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SubjectRe: [PATCH v3 14/28] userfaultfd: wp: handle COW properly for uffd-wp
On Mon, Apr 22, 2019 at 08:20:10PM +0800, Peter Xu wrote:
> On Fri, Apr 19, 2019 at 11:02:53AM -0400, Jerome Glisse wrote:
>
> [...]
>
> > > > > + 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.
>
> Yeah it makes sense to me. Then here's my plan:
>
> - I'll need to drop previous patch "export wp_page_copy" since then
> it'll be not needed
>
> - I'll introduce another patch to split current do_wp_page() and
> introduce function "wp_page_copy_cont" (better suggestion on the
> naming would be welcomed) which contains most of the wp handling
> that'll be needed for change_pte_range() in this patch and isolate
> the uffd handling:
>
> static vm_fault_t do_wp_page(struct vm_fault *vmf)
> __releases(vmf->ptl)
> {
> struct vm_area_struct *vma = vmf->vma;
>
> if (userfaultfd_pte_wp(vma, *vmf->pte)) {
> pte_unmap_unlock(vmf->pte, vmf->ptl);
> return handle_userfault(vmf, VM_UFFD_WP);
> }
>
> return do_wp_page_cont(vmf);
> }
>
> Then I can probably use do_wp_page_cont() in this patch.

Instead i would keep the do_wp_page name and do:
static vm_fault_t do_userfaultfd_wp_page(struct vm_fault *vmf) {
... // what you have above
return do_wp_page(vmf);
}

Naming wise i think it would be better to keep do_wp_page() as
is.

Cheers,
Jérôme

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