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SubjectRe: [PATCH v2] mm: fix double page fault on arm64 if PTE_AF is cleared
On Fri, Sep 06, 2019 at 09:57:47PM +0800, Jia He wrote:
> * This really shouldn't fail, because the page is there
> * in the page tables. But it might just be unreadable,
> * in which case we just give up and fill the result with
> - * zeroes.
> + * zeroes. If PTE_AF is cleared on arm64, it might
> + * cause double page fault. So makes pte young here

How about:
* zeroes. On architectures with software "accessed" bits,
* we would take a double page fault here, so mark it
* accessed here.

> */
> + if (!pte_young(vmf->orig_pte)) {

Let's guard this with:

if (arch_sw_access_bit && !pte_young(vmf->orig_pte)) {

#define arch_sw_access_bit 0
by default and have arm64 override it (either to a variable or a constant
... your choice). Also, please somebody decide on a better name than
arch_sw_access_bit.

> + entry = pte_mkyoung(vmf->orig_pte);
> + if (ptep_set_access_flags(vmf->vma, vmf->address,
> + vmf->pte, entry, 0))

This indentation is wrong; it makes vmf->pte look like part of the subsequent
statement instead of part of the condition.

> + update_mmu_cache(vmf->vma, vmf->address,
> + vmf->pte);
> + }
> +

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