Re: [PATCH v2 14/26] userfaultfd: wp: handle COW properly for uffd-wp

[Peter Xu <peterx@redhat.com>]

On Fri, Feb 22, 2019 at 10:35:09AM -0500, Jerome Glisse wrote:
> On Fri, Feb 22, 2019 at 04:46:03PM +0800, Peter Xu wrote:
> > On Thu, Feb 21, 2019 at 01:04:24PM -0500, Jerome Glisse wrote:
> > > On Tue, Feb 12, 2019 at 10:56:20AM +0800, Peter Xu wrote:
> > > > This allows uffd-wp to support write-protected pages for COW.
> 
> [...]
> 
> > > > diff --git a/mm/mprotect.c b/mm/mprotect.c
> > > > index 9d4433044c21..ae93721f3795 100644
> > > > --- a/mm/mprotect.c
> > > > +++ b/mm/mprotect.c
> > > > @@ -77,14 +77,13 @@ static unsigned long change_pte_range(struct vm_area_struct *vma, pmd_t *pmd,
> > > >  		if (pte_present(oldpte)) {
> > > >  			pte_t ptent;
> > > >  			bool preserve_write = prot_numa && pte_write(oldpte);
> > > > +			struct page *page;
> > > >  
> > > >  			/*
> > > >  			 * Avoid trapping faults against the zero or KSM
> > > >  			 * pages. See similar comment in change_huge_pmd.
> > > >  			 */
> > > >  			if (prot_numa) {
> > > > -				struct page *page;
> > > > -
> > > >  				page = vm_normal_page(vma, addr, oldpte);
> > > >  				if (!page || PageKsm(page))
> > > >  					continue;
> > > > @@ -114,6 +113,46 @@ static unsigned long change_pte_range(struct vm_area_struct *vma, pmd_t *pmd,
> > > >  					continue;
> > > >  			}
> > > >  
> > > > +			/*
> > > > +			 * Detect whether we'll need to COW before
> > > > +			 * resolving an uffd-wp fault.  Note that this
> > > > +			 * includes detection of the zero page (where
> > > > +			 * page==NULL)
> > > > +			 */
> > > > +			if (uffd_wp_resolve) {
> > > > +				/* If the fault is resolved already, skip */
> > > > +				if (!pte_uffd_wp(*pte))
> > > > +					continue;
> > > > +				page = vm_normal_page(vma, addr, oldpte);
> > > > +				if (!page || page_mapcount(page) > 1) {
> > > 
> > > This is wrong, if you allow page to be NULL then you gonna segfault
> > > in wp_page_copy() down below. Are you sure you want to test for
> > > special page ? For anonymous memory this should never happens ie
> > > anon page always are regular page. So if you allow userfaulfd to
> > > write protect only anonymous vma then there is no point in testing
> > > here beside maybe a BUG_ON() just in case ...
> > 
> > It's majorly for zero pages where page can be NULL.  Would this be
> > clearer:
> > 
> >   if (is_zero_pfn(pte_pfn(old_pte)) || (page && page_mapcount(page)))
> > 
> > ?
> > 
> > Now we treat zero pages as normal COW pages so we'll do COW here even
> > for zero pages.  I think maybe we can do special handling on all over
> > the places for zero pages (e.g., we don't write protect a PTE if we
> > detected that this is the zero PFN) but I'm uncertain on whether
> > that's what we want, so I chose to start with current solution at
> > least to achieve functionality first.
> 
> You can keep the vm_normal_page() in that case but split the if
> between page == NULL and page != NULL with mapcount > 1. As other-
> wise you will segfault below.

Could I ask what's the segfault you mentioned?  My understanding is
that below code has taken page==NULL into consideration already, e.g.,
we only do get_page() if page!=NULL, and inside wp_page_copy() it has
similar considerations.

> 
> 
> > 
> > > 
> > > > +					struct vm_fault vmf = {
> > > > +						.vma = vma,
> > > > +						.address = addr & PAGE_MASK,
> > > > +						.page = page,
> > > > +						.orig_pte = oldpte,
> > > > +						.pmd = pmd,
> > > > +						/* pte and ptl not needed */
> > > > +					};
> > > > +					vm_fault_t ret;
> > > > +
> > > > +					if (page)
> > > > +						get_page(page);
> > > > +					arch_leave_lazy_mmu_mode();
> > > > +					pte_unmap_unlock(pte, ptl);
> > > > +					ret = wp_page_copy(&vmf);
> > > > +					/* PTE is changed, or OOM */
> > > > +					if (ret == 0)
> > > > +						/* It's done by others */
> > > > +						continue;
> > > > +					else if (WARN_ON(ret != VM_FAULT_WRITE))
> > > > +						return pages;
> > > > +					pte = pte_offset_map_lock(vma->vm_mm,
> > > > +								  pmd, addr,
> > > > +								  &ptl);
> > > 
> > > Here you remap the pte locked but you are not checking if the pte is
> > > the one you expect ie is it pointing to the copied page and does it
> > > have expect uffd_wp flag. Another thread might have raced between the
> > > time you called wp_page_copy() and the time you pte_offset_map_lock()
> > > I have not check the mmap_sem so maybe you are protected by it as
> > > mprotect is taking it in write mode IIRC, if so you should add a
> > > comments at very least so people do not see this as a bug.
> > 
> > Thanks for spotting this.  With nornal uffd-wp page fault handling
> > path we're only with read lock held (and I would suspect it's racy
> > even with write lock...).  I agree that there can be a race right
> > after the COW has done.
> > 
> > Here IMHO we'll be fine as long as it's still a present PTE, in other
> > words, we should be able to tolerate PTE changes as long as it's still
> > present otherwise we'll need to retry this single PTE (e.g., the page
> > can be quickly marked as migrating swap entry, or even the page could
> > be freed beneath us).  Do you think below change look good to you to
> > be squashed into this patch?
> 
> Ok, but below if must be after arch_enter_lazy_mmu_mode(); not before.

Oops... you are right. :)

Thanks,

> 
> > 
> > diff --git a/mm/mprotect.c b/mm/mprotect.c
> > index 73a65f07fe41..3423f9692838 100644
> > --- a/mm/mprotect.c
> > +++ b/mm/mprotect.c
> > @@ -73,6 +73,7 @@ static unsigned long change_pte_range(struct vm_area_struct *vma, pmd_t *pmd,                                                              
> >         flush_tlb_batched_pending(vma->vm_mm);
> >         arch_enter_lazy_mmu_mode();
> >         do {
> > +retry_pte:
> >                 oldpte = *pte;
> >                 if (pte_present(oldpte)) {
> >                         pte_t ptent;
> > @@ -149,6 +150,13 @@ static unsigned long change_pte_range(struct vm_area_struct *vma, pmd_t *pmd,                                                           
> >                                         pte = pte_offset_map_lock(vma->vm_mm,
> >                                                                   pmd, addr,
> >                                                                   &ptl);
> > +                                       if (!pte_present(*pte))
> > +                                               /*
> > +                                                * This PTE could have
> > +                                                * been modified when COW;
> > +                                                * retry it
> > +                                                */
> > +                                               goto retry_pte;
> >                                         arch_enter_lazy_mmu_mode();
> >                                 }
> >                         }

-- 
Peter Xu