Re: [PATCH v3 14/28] userfaultfd: wp: handle COW properly for uffd-wp

[Jerome Glisse <jglisse@redhat.com>]

On Fri, Apr 19, 2019 at 02:26:50PM +0800, Peter Xu wrote:
> On Thu, Apr 18, 2019 at 04:51:15PM -0400, Jerome Glisse wrote:
> > On Wed, Mar 20, 2019 at 10:06:28AM +0800, Peter Xu wrote:
> > > This allows uffd-wp to support write-protected pages for COW.
> > > 
> > > For example, the uffd write-protected PTE could also be write-protected
> > > by other usages like COW or zero pages.  When that happens, we can't
> > > simply set the write bit in the PTE since otherwise it'll change the
> > > content of every single reference to the page.  Instead, we should do
> > > the COW first if necessary, then handle the uffd-wp fault.
> > > 
> > > To correctly copy the page, we'll also need to carry over the
> > > _PAGE_UFFD_WP bit if it was set in the original PTE.
> > > 
> > > For huge PMDs, we just simply split the huge PMDs where we want to
> > > resolve an uffd-wp page fault always.  That matches what we do with
> > > general huge PMD write protections.  In that way, we resolved the huge
> > > PMD copy-on-write issue into PTE copy-on-write.
> > > 
> > > Signed-off-by: Peter Xu <peterx@redhat.com>
> > 
> > This one has a bug see below.
> > 
> > 
> > > ---
> > >  mm/memory.c   |  5 +++-
> > >  mm/mprotect.c | 64 ++++++++++++++++++++++++++++++++++++++++++++++++---
> > >  2 files changed, 65 insertions(+), 4 deletions(-)
> > > 
> > > diff --git a/mm/memory.c b/mm/memory.c
> > > index e7a4b9650225..b8a4c0bab461 100644
> > > --- a/mm/memory.c
> > > +++ b/mm/memory.c
> > > @@ -2291,7 +2291,10 @@ vm_fault_t wp_page_copy(struct vm_fault *vmf)
> > >  		}
> > >  		flush_cache_page(vma, vmf->address, pte_pfn(vmf->orig_pte));
> > >  		entry = mk_pte(new_page, vma->vm_page_prot);
> > > -		entry = maybe_mkwrite(pte_mkdirty(entry), vma);
> > > +		if (pte_uffd_wp(vmf->orig_pte))
> > > +			entry = pte_mkuffd_wp(entry);
> > > +		else
> > > +			entry = maybe_mkwrite(pte_mkdirty(entry), vma);
> > >  		/*
> > >  		 * Clear the pte entry and flush it first, before updating the
> > >  		 * pte with the new entry. This will avoid a race condition
> > > diff --git a/mm/mprotect.c b/mm/mprotect.c
> > > index 9d4433044c21..855dddb07ff2 100644
> > > --- a/mm/mprotect.c
> > > +++ b/mm/mprotect.c
> > > @@ -73,18 +73,18 @@ 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;
> > >  			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 +114,54 @@ 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) {
> > > +					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;
> > 
> > This is wrong if ret == 0 you still need to remap the pte before
> > continuing as otherwise you will go to next pte without the page
> > table lock for the directory. So 0 case must be handled after
> > arch_enter_lazy_mmu_mode() below.
> > 
> > Sorry i should have catch that in previous review.
> 
> My fault to not have noticed it since the very beginning... thanks for
> spotting that.
> 
> I'm squashing below changes into the patch:


Well thinking of this some more i think you should use do_wp_page() and
not wp_page_copy() it would avoid bunch of code above and also you are
not properly handling KSM page or page in the swap cache. Instead of
duplicating same code that is in do_wp_page() it would be better to call
it here.


> 
> diff --git a/mm/mprotect.c b/mm/mprotect.c
> index 3cddfd6627b8..13d493b836bb 100644
> --- a/mm/mprotect.c
> +++ b/mm/mprotect.c
> @@ -141,22 +141,19 @@ static unsigned long change_pte_range(struct vm_area_struct *vma, pmd_t *pmd,
>                                         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))
> +                                       if (ret != VM_FAULT_WRITE && ret != 0)
> +                                               /* Probably OOM */
>                                                 return pages;
>                                         pte = pte_offset_map_lock(vma->vm_mm,
>                                                                   pmd, addr,
>                                                                   &ptl);
>                                         arch_enter_lazy_mmu_mode();
> -                                       if (!pte_present(*pte))
> +                                       if (ret == 0 || !pte_present(*pte))
>                                                 /*
>                                                  * This PTE could have been
> -                                                * modified after COW
> -                                                * before we have taken the
> -                                                * lock; retry this PTE
> +                                                * modified during or after
> +                                                * COW before take the lock;
> +                                                * retry.
>                                                  */
>                                                 goto retry_pte;
>                                 }
> 
> [...]
> 
> > >  		if (is_swap_pmd(*pmd) || pmd_trans_huge(*pmd) || pmd_devmap(*pmd)) {
> > > -			if (next - addr != HPAGE_PMD_SIZE) {
> > > +			/*
> > > +			 * When resolving an userfaultfd write
> > > +			 * protection fault, it's not easy to identify
> > > +			 * whether a THP is shared with others and
> > > +			 * whether we'll need to do copy-on-write, so
> > > +			 * just split it always for now to simply the
> > > +			 * procedure.  And that's the policy too for
> > > +			 * general THP write-protect in af9e4d5f2de2.
> > > +			 */
> > > +			if (next - addr != HPAGE_PMD_SIZE || uffd_wp_resolve) {
> > 
> > Just a nit pick can you please add () to next - addr ie:
> > if ((next - addr) != HPAGE_PMD_SIZE || uffd_wp_resolve) {
> > 
> > I know it is not needed but each time i bump into this i
> > have to scratch my head for second to remember the operator
> > rules :)
> 
> Sure, as usual. :) And I tend to agree it's a good habit.  It's just
> me that always forgot about it.
> 
> Thanks,
> 
> -- 
> Peter Xu