RE: [PATCH v3 2/2] mm: fix double page fault on arm64 if PTE_AF is cleared

["Justin He (Arm Technology China)" <Justin.He@arm.com>]

Hi Kirill
> -----Original Message-----
> From: Kirill A. Shutemov <kirill@shutemov.name>
> Sent: 2019年9月16日 17:16
> To: Justin He (Arm Technology China) <Justin.He@arm.com>
> Cc: Catalin Marinas <Catalin.Marinas@arm.com>; Will Deacon
> <will@kernel.org>; Mark Rutland <Mark.Rutland@arm.com>; James Morse
> <James.Morse@arm.com>; Marc Zyngier <maz@kernel.org>; Matthew
> Wilcox <willy@infradead.org>; Kirill A. Shutemov
> <kirill.shutemov@linux.intel.com>; linux-arm-kernel@lists.infradead.org;
> linux-kernel@vger.kernel.org; linux-mm@kvack.org; Punit Agrawal
> <punitagrawal@gmail.com>; Anshuman Khandual
> <Anshuman.Khandual@arm.com>; Jun Yao <yaojun8558363@gmail.com>;
> Alex Van Brunt <avanbrunt@nvidia.com>; Robin Murphy
> <Robin.Murphy@arm.com>; Thomas Gleixner <tglx@linutronix.de>;
> Andrew Morton <akpm@linux-foundation.org>; Jérôme Glisse
> <jglisse@redhat.com>; Ralph Campbell <rcampbell@nvidia.com>;
> hejianet@gmail.com
> Subject: Re: [PATCH v3 2/2] mm: fix double page fault on arm64 if PTE_AF
> is cleared
>
> On Sat, Sep 14, 2019 at 12:32:39AM +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@arm.com>
> > Signed-off-by: Jia He <justin.he@arm.com>
> > ---
> >  mm/memory.c | 30 +++++++++++++++++++++++++-----
> >  1 file changed, 25 insertions(+), 5 deletions(-)
> >
> > diff --git a/mm/memory.c b/mm/memory.c
> > index e2bb51b6242e..a64af6495f71 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;
> > @@ -2140,7 +2147,8 @@ 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)
> >  {
> >     debug_dma_assert_idle(src);
> >
> > @@ -2152,20 +2160,32 @@ 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 *)(vmf->address &
> 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. If PTE_AF is cleared on arm64, it might
> > +            * cause double page fault. So makes pte young here
> >              */
> > +           if (arch_faults_on_old_pte() && !pte_young(vmf->orig_pte))
> {
> > +                   spin_lock(vmf->ptl);
> > +                   entry = pte_mkyoung(vmf->orig_pte);
>
> Should't you re-validate that orig_pte after re-taking ptl? It can be
> stale by now.
Thanks, do you mean flush_cache_page(vma, vmf->address, pte_pfn(vmf->orig_pte))
before pte_mkyoung?

--
Cheers,
Justin (Jia He)


>
> > +                   if (ptep_set_access_flags(vmf->vma, vmf->address,
> > +                                             vmf->pte, entry, 0))
> > +                           update_mmu_cache(vmf->vma, vmf-
> >address,
> > +                                            vmf->pte);
> > +                   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, vmf->address, vmf->vma);
> >  }
> >
> >  static gfp_t __get_fault_gfp_mask(struct vm_area_struct *vma)
> > @@ -2318,7 +2338,7 @@ 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
> >
> >
>
> --
>  Kirill A. Shutemov
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