Re: [PATCH v9 23/24] x86/mm: Add speculative pagefault handling

[David Rientjes]

On Tue, 13 Mar 2018, Laurent Dufour wrote:

> diff --git a/arch/x86/mm/fault.c b/arch/x86/mm/fault.c
> index e6af2b464c3d..a73cf227edd6 100644
> --- a/arch/x86/mm/fault.c
> +++ b/arch/x86/mm/fault.c
> @@ -1239,6 +1239,9 @@ __do_page_fault(struct pt_regs *regs, unsigned long 
> error_code,
>   unsigned long address)
>  {
>   struct vm_area_struct *vma;
> +#ifdef CONFIG_SPECULATIVE_PAGE_FAULT
> + struct vm_area_struct *spf_vma = NULL;
> +#endif
>   struct task_struct *tsk;
>   struct mm_struct *mm;
>   int fault, major = 0;
> @@ -1332,6 +1335,27 @@ __do_page_fault(struct pt_regs *regs, unsigned long 
> error_code,
>   if (error_code & X86_PF_INSTR)
>   flags |= FAULT_FLAG_INSTRUCTION;
>  
> +#ifdef CONFIG_SPECULATIVE_PAGE_FAULT
> + if ((error_code & X86_PF_USER) && (atomic_read(>mm_users) > 1)) {
> + fault = handle_speculative_fault(mm, address, flags,
> +  _vma);
> +
> + if (!(fault & VM_FAULT_RETRY)) {
> + if (!(fault & VM_FAULT_ERROR)) {
> + perf_sw_event(PERF_COUNT_SW_SPF, 1,
> +   regs, address);
> + goto done;
> + }
> + /*
> +  * In case of error we need the pkey value, but
> +  * can't get it from the spf_vma as it is only returned
> +  * when VM_FAULT_RETRY is returned. So we have to
> +  * retry the page fault with the mmap_sem grabbed.
> +  */
> + }
> + }
> +#endif /* CONFIG_SPECULATIVE_PAGE_FAULT */

All the comments from the powerpc version will apply here as well, the 
only interesting point is whether VM_FAULT_FALLBACK can be returned from 
handle_speculative_fault() to indicate its not possible.

> +
>   /*
>* When running in the kernel we expect faults to occur only to
>* addresses in user space.  All other faults represent errors in
> @@ -1365,7 +1389,16 @@ __do_page_fault(struct pt_regs *regs, unsigned long 
> error_code,
>   might_sleep();
>   }
>  
> - vma = find_vma(mm, address);
> +#ifdef CONFIG_SPECULATIVE_PAGE_FAULT
> + if (spf_vma) {
> + if (can_reuse_spf_vma(spf_vma, address))
> + vma = spf_vma;
> + else
> + vma = find_vma(mm, address);
> + spf_vma = NULL;
> + } else
> +#endif
> + vma = find_vma(mm, address);
>   if (unlikely(!vma)) {
>   bad_area(regs, error_code, address);
>   return;
> @@ -1451,6 +1484,9 @@ __do_page_fault(struct pt_regs *regs, unsigned long 
> error_code,
>   return;
>   }
>  
> +#ifdef CONFIG_SPECULATIVE_PAGE_FAULT
> +done:
> +#endif
>   /*
>* Major/minor page fault accounting. If any of the events
>* returned VM_FAULT_MAJOR, we account it as a major fault.

[PATCH v9 23/24] x86/mm: Add speculative pagefault handling

[Laurent Dufour]

From: Peter Zijlstra 

Try a speculative fault before acquiring mmap_sem, if it returns with
VM_FAULT_RETRY continue with the mmap_sem acquisition and do the
traditional fault.

Signed-off-by: Peter Zijlstra (Intel) 

[Clearing of FAULT_FLAG_ALLOW_RETRY is now done in
 handle_speculative_fault()]
[Retry with usual fault path in the case VM_ERROR is returned by
 handle_speculative_fault(). This allows signal to be delivered]
[Don't build SPF call if !CONFIG_SPECULATIVE_PAGE_FAULT]
[Try speculative fault path only for multi threaded processes]
[Try to the VMA fetch during the speculative path in case of retry]
Signed-off-by: Laurent Dufour 
---
 arch/x86/mm/fault.c | 38 +-
 1 file changed, 37 insertions(+), 1 deletion(-)

diff --git a/arch/x86/mm/fault.c b/arch/x86/mm/fault.c
index e6af2b464c3d..a73cf227edd6 100644
--- a/arch/x86/mm/fault.c
+++ b/arch/x86/mm/fault.c
@@ -1239,6 +1239,9 @@ __do_page_fault(struct pt_regs *regs, unsigned long 
error_code,
unsigned long address)
 {
struct vm_area_struct *vma;
+#ifdef CONFIG_SPECULATIVE_PAGE_FAULT
+   struct vm_area_struct *spf_vma = NULL;
+#endif
struct task_struct *tsk;
struct mm_struct *mm;
int fault, major = 0;
@@ -1332,6 +1335,27 @@ __do_page_fault(struct pt_regs *regs, unsigned long 
error_code,
if (error_code & X86_PF_INSTR)
flags |= FAULT_FLAG_INSTRUCTION;
 
+#ifdef CONFIG_SPECULATIVE_PAGE_FAULT
+   if ((error_code & X86_PF_USER) && (atomic_read(>mm_users) > 1)) {
+   fault = handle_speculative_fault(mm, address, flags,
+_vma);
+
+   if (!(fault & VM_FAULT_RETRY)) {
+   if (!(fault & VM_FAULT_ERROR)) {
+   perf_sw_event(PERF_COUNT_SW_SPF, 1,
+ regs, address);
+   goto done;
+   }
+   /*
+* In case of error we need the pkey value, but
+* can't get it from the spf_vma as it is only returned
+* when VM_FAULT_RETRY is returned. So we have to
+* retry the page fault with the mmap_sem grabbed.
+*/
+   }
+   }
+#endif /* CONFIG_SPECULATIVE_PAGE_FAULT */
+
/*
 * When running in the kernel we expect faults to occur only to
 * addresses in user space.  All other faults represent errors in
@@ -1365,7 +1389,16 @@ __do_page_fault(struct pt_regs *regs, unsigned long 
error_code,
might_sleep();
}
 
-   vma = find_vma(mm, address);
+#ifdef CONFIG_SPECULATIVE_PAGE_FAULT
+   if (spf_vma) {
+   if (can_reuse_spf_vma(spf_vma, address))
+   vma = spf_vma;
+   else
+   vma = find_vma(mm, address);
+   spf_vma = NULL;
+   } else
+#endif
+   vma = find_vma(mm, address);
if (unlikely(!vma)) {
bad_area(regs, error_code, address);
return;
@@ -1451,6 +1484,9 @@ __do_page_fault(struct pt_regs *regs, unsigned long 
error_code,
return;
}
 
+#ifdef CONFIG_SPECULATIVE_PAGE_FAULT
+done:
+#endif
/*
 * Major/minor page fault accounting. If any of the events
 * returned VM_FAULT_MAJOR, we account it as a major fault.
-- 
2.7.4