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 | 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); + 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
