Re: [PATCH v6 06/23] mm/shmem: Handle uffd-wp special pte in page fault handler

[Alistair Popple <apopple@nvidia.com>]

On Thursday, 16 December 2021 5:17:30 PM AEDT Peter Xu wrote:
> On Thu, Dec 16, 2021 at 04:56:42PM +1100, Alistair Popple wrote:
> > On Monday, 15 November 2021 6:55:05 PM AEDT Peter Xu wrote:
> > 
> > [...]
> > 
> > > diff --git a/mm/memory.c b/mm/memory.c
> > > index d5966d9e24c3..e8557d43a87d 100644
> > > --- a/mm/memory.c
> > > +++ b/mm/memory.c
> > > @@ -3452,6 +3452,43 @@ static vm_fault_t remove_device_exclusive_entry(struct vm_fault *vmf)
> > >  	return 0;
> > >  }
> > >  
> > > +static vm_fault_t pte_marker_clear(struct vm_fault *vmf)
> > > +{
> > > +	vmf->pte = pte_offset_map_lock(vmf->vma->vm_mm, vmf->pmd,
> > > +				       vmf->address, &vmf->ptl);
> > > +	/*
> > > +	 * Be careful so that we will only recover a special uffd-wp pte into a
> > > +	 * none pte.  Otherwise it means the pte could have changed, so retry.
> > > +	 */
> > > +	if (is_pte_marker(*vmf->pte))
> > > +		pte_clear(vmf->vma->vm_mm, vmf->address, vmf->pte);
> > > +	pte_unmap_unlock(vmf->pte, vmf->ptl);
> > > +	return 0;
> > > +}
> > > +
> > > +/*
> > > + * This is actually a page-missing access, but with uffd-wp special pte
> > > + * installed.  It means this pte was wr-protected before being unmapped.
> > > + */
> > > +static vm_fault_t pte_marker_handle_uffd_wp(struct vm_fault *vmf)
> > > +{
> > > +	/* Careful!  vmf->pte unmapped after return */
> > > +	if (!pte_unmap_same(vmf))
> > 
> > Hasn't vmf->pte already been unmapped by do_swap_page() by the time we get
> > here?
> 
> Great catch, thanks!
> 
> It was needed before with the "swap special pte" version because that was
> handled outside do_swap_page().  After the rebase I forgot to remove it.

No worries, and for what it's worth IMHO this version that handles it inside
do_swap_page() along with all the other "special" cases is much nicer.

> I believe it didn't crash simply because we've got commit 2ca99358671a ("mm:
> clear vmf->pte after pte_unmap_same() returns", 2021-11-06) very recently so it
> just became a safe no-op, so all things will still work.
> 
> I'll drop it.
> 
> > 
> > > +		return 0;
> > > +
> > > +	/*
> > > +	 * Just in case there're leftover special ptes even after the region
> > > +	 * got unregistered - we can simply clear them.  We can also do that
> > > +	 * proactively when e.g. when we do UFFDIO_UNREGISTER upon some uffd-wp
> > > +	 * ranges, but it should be more efficient to be done lazily here.
> > > +	 */
> > > +	if (unlikely(!userfaultfd_wp(vmf->vma) || vma_is_anonymous(vmf->vma)))
> > > +		return pte_marker_clear(vmf);
> > > +
> > > +	/* do_fault() can handle pte markers too like none pte */
> > > +	return do_fault(vmf);
> > > +}
> > > +
> > >  static vm_fault_t handle_pte_marker(struct vm_fault *vmf)
> > >  {
> > >  	swp_entry_t entry = pte_to_swp_entry(vmf->orig_pte);
> > > @@ -3465,8 +3502,11 @@ static vm_fault_t handle_pte_marker(struct vm_fault *vmf)
> > >  	if (WARN_ON_ONCE(vma_is_anonymous(vmf->vma) || !marker))
> > >  		return VM_FAULT_SIGBUS;
> > >  
> > > -	/* TODO: handle pte markers */
> > > -	return 0;
> > > +	if (marker & PTE_MARKER_UFFD_WP)
> > 
> > Can we make this check `marker == PTE_MARKER_UFFD_WP`? There is currently only
> > one user of pte markers, and from what I can tell pte_marker_handle_uffd_wp()
> > wouldn't do the correct thing if other users were added because it could clear
> > non-uffd-wp markers. I don't think it's worth making it do the right thing now,
> > but a comment noting that would be helpful.
> 
> Sure thing, and yeah I agree it's trivial and shouldn't matter in real-life.
> 
> I'll change it to "marker == PTE_MARKER_UFFD_WP" as you suggested, so if
> there's surprise we'll get a sigbus.
> 
> Thanks,
> 
> > 
> > > +		return pte_marker_handle_uffd_wp(vmf);
> > > +
> > > +	/* This is an unknown pte marker */
> > > +	return VM_FAULT_SIGBUS;
> > >  }
> 
>