Re: [PATCH v4 3/3] mm: fix double page fault on arm64 if PTE_AF is cleared

[Jia He]

Hi Kirill

[On behalf of justin...@arm.com because some mails are filted...]

On 2019/9/18 22:00, Kirill A. Shutemov wrote:

On Wed, Sep 18, 2019 at 09:19:14PM +0800, Jia He wrote:

When we tested pmdk unit test [1] vmmalloc_fork TEST1 in arm64 guest, there
will be a double page fault in __copy_from_user_inatomic of cow_user_page.

Below call trace is from arm64 do_page_fault for debugging purpose
[  110.016195] Call trace:
[  110.016826]  do_page_fault+0x5a4/0x690
[  110.017812]  do_mem_abort+0x50/0xb0
[  110.018726]  el1_da+0x20/0xc4
[  110.019492]  __arch_copy_from_user+0x180/0x280
[  110.020646]  do_wp_page+0xb0/0x860
[  110.021517]  __handle_mm_fault+0x994/0x1338
[  110.022606]  handle_mm_fault+0xe8/0x180
[  110.023584]  do_page_fault+0x240/0x690
[  110.024535]  do_mem_abort+0x50/0xb0
[  110.025423]  el0_da+0x20/0x24

The pte info before __copy_from_user_inatomic is (PTE_AF is cleared):
[ffff9b007000] pgd=000000023d4f8003, pud=000000023da9b003, 
pmd=000000023d4b3003, pte=360000298607bd3

As told by Catalin: "On arm64 without hardware Access Flag, copying from
user will fail because the pte is old and cannot be marked young. So we
always end up with zeroed page after fork() + CoW for pfn mappings. we
don't always have a hardware-managed access flag on arm64."

This patch fix it by calling pte_mkyoung. Also, the parameter is
changed because vmf should be passed to cow_user_page()

[1] https://github.com/pmem/pmdk/tree/master/src/test/vmmalloc_fork

Reported-by: Yibo Cai <yibo....@arm.com>
Signed-off-by: Jia He <justin...@arm.com>
---
  mm/memory.c | 35 ++++++++++++++++++++++++++++++-----
  1 file changed, 30 insertions(+), 5 deletions(-)

diff --git a/mm/memory.c b/mm/memory.c
index e2bb51b6242e..d2c130a5883b 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -118,6 +118,13 @@ int randomize_va_space __read_mostly =
                                        2;
  #endif

+#ifndef arch_faults_on_old_pte

+static inline bool arch_faults_on_old_pte(void)
+{
+       return false;
+}
+#endif
+
  static int __init disable_randmaps(char *s)
  {
        randomize_va_space = 0;
@@ -2140,8 +2147,12 @@ static inline int pte_unmap_same(struct mm_struct *mm, 
pmd_t *pmd,
        return same;
  }

-static inline void cow_user_page(struct page *dst, struct page *src, unsigned long va, struct vm_area_struct *vma)

+static inline void cow_user_page(struct page *dst, struct page *src,
+                                struct vm_fault *vmf)
  {
+       struct vm_area_struct *vma = vmf->vma;
+       unsigned long addr = vmf->address;
+
        debug_dma_assert_idle(src);

/*

@@ -2152,20 +2163,34 @@ static inline void cow_user_page(struct page *dst, 
struct page *src, unsigned lo
         */
        if (unlikely(!src)) {
                void *kaddr = kmap_atomic(dst);
-               void __user *uaddr = (void __user *)(va & PAGE_MASK);
+               void __user *uaddr = (void __user *)(addr & PAGE_MASK);
+               pte_t entry;

/*

                 * This really shouldn't fail, because the page is there
                 * in the page tables. But it might just be unreadable,
                 * in which case we just give up and fill the result with
-                * zeroes.
+                * zeroes. On architectures with software "accessed" bits,
+                * we would take a double page fault here, so mark it
+                * accessed here.
                 */
+               if (arch_faults_on_old_pte() && !pte_young(vmf->orig_pte)) {
+                       spin_lock(vmf->ptl);
+                       if (likely(pte_same(*vmf->pte, vmf->orig_pte))) {
+                               entry = pte_mkyoung(vmf->orig_pte);
+                               if (ptep_set_access_flags(vma, addr,
+                                                         vmf->pte, entry, 0))
+                                       update_mmu_cache(vma, addr, vmf->pte);
+                       }

I don't follow.

So if pte has changed under you, you don't set the accessed bit, but never
the less copy from the user.

What makes you think it will not trigger the same problem?

I think we need to make cow_user_page() fail in this case and caller --
wp_page_copy() -- return zero. If the fault was solved by other thread, we
are fine. If not userspace would re-fault on the same address and we will
handle the fault from the second attempt.

Thanks for the pointing. How about make cow_user_page() be returned

VM_FAULT_RETRY? Then in do_page_fault(), it can retry the page fault?

---
Cheers,
Justin (Jia He)

+                       spin_unlock(vmf->ptl);
+               }
+
                if (__copy_from_user_inatomic(kaddr, uaddr, PAGE_SIZE))
                        clear_page(kaddr);
                kunmap_atomic(kaddr);
                flush_dcache_page(dst);
        } else
-               copy_user_highpage(dst, src, va, vma);
+               copy_user_highpage(dst, src, addr, vma);
  }

static gfp_t __get_fault_gfp_mask(struct vm_area_struct *vma)

@@ -2318,7 +2343,7 @@ static vm_fault_t wp_page_copy(struct vm_fault *vmf)
                                vmf->address);
                if (!new_page)
                        goto oom;
-               cow_user_page(new_page, old_page, vmf->address, vma);
+               cow_user_page(new_page, old_page, vmf);
        }

if (mem_cgroup_try_charge_delay(new_page, mm, GFP_KERNEL, &memcg, false))

--
2.17.1

--