| Message ID | 20190920135437.25622-4-justin.he@arm.com (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | fix double page fault on arm64 | expand |
On Fri, Sep 20, 2019 at 09:54:37PM +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 > > Reported-by: Yibo Cai <Yibo.Cai@arm.com> > Signed-off-by: Jia He <justin.he@arm.com> Acked-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Thanks for your patent review
On Fri, Sep 20, 2019 at 09:54:37PM +0800, Jia He wrote: > -static inline void cow_user_page(struct page *dst, struct page *src, unsigned long va, struct vm_area_struct *vma) > +static inline int cow_user_page(struct page *dst, struct page *src, > + struct vm_fault *vmf) > { Can we talk about the return type here? > + } else { > + /* 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. > + */ > + pte_unmap_unlock(vmf->pte, vmf->ptl); > + return -1; ... > + return 0; > } So -1 for "try again" and 0 for "succeeded". > + if (cow_user_page(new_page, old_page, vmf)) { Then we use it like a bool. But it's kind of backwards from a bool because false is success. > + /* 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; And we don't use the return value; in fact we invert it. Would this make more sense: static inline bool cow_user_page(struct page *dst, struct page *src, struct vm_fault *vmf) ... pte_unmap_unlock(vmf->pte, vmf->ptl); return false; ... return true; ... if (!cow_user_page(new_page, old_page, vmf)) { That reads more sensibly for me. We could also go with returning a vm_fault_t, but that would be more complex than needed today, I think.
On Fri, Sep 20, 2019 at 08:53:00AM -0700, Matthew Wilcox wrote: > On Fri, Sep 20, 2019 at 09:54:37PM +0800, Jia He wrote: > > -static inline void cow_user_page(struct page *dst, struct page *src, unsigned long va, struct vm_area_struct *vma) > > +static inline int cow_user_page(struct page *dst, struct page *src, > > + struct vm_fault *vmf) > > { > > Can we talk about the return type here? > > > + } else { > > + /* 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. > > + */ > > + pte_unmap_unlock(vmf->pte, vmf->ptl); > > + return -1; > ... > > + return 0; > > } > > So -1 for "try again" and 0 for "succeeded". > > > + if (cow_user_page(new_page, old_page, vmf)) { > > Then we use it like a bool. But it's kind of backwards from a bool because > false is success. > > > + /* 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; > > And we don't use the return value; in fact we invert it. > > Would this make more sense: > > static inline bool cow_user_page(struct page *dst, struct page *src, > struct vm_fault *vmf) > ... > pte_unmap_unlock(vmf->pte, vmf->ptl); > return false; > ... > return true; > ... > if (!cow_user_page(new_page, old_page, vmf)) { > > That reads more sensibly for me. I like this idea too.
[On behalf of justin.he@arm.com] Hi Matthew On 2019/9/20 23:53, Matthew Wilcox wrote: > On Fri, Sep 20, 2019 at 09:54:37PM +0800, Jia He wrote: >> -static inline void cow_user_page(struct page *dst, struct page *src, unsigned long va, struct vm_area_struct *vma) >> +static inline int cow_user_page(struct page *dst, struct page *src, >> + struct vm_fault *vmf) >> { > Can we talk about the return type here? > >> + } else { >> + /* 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. >> + */ >> + pte_unmap_unlock(vmf->pte, vmf->ptl); >> + return -1; > ... >> + return 0; >> } > So -1 for "try again" and 0 for "succeeded". > >> + if (cow_user_page(new_page, old_page, vmf)) { > Then we use it like a bool. But it's kind of backwards from a bool because > false is success. > >> + /* 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; > And we don't use the return value; in fact we invert it. > > Would this make more sense: > > static inline bool cow_user_page(struct page *dst, struct page *src, > struct vm_fault *vmf) > ... > pte_unmap_unlock(vmf->pte, vmf->ptl); > return false; > ... > return true; > ... > if (!cow_user_page(new_page, old_page, vmf)) { > > That reads more sensibly for me. We could also go with returning a > vm_fault_t, but that would be more complex than needed today, I think. Ok, will change the return type to bool as you suggested. Thanks --- Cheers, Justin (Jia He)
diff --git a/mm/memory.c b/mm/memory.c index e2bb51b6242e..3e39e40fee87 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 +2147,13 @@ 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 int cow_user_page(struct page *dst, struct page *src, + struct vm_fault *vmf) { + struct vm_area_struct *vma = vmf->vma; + struct mm_struct *mm = vma->vm_mm; + unsigned long addr = vmf->address; + debug_dma_assert_idle(src); /* @@ -2151,21 +2163,53 @@ static inline void cow_user_page(struct page *dst, struct page *src, unsigned lo * 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); + void *kaddr; + pte_t entry; + void __user *uaddr = (void __user *)(addr & PAGE_MASK); + /* On architectures with software "accessed" bits, we would + * take a double page fault, so mark it accessed here. + */ + if (arch_faults_on_old_pte() && !pte_young(vmf->orig_pte)) { + vmf->pte = pte_offset_map_lock(mm, vmf->pmd, addr, + &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); + } else { + /* 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. + */ + pte_unmap_unlock(vmf->pte, vmf->ptl); + return -1; + } + pte_unmap_unlock(vmf->pte, vmf->ptl); + } + + kaddr = kmap_atomic(dst); /* * 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)) + if (__copy_from_user_inatomic(kaddr, uaddr, PAGE_SIZE)) { + /* Give a warn in case there can be some obscure + * use-case + */ + WARN_ON_ONCE(1); 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); + + return 0; } static gfp_t __get_fault_gfp_mask(struct vm_area_struct *vma) @@ -2318,7 +2362,18 @@ 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))
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 Reported-by: Yibo Cai <Yibo.Cai@arm.com> Signed-off-by: Jia He <justin.he@arm.com> --- mm/memory.c | 67 ++++++++++++++++++++++++++++++++++++++++++++++++----- 1 file changed, 61 insertions(+), 6 deletions(-)