On Wed, Sep 25, 2019 at 10:59:22AM +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() > > Add a WARN_ON_ONCE when __copy_from_user_inatomic() returns error > in case there can be some obscure use-case.(by Kirill) > > [1] https://github.com/pmem/pmdk/tree/master/src/test/vmmalloc_fork > > Signed-off-by: Jia He <justin.he@xxxxxxx> > Reported-by: Yibo Cai <Yibo.Cai@xxxxxxx> Reviewed-by: Catalin Marinas <catalin.marinas@xxxxxxx> > mm/memory.c | 99 +++++++++++++++++++++++++++++++++++++++++++++-------- > 1 file changed, 84 insertions(+), 15 deletions(-) > > diff --git a/mm/memory.c b/mm/memory.c > index e2bb51b6242e..a0a381b36ff2 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; > @@ -2140,32 +2147,82 @@ 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 bool cow_user_page(struct page *dst, struct page *src, > + struct vm_fault *vmf) > { > + bool ret; > + void *kaddr; > + void __user *uaddr; > + bool force_mkyoung; > + struct vm_area_struct *vma = vmf->vma; > + struct mm_struct *mm = vma->vm_mm; > + unsigned long addr = vmf->address; > + > debug_dma_assert_idle(src); > > + if (likely(src)) { > + copy_user_highpage(dst, src, addr, vma); > + return true; > + } > + > /* > * If the source page was a PFN mapping, we don't have > * a "struct page" for it. We do a best-effort copy by > * just copying from the original user address. If that > * fails, we just zero-fill it. Live with it. > */ > - if (unlikely(!src)) { > - void *kaddr = kmap_atomic(dst); > - void __user *uaddr = (void __user *)(va & PAGE_MASK); > + kaddr = kmap_atomic(dst); > + uaddr = (void __user *)(addr & PAGE_MASK); > + > + /* > + * On architectures with software "accessed" bits, we would > + * take a double page fault, so mark it accessed here. > + */ > + force_mkyoung = arch_faults_on_old_pte() && !pte_young(vmf->orig_pte); > + if (force_mkyoung) { > + pte_t entry; > + > + vmf->pte = pte_offset_map_lock(mm, vmf->pmd, addr, &vmf->ptl); > + if (!likely(pte_same(*vmf->pte, vmf->orig_pte))) { > + /* > + * 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. > + */ > + ret = false; > + goto pte_unlock; > + } > + > + entry = pte_mkyoung(vmf->orig_pte); > + if (ptep_set_access_flags(vma, addr, vmf->pte, entry, 0)) > + update_mmu_cache(vma, addr, vmf->pte); > + } > > + /* > + * 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. > + */ > + if (__copy_from_user_inatomic(kaddr, uaddr, PAGE_SIZE)) { > /* > - * 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. > + * Give a warn in case there can be some obscure > + * use-case > */ > - 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); > + WARN_ON_ONCE(1); > + clear_page(kaddr); > + } > + > + ret = true; > + > +pte_unlock: > + if (force_mkyoung) > + pte_unmap_unlock(vmf->pte, vmf->ptl); > + kunmap_atomic(kaddr); > + flush_dcache_page(dst); > + > + return ret; > } > > static gfp_t __get_fault_gfp_mask(struct vm_area_struct *vma) > @@ -2318,7 +2375,19 @@ 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); > + > + if (!cow_user_page(new_page, old_page, vmf)) { > + /* > + * COW failed, if the fault was solved by other, > + * it's fine. If not, userspace would re-fault on > + * the same address and we will handle the fault > + * from the second attempt. > + */ > + put_page(new_page); > + if (old_page) > + put_page(old_page); > + return 0; > + } > } > > if (mem_cgroup_try_charge_delay(new_page, mm, GFP_KERNEL, &memcg, false)) > -- > 2.17.1 > -- Catalin
