work
> to convert it yet)
>
Got it.

>> +++ b/mm/huge_memory.c
>> @@ -791,6 +791,8 @@ static void insert_pfn_pmd(struct vm_area_struct *vma, unsigned long addr,
>> entry = pmd_mkhuge(pfn_t_pmd(pfn, prot));
>> if (pfn_t_devmap(pfn))
>> entry = pmd_mkdevmap(entry);
>> + else if (pfn_t_special(pfn))
>> + entry = pmd_mkspecial(entry);
>
> Again, we'll need a generic one.
>
Will add it.

>> @@ -823,8 +825,7 @@ vm_fault_t vmf_insert_pfn_pmd(struct vm_fault *vmf, pfn_t pfn, bool write)
>> * but we need to be consistent with PTEs and architectures that
>> * can't support a 'special' bit.
>> */
>> - BUG_ON(!(vma->vm_flags & (VM_PFNMAP|VM_MIXEDMAP)) &&
>> - !pfn_t_devmap(pfn));
>> + BUG_ON(!(vma->vm_flags & (VM_PFNMAP|VM_MIXEDMAP)));
>
> Should that rather be ...
>
> + BUG_ON(!(vma->vm_flags & (VM_PFNMAP|VM_MIXEDMAP)) &&
> + !pfn_t_devmap(pfn) && !pfn_t_special(pfn));
>
Yes. That is indeed a mistake I had already fixed for v2. Patch 3 does the exact
same, so as the other comments you mentioned here too so will adjust that
accordingly.

> I also think this comment needs adjusting:
>
> /*
> * There is no pmd_special() but there may be special pmds, e.g.
> * in a direct-access (dax) mapping, so let's just replicate the
> * !CONFIG_ARCH_HAS_PTE_SPECIAL case from vm_normal_page() here.
> */
>
>
I'll replace with what vm_normal_page() equivalent has:

/* !CONFIG_ARCH_HAS_PTE_SPECIAL case follows: */

Joao

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