Messages in this thread Patch in this message | | | Subject | Re: [PATCH v4 3/3] mm: fix double page fault on arm64 if PTE_AF is cleared | From | Jia He <> | Date | Thu, 19 Sep 2019 10:16:34 +0800 |
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Hi Kirill
[On behalf of justin.he@arm.com 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@arm.com> >> Signed-off-by: Jia He <justin.he@arm.com> >> --- >> 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?
--- Cheers, Justin (Jia He)
> >> + 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 >> >> --
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