Hi Anshuman, thanks for the comments, see below please
> -----Original Message-----
> From: Anshuman Khandual <anshuman.khandual@xxxxxxx>
> Sent: 2019年9月4日 12:38
> To: Justin He (Arm Technology China) <Justin.He@xxxxxxx>; Andrew
> Morton <akpm@xxxxxxxxxxxxxxxxxxxx>; Matthew Wilcox
> <willy@xxxxxxxxxxxxx>; Jérôme Glisse <jglisse@xxxxxxxxxx>; Ralph
> Campbell <rcampbell@xxxxxxxxxx>; Jason Gunthorpe <jgg@xxxxxxxx>;
> Peter Zijlstra <peterz@xxxxxxxxxxxxx>; Dave Airlie <airlied@xxxxxxxxxx>;
> Aneesh Kumar K.V <aneesh.kumar@xxxxxxxxxxxxx>; Thomas Hellstrom
> <thellstrom@xxxxxxxxxx>; Souptick Joarder <jrdr.linux@xxxxxxxxx>;
> linux-mm@xxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx
> Subject: Re: [PATCH] mm: fix double page fault on arm64 if PTE_AF is
> cleared
>
>
>
> 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.
I agree that a generic interface here is needed but not the arch_map_pte().
In this case, I thought all the pgd/pud/pmd/pte had been set correctly except
for the PTE_AF bit.
How about arch_hw_access_flag()?
If non-arm64, arch_hw_access_flag() == true
If arm64 with hardware-managed access flag supported, == true
else == false?
--
Cheers,
Justin (Jia He)
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