Re: [PATCH v2] mm: fix double page fault on arm64 if PTE_AF is cleared
On Fri, Sep 06, 2019 at 09:57:47PM +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):
> [9b007000] pgd=00023d4f8003, pud=00023da9b003,
> pmd=00023d4b3003, pte=36298607bd3
>
> 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
> Signed-off-by: Jia He
> ---
> Changes
> v2: remove FAULT_FLAG_WRITE when setting pte access flag (by Catalin)
>
> mm/memory.c | 21 -
> 1 file changed, 16 insertions(+), 5 deletions(-)
>
> diff --git a/mm/memory.c b/mm/memory.c
> index e2bb51b6242e..63d4fd285e8e 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -2140,7 +2140,8 @@ 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 +2153,30 @@ 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 (!pte_young(vmf->orig_pte)) {
> + 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);
> + }
> +
I don't see where you take ptl.
--
Kirill A. Shutemov
Re: [PATCH v2] mm: fix double page fault on arm64 if PTE_AF is cleared
On Fri, Sep 06, 2019 at 07:57:42AM -0700, Matthew Wilcox wrote:
> On Fri, Sep 06, 2019 at 09:57:47PM +0800, Jia He wrote:
> > * 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
>
> How about:
>* zeroes. On architectures with software "accessed" bits,
>* we would take a double page fault here, so mark it
>* accessed here.
>
> > */
> > + if (!pte_young(vmf->orig_pte)) {
>
> Let's guard this with:
>
> if (arch_sw_access_bit && !pte_young(vmf->orig_pte)) {
>
> #define arch_sw_access_bit0
> by default and have arm64 override it (either to a variable or a constant
> ... your choice). Also, please somebody decide on a better name than
> arch_sw_access_bit.
I'm not good at names either (is arch_faults_on_old_pte any better?) but
I'd make this a 0 args call: arch_sw_access_bit(). This way we can make
it a static inline function on arm64 with some static label check.
--
Catalin
Re: [PATCH v2] mm: fix double page fault on arm64 if PTE_AF is cleared
On Mon, Sep 09, 2019 at 02:27:12PM -0700, Matthew Wilcox wrote:
> On Fri, Sep 06, 2019 at 09:57:47PM +0800, Jia He wrote:
> > + if (!pte_young(vmf->orig_pte)) {
> > + 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);
> > + }
> > +
>
> Oh, btw, why call update_mmu_cache() here? All you've done is changed
> the 'accessed' bit. What is any architecture supposed to do in response
> to this?
For arm64 and x86 that's a no-op but an architecture with software TLBs
may preload them to avoid a subsequent fault on access after the pte was
made young.
--
Catalin
Re: [PATCH v2] mm: fix double page fault on arm64 if PTE_AF is cleared
On Fri, Sep 06, 2019 at 09:57:47PM +0800, Jia He wrote:
> + if (!pte_young(vmf->orig_pte)) {
> + 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);
> + }
> +
Oh, btw, why call update_mmu_cache() here? All you've done is changed
the 'accessed' bit. What is any architecture supposed to do in response
to this?
Re: [PATCH v2] mm: fix double page fault on arm64 if PTE_AF is cleared
On Fri, Sep 06, 2019 at 09:57:47PM +0800, Jia He wrote:
>* 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
How about:
* zeroes. On architectures with software "accessed" bits,
* we would take a double page fault here, so mark it
* accessed here.
>*/
> + if (!pte_young(vmf->orig_pte)) {
Let's guard this with:
if (arch_sw_access_bit && !pte_young(vmf->orig_pte)) {
#define arch_sw_access_bit 0
by default and have arm64 override it (either to a variable or a constant
... your choice). Also, please somebody decide on a better name than
arch_sw_access_bit.
> + entry = pte_mkyoung(vmf->orig_pte);
> + if (ptep_set_access_flags(vmf->vma, vmf->address,
> + vmf->pte, entry, 0))
This indentation is wrong; it makes vmf->pte look like part of the subsequent
statement instead of part of the condition.
> + update_mmu_cache(vmf->vma, vmf->address,
> + vmf->pte);
> + }
> +
[PATCH v2] mm: fix double page fault on arm64 if PTE_AF is cleared
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):
[9b007000] pgd=00023d4f8003, pud=00023da9b003,
pmd=00023d4b3003, pte=36298607bd3
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
Signed-off-by: Jia He
---
Changes
v2: remove FAULT_FLAG_WRITE when setting pte access flag (by Catalin)
mm/memory.c | 21 -
1 file changed, 16 insertions(+), 5 deletions(-)
diff --git a/mm/memory.c b/mm/memory.c
index e2bb51b6242e..63d4fd285e8e 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -2140,7 +2140,8 @@ 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 +2153,30 @@ 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 (!pte_young(vmf->orig_pte)) {
+ 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);
+ }
+
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 +2329,7 @@ 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, ,
false))
--
2.17.1
