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. > + 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 > > -- Kirill A. Shutemov
