[v2] mm: fix double page fault on arm64 if PTE_AF is cleared
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Message ID 20190906135747.211836-1-justin.he@arm.com
State New
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Series
  • [v2] mm: fix double page fault on arm64 if PTE_AF is cleared
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Commit Message

Jia He Sept. 6, 2019, 1:57 p.m. UTC
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>
---
Changes
v2: remove FAULT_FLAG_WRITE when setting pte access flag (by Catalin)

 mm/memory.c | 21 ++++++++++++++++-----
 1 file changed, 16 insertions(+), 5 deletions(-)

Comments

Matthew Wilcox Sept. 6, 2019, 2:57 p.m. UTC | #1
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);
> +		}
> +
Matthew Wilcox Sept. 9, 2019, 9:27 p.m. UTC | #2
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?
Catalin Marinas Sept. 10, 2019, 9:08 a.m. UTC | #3
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 Marinas Sept. 10, 2019, 9:15 a.m. UTC | #4
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_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.

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.
Kirill A. Shutemov Sept. 10, 2019, 9:29 a.m. UTC | #5
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):
> [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>
> ---
> 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.

Patch
diff mbox series

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, &memcg, false))