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

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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