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SubjectRe: [PATCH v3 14/28] userfaultfd: wp: handle COW properly for uffd-wp
On Tue, Apr 23, 2019 at 11:34:56AM -0400, Jerome Glisse wrote:
> On Tue, Apr 23, 2019 at 11:00:30AM +0800, Peter Xu wrote:
> > On Mon, Apr 22, 2019 at 10:54:02AM -0400, Jerome Glisse wrote:
> > > 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.
> >
> > In case I misunderstood... what I've proposed will be simply:
> >
> > diff --git a/mm/memory.c b/mm/memory.c
> > index 64bd8075f054..ab98a1eb4702 100644
> > --- a/mm/memory.c
> > +++ b/mm/memory.c
> > @@ -2497,6 +2497,14 @@ static vm_fault_t do_wp_page(struct vm_fault *vmf)
> > return handle_userfault(vmf, VM_UFFD_WP);
> > }
> >
> > + return do_wp_page_cont(vmf);
> > +}
> > +
> > +vm_fault_t do_wp_page_cont(struct vm_fault *vmf)
> > + __releases(vmf->ptl)
> > +{
> > + struct vm_area_struct *vma = vmf->vma;
> > +
> > vmf->page = vm_normal_page(vma, vmf->address, vmf->orig_pte);
> > if (!vmf->page) {
> > /*
> >
> > And the other proposal is:
> >
> > diff --git a/mm/memory.c b/mm/memory.c
> > index 64bd8075f054..a73792127553 100644
> > --- a/mm/memory.c
> > +++ b/mm/memory.c
> > @@ -2469,6 +2469,8 @@ static vm_fault_t wp_page_shared(struct vm_fault *vmf)
> > return VM_FAULT_WRITE;
> > }
> >
> > +static vm_fault_t do_wp_page(struct vm_fault *vmf);
> > +
> > /*
> > * This routine handles present pages, when users try to write
> > * to a shared page. It is done by copying the page to a new address
> > @@ -2487,7 +2489,7 @@ static vm_fault_t wp_page_shared(struct vm_fault *vmf)
> > * but allow concurrent faults), with pte both mapped and locked.
> > * We return with mmap_sem still held, but pte unmapped and unlocked.
> > */
> > -static vm_fault_t do_wp_page(struct vm_fault *vmf)
> > +static vm_fault_t do_userfaultfd_wp_page(struct vm_fault *vmf)
> > __releases(vmf->ptl)
> > {
> > struct vm_area_struct *vma = vmf->vma;
> > @@ -2497,6 +2499,14 @@ static vm_fault_t do_wp_page(struct vm_fault *vmf)
> > return handle_userfault(vmf, VM_UFFD_WP);
> > }
> >
> > + return do_wp_page(vmf);
> > +}
> > +
> > +static vm_fault_t do_wp_page(struct vm_fault *vmf)
> > + __releases(vmf->ptl)
> > +{
> > + struct vm_area_struct *vma = vmf->vma;
> > +
> > vmf->page = vm_normal_page(vma, vmf->address, vmf->orig_pte);
> > if (!vmf->page) {
> > /*
> > @@ -2869,7 +2879,7 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
> > }
> >
> > if (vmf->flags & FAULT_FLAG_WRITE) {
> > - ret |= do_wp_page(vmf);
> > + ret |= do_userfaultfd_wp_page(vmf);
> > if (ret & VM_FAULT_ERROR)
> > ret &= VM_FAULT_ERROR;
> > goto out;
> > @@ -3831,7 +3841,7 @@ static vm_fault_t handle_pte_fault(struct vm_fault *vmf)
> > goto unlock;
> > if (vmf->flags & FAULT_FLAG_WRITE) {
> > if (!pte_write(entry))
> > - return do_wp_page(vmf);
> > + return do_userfaultfd_wp_page(vmf);
> > entry = pte_mkdirty(entry);
> > }
> > entry = pte_mkyoung(entry);
> >
> > I would prefer the 1st approach since it not only contains fewer lines
> > of changes because it does not touch callers, and also the naming in
> > the 2nd approach can be a bit confusing (calling
> > do_userfaultfd_wp_page in handle_pte_fault may let people think of an
> > userfault-only path but actually it covers the general path). But if
> > you really like the 2nd one I can use that too.
>
> Maybe move the userfaultfd code to a small helper, call it first in
> call site of do_wp_page() and do_wp_page() if it does not fire ie:
>
> bool do_userfaultfd_wp(struct vm_fault *vmf, int ret)
> {
> if (handleuserfault) return true;
> return false;
> }
>
> then
> if (vmf->flags & FAULT_FLAG_WRITE) {
> if (do_userfaultfd_wp(vmf, tmp)) {
> ret |= tmp;
> } else
> ret |= do_wp_page(vmf);
> if (ret & VM_FAULT_ERROR)
> ret &= VM_FAULT_ERROR;
> goto out;
>
> and:
> if (vmf->flags & FAULT_FLAG_WRITE) {
> if (!pte_write(entry)) {
> if (do_userfaultfd_wp(vmf, ret))
> return ret;
> else
> return do_wp_page(vmf);
> }

But then we will be duplicating the code patterns somehow? :-/

I'll think them over... Thanks for all these suggestions!

--
Peter Xu

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