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

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