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

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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.

Thanks,

-- 
Peter Xu




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