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SubjectRe: [PATCH v9 15/24] mm: Introduce __vm_normal_page()
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.

Cheers,
Jérôme

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