Re: [PATCH 1/2] mm: Allow architectures to request 'old' entries when prefaulting

[Will Deacon]

On Wed, Dec 09, 2020 at 09:58:12AM -0800, Linus Torvalds wrote:
> On Wed, Dec 9, 2020 at 8:40 AM Will Deacon <will@xxxxxxxxxx> wrote:
> >
> > @@ -3978,8 +3994,17 @@ static vm_fault_t do_fault_around(struct vm_fault *vmf)
> >
> >         /* check if the page fault is solved */
> >         vmf->pte -= (vmf->address >> PAGE_SHIFT) - (address >> PAGE_SHIFT);
> > -       if (!pte_none(*vmf->pte))
> > -               ret = VM_FAULT_NOPAGE;
> > +       if (pte_none(*vmf->pte))
> > +               goto out_unlock;
> > +
> > +       if (vmf->flags & FAULT_FLAG_PREFAULT_OLD) {
> > +               pte_t pte = pte_mkyoung(*vmf->pte);
> > +               if (ptep_set_access_flags(vmf->vma, address, vmf->pte, pte, 0))
> > +                       update_mmu_cache(vmf->vma, address, vmf->pte);
> > +       }
> 
> Oh, please dear God no.
> 
> First you incorrectly set it old, and then you conditionally make it
> young again and as a result force an atomic rwm update and another TLB
> flush for no good reason.

There shouldn't be a TLB flush here, but I agree that it would have to
go and nobble the hash for PowerPC if they wanted to enable this.

> Just make sure that the FAULT_FLAG_PREFAULT_OLD never sets the
> *actual* address to old.
> 
> And yes, that probably means that you need to change "alloc_set_pte()"
> to actually pass in the real address, and leave "vmf->address" alone -
> so that it can know which ones are prefaulted and which one is real,
> but that sounds like a good idea anyway.

Right, I deliberately avoided that based on the feedback from Jan on an
older version [1], but I can certainly look at it again.

> Then you can just make alloc_set_pte() do the right thing in the first
> place, instead of doing this nasty "lets do it wrong and fix it up
> later" horror.

I'll have a crack at this in v2.

Cheers,

Will

[1] https://patchwork.kernel.org/project/linux-arm-kernel/patch/1511845670-12133-1-git-send-email-vinmenon@xxxxxxxxxxxxxx/