Re: [PATCH v8 1/4] userfaultfd: Add UFFD WP Async support

[Peter Xu]

On Mon, Jan 30, 2023 at 01:38:16PM +0500, Muhammad Usama Anjum wrote:
> On 1/27/23 8:32 PM, Peter Xu wrote:
> > On Fri, Jan 27, 2023 at 11:47:14AM +0500, Muhammad Usama Anjum wrote:
> >>>> diff --git a/mm/memory.c b/mm/memory.c
> >>>> index 4000e9f017e0..8c03b133d483 100644
> >>>> --- a/mm/memory.c
> >>>> +++ b/mm/memory.c
> >>>> @@ -3351,6 +3351,18 @@ static vm_fault_t do_wp_page(struct vm_fault *vmf)
> >>>>  
> >>>>  	if (likely(!unshare)) {
> >>>>  		if (userfaultfd_pte_wp(vma, *vmf->pte)) {
> >>>> +			if (userfaultfd_wp_async(vma)) {
> >>>> +				/*
> >>>> +				 * Nothing needed (cache flush, TLB invalidations,
> >>>> +				 * etc.) because we're only removing the uffd-wp bit,
> >>>> +				 * which is completely invisible to the user. This
> >>>> +				 * falls through to possible CoW.
> >>>
> >>> Here it says it falls through to CoW, but..
> >>>
> >>>> +				 */
> >>>> +				pte_unmap_unlock(vmf->pte, vmf->ptl);
> >>>> +				set_pte_at(vma->vm_mm, vmf->address, vmf->pte,
> >>>> +					   pte_clear_uffd_wp(*vmf->pte));
> >>>> +				return 0;
> >>>
> >>> ... it's not doing so.  The original lines should do:
> >>>
> >>> https://lore.kernel.org/all/Y8qq0dKIJBshua+X@x1n/
> > 
> > [1]
> > 
> >>>
> >>> Side note: you cannot modify pgtable after releasing the pgtable lock.
> >>> It's racy.
> >> If I don't unlock and return after removing the UFFD_WP flag in case of
> >> async wp, the target just gets stuck. Maybe the pte lock is not unlocked in
> >> some path.
> >>
> >> If I unlock and don't return, the crash happens.
> >>
> >> So I'd put unlock and return from here. Please comment on the below patch
> >> and what do you think should be done. I've missed something.
> > 
> > Have you tried to just use exactly what I suggested in [1]?  I'll paste
> > again:
> > 
> > ---8<---
> > diff --git a/mm/memory.c b/mm/memory.c
> > index 4000e9f017e0..09aab434654c 100644
> > --- a/mm/memory.c
> > +++ b/mm/memory.c
> > @@ -3351,8 +3351,20 @@ static vm_fault_t do_wp_page(struct vm_fault *vmf)
> > 
> >         if (likely(!unshare)) {
> >                 if (userfaultfd_pte_wp(vma, *vmf->pte)) {
> > -                       pte_unmap_unlock(vmf->pte, vmf->ptl);
> > -                       return handle_userfault(vmf, VM_UFFD_WP);
> > +                       if (userfaultfd_uffd_wp_async(vma)) {
> > +                               /*
> > +                                * Nothing needed (cache flush, TLB
> > +                                * invalidations, etc.) because we're only
> > +                                * removing the uffd-wp bit, which is
> > +                                * completely invisible to the user.
> > +                                * This falls through to possible CoW.
> > +                                */
> > +                               set_pte_at(vma->vm_mm, vmf->address, vmf->pte,
> > +                                          pte_clear_uffd_wp(*vmf->pte));
> > +                       } else {
> > +                               pte_unmap_unlock(vmf->pte, vmf->ptl);
> > +                               return handle_userfault(vmf, VM_UFFD_WP);
> > +                       }
> >                 }
> > ---8<---
> > 
> > Note that there's no "return", neither the unlock.  The lock is used in the
> > follow up write fault resolution and it's released later.
> I've tried out the exact patch above. This doesn't work. The pages keep
> their WP flag even after being resolved in do_wp_page() while is written on
> the page.
> 
> So I'd added pte_unmap_unlock() and return 0 from here. This makes the
> patch to work. Maybe you can try this on your end to see what I'm seeing here?

Oh maybe it's because it didn't update orig_pte.  If you want, you can try
again with doing so by changing:

  set_pte_at(vma->vm_mm, vmf->address, vmf->pte,
             pte_clear_uffd_wp(*vmf->pte));

into:

  pte_t pte = pte_clear_uffd_wp(*vmf->pte);
  set_pte_at(vma->vm_mm, vmf->address, vmf->pte, pte);
  /* Update this to be prepared for following up CoW handling */
  vmf->orig_pte = pte;

> 
> > 
> > Meanwhile please fully digest how pgtable lock is used in this path before
> > moving forward on any of such changes.
> > 
> >>
> >>>
> >>>> +			}
> >>>>  			pte_unmap_unlock(vmf->pte, vmf->ptl);
> >>>>  			return handle_userfault(vmf, VM_UFFD_WP);
> >>>>  		}
> >>>> @@ -4812,8 +4824,21 @@ static inline vm_fault_t wp_huge_pmd(struct vm_fault *vmf)
> >>>>  
> >>>>  	if (vma_is_anonymous(vmf->vma)) {
> >>>>  		if (likely(!unshare) &&
> >>>> -		    userfaultfd_huge_pmd_wp(vmf->vma, vmf->orig_pmd))
> >>>> -			return handle_userfault(vmf, VM_UFFD_WP);
> >>>> +		    userfaultfd_huge_pmd_wp(vmf->vma, vmf->orig_pmd)) {
> >>>> +			if (userfaultfd_wp_async(vmf->vma)) {
> >>>> +				/*
> >>>> +				 * Nothing needed (cache flush, TLB invalidations,
> >>>> +				 * etc.) because we're only removing the uffd-wp bit,
> >>>> +				 * which is completely invisible to the user. This
> >>>> +				 * falls through to possible CoW.
> >>>> +				 */
> >>>> +				set_pmd_at(vmf->vma->vm_mm, vmf->address, vmf->pmd,
> >>>> +					   pmd_clear_uffd_wp(*vmf->pmd));
> >>>
> >>> This is for THP, not hugetlb.
> >>>
> >>> Clearing uffd-wp bit here for the whole pmd is wrong to me, because we
> >>> track writes in small page sizes only.  We should just split.
> >> By detecting if the fault is async wp, just splitting the PMD doesn't work.
> >> The below given snippit is working right now. But definately, the fault of
> >> the whole PMD is being resolved which if we can bypass by correctly
> >> splitting would be highly desirable. Can you please take a look on UFFD
> >> side and suggest the changes? It would be much appreciated. I'm attaching
> >> WIP v9 patches for you to apply on next(next-20230105) and pagemap_ioctl
> >> selftest can be ran to test things after making changes.
> > 
> > Can you elaborate why thp split didn't work?  Or if you want, I can look
> > into this and provide the patch to enable uffd async mode.
> Sorry, I was doing the wrong way. Splitting the page does work. What do you
> think about the following:
> 
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -3351,6 +3351,17 @@ static vm_fault_t do_wp_page(struct vm_fault *vmf)
> 
>  	if (likely(!unshare)) {
>  		if (userfaultfd_pte_wp(vma, *vmf->pte)) {
> +			if (userfaultfd_wp_async(vma)) {
> +				/*
> +				 * Nothing needed (cache flush, TLB invalidations,
> +				 * etc.) because we're only removing the uffd-wp bit,
> +				 * which is completely invisible to the user.
> +				 */
> +				set_pte_at(vma->vm_mm, vmf->address, vmf->pte,
> +					   pte_clear_uffd_wp(*vmf->pte));
> +				pte_unmap_unlock(vmf->pte, vmf->ptl);
> +				return 0;

Please give it a shot with above to see whether we can avoid the "return 0"
here.

> +			}
>  			pte_unmap_unlock(vmf->pte, vmf->ptl);
>  			return handle_userfault(vmf, VM_UFFD_WP);
>  		}
> @@ -4812,8 +4823,13 @@ static inline vm_fault_t wp_huge_pmd(struct vm_fault
> *vmf)
> 
>  	if (vma_is_anonymous(vmf->vma)) {
>  		if (likely(!unshare) &&
> -		    userfaultfd_huge_pmd_wp(vmf->vma, vmf->orig_pmd))
> +		    userfaultfd_huge_pmd_wp(vmf->vma, vmf->orig_pmd)) {
> +			if (userfaultfd_wp_async(vmf->vma)) {
> +				__split_huge_pmd(vmf->vma, vmf->pmd, vmf->address, false, NULL);
> +				return 0;

Same here, I hope it'll work for you if you just goto __split_huge_pmd()
right below and return with VM_FAULT_FALLBACK.  It avoids one more round of
fault just like the pte case above.

> +			}
>  			return handle_userfault(vmf, VM_UFFD_WP);
> +		}
>  		return do_huge_pmd_wp_page(vmf);
>  	}

-- 
Peter Xu