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

[Justin He (Arm Technology China)]

Hi Kirill
Thanks for the detailed explanation.

--
Cheers,
Justin (Jia He)



> -----Original Message-----
> From: Kirill A. Shutemov <kirill@xxxxxxxxxxxxx>
> Sent: 2019å9æ19æ 22:58
> To: Jia He <hejianet@xxxxxxxxx>
> Cc: Justin He (Arm Technology China) <Justin.He@xxxxxxx>; Catalin
> Marinas <Catalin.Marinas@xxxxxxx>; Will Deacon <will@xxxxxxxxxx>; Mark
> Rutland <Mark.Rutland@xxxxxxx>; James Morse
> <James.Morse@xxxxxxx>; Marc Zyngier <maz@xxxxxxxxxx>; Matthew
> Wilcox <willy@xxxxxxxxxxxxx>; Kirill A. Shutemov
> <kirill.shutemov@xxxxxxxxxxxxxxx>; linux-arm-kernel@xxxxxxxxxxxxxxxxxxx;
> linux-kernel@xxxxxxxxxxxxxxx; linux-mm@xxxxxxxxx; Suzuki Poulose
> <Suzuki.Poulose@xxxxxxx>; Punit Agrawal <punitagrawal@xxxxxxxxx>;
> Anshuman Khandual <Anshuman.Khandual@xxxxxxx>; Jun Yao
> <yaojun8558363@xxxxxxxxx>; Alex Van Brunt <avanbrunt@xxxxxxxxxx>;
> Robin Murphy <Robin.Murphy@xxxxxxx>; Thomas Gleixner
> <tglx@xxxxxxxxxxxxx>; Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>;
> JÃrÃme Glisse <jglisse@xxxxxxxxxx>; Ralph Campbell
> <rcampbell@xxxxxxxxxx>; Kaly Xin (Arm Technology China)
> <Kaly.Xin@xxxxxxx>
> Subject: Re: [PATCH v4 3/3] mm: fix double page fault on arm64 if PTE_AF is
> cleared
>
> On Thu, Sep 19, 2019 at 10:16:34AM +0800, Jia He wrote:
> > Hi Kirill
> >
> > [On behalf of justin.he@xxxxxxx 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.Cai@xxxxxxx>
> > > > Signed-off-by: Jia He <justin.he@xxxxxxx>
> > > > ---
> > > >   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?
>
> No. VM_FAULT_RETRY has different semantics: we have to drop
> mmap_sem(), so
> let's try to take it again and handle the fault. In this case the more
> likely scenario is that other thread has already handled the fault and we
> don't need to do anything. If it's not the case, the fault will be
> triggered again on the same address.
>
> --
>  Kirill A. Shutemov
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