Re: [PATCH] mm: fix double page fault on arm64 if PTE_AF is cleared

[Anshuman Khandual <anshuman.khandual@arm.com>]

On 09/04/2019 08:49 AM, Anshuman Khandual 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 here. so makes pte young here
>  		 */
> +		if (!pte_young(vmf->orig_pte)) {
> +			entry = pte_mkyoung(vmf->orig_pte);
> +			if (ptep_set_access_flags(vmf->vma, vmf->address,
> +				vmf->pte, entry, vmf->flags & FAULT_FLAG_WRITE))
> +				update_mmu_cache(vmf->vma, vmf->address,
> +						vmf->pte);
> +		}

This looks correct where it updates the pte entry with PTE_AF which
will prevent a subsequent page fault. But I think what we really need
here is to make sure 'uaddr' is mapped correctly at vma->pte. Probably
a generic function arch_map_pte() when defined for arm64 should check
CPU version and ensure continuance of PTE_AF if required. The comment
above also need to be updated saying not only the page should be there
in the page table, it needs to mapped appropriately as well.