Re: [PATCH 2/3] powerpc/mm: handle VM_FAULT_RETRY earlier

[Laurent Dufour]

On 21/03/2017 10:12, Aneesh Kumar K.V wrote:
> Laurent Dufour  writes:
> 
>> In do_page_fault() if handle_mm_fault() returns VM_FAULT_RETRY, retry
>> the page fault handling before anything else.
>>
>> This would simplify the handling of the mmap_sem lock in this part of
>> the code.
>>
>> Signed-off-by: Laurent Dufour 
>> ---
>>  arch/powerpc/mm/fault.c | 67 
>> -
>>  1 file changed, 38 insertions(+), 29 deletions(-)
>>
>> diff --git a/arch/powerpc/mm/fault.c b/arch/powerpc/mm/fault.c
>> index ee09604bbe12..2a6bc7e6e69a 100644
>> --- a/arch/powerpc/mm/fault.c
>> +++ b/arch/powerpc/mm/fault.c
>> @@ -434,6 +434,26 @@ int do_page_fault(struct pt_regs *regs, unsigned long 
>> address,
>>   * the fault.
>>   */
>>  fault = handle_mm_fault(vma, address, flags);
>> +
>> +/*
>> + * Handle the retry right now, the mmap_sem has been released in that
>> + * case.
>> + */
>> +if (unlikely(fault & VM_FAULT_RETRY)) {
>> +/* We retry only once */
>> +if (flags & FAULT_FLAG_ALLOW_RETRY) {
>> +/*
>> + * Clear FAULT_FLAG_ALLOW_RETRY to avoid any risk
>> + * of starvation.
>> + */
>> +flags &= ~FAULT_FLAG_ALLOW_RETRY;
>> +flags |= FAULT_FLAG_TRIED;
>> +if (!fatal_signal_pending(current))
>> +goto retry;
>> +}
>> +/* We will enter mm_fault_error() below */
>> +}
>> +
>>  if (unlikely(fault & (VM_FAULT_RETRY|VM_FAULT_ERROR))) {
>>  if (fault & VM_FAULT_SIGSEGV)
>>  goto bad_area;
>> @@ -445,38 +465,27 @@ int do_page_fault(struct pt_regs *regs, unsigned long 
>> address,
>>  }
> 
> We could make it further simpler, by handling the FAULT_RETRY without
> FLAG_ALLOW_RETRY set earlier. But i guess that can be done later ?


Thanks for the review,

I agree that double checking against VM_FAULT_RETRY is confusing here.

But handling all the retry path in the first if() statement means that
we'll have to handle part of the mm_fault_error() code and segv here...
Unless we can't identify what is really relevant in that retry path.

It would take time to review all that tricky part, but I agree it should
be simplified later.

> 
> Reviewed-by: Aneesh Kumar K.V 
> 
> 
>>
>>  /*
>> - * Major/minor page fault accounting is only done on the
>> - * initial attempt. If we go through a retry, it is extremely
>> - * likely that the page will be found in page cache at that point.
>> + * Major/minor page fault accounting.
>>   */
>> -if (flags & FAULT_FLAG_ALLOW_RETRY) {
>> -if (fault & VM_FAULT_MAJOR) {
>> -current->maj_flt++;
>> -perf_sw_event(PERF_COUNT_SW_PAGE_FAULTS_MAJ, 1,
>> -  regs, address);
>> +if (fault & VM_FAULT_MAJOR) {
>> +current->maj_flt++;
>> +perf_sw_event(PERF_COUNT_SW_PAGE_FAULTS_MAJ, 1,
>> +  regs, address);
>>  #ifdef CONFIG_PPC_SMLPAR
>> -if (firmware_has_feature(FW_FEATURE_CMO)) {
>> -u32 page_ins;
>> -
>> -preempt_disable();
>> -page_ins = be32_to_cpu(get_lppaca()->page_ins);
>> -page_ins += 1 << PAGE_FACTOR;
>> -get_lppaca()->page_ins = cpu_to_be32(page_ins);
>> -preempt_enable();
>> -}
>> -#endif /* CONFIG_PPC_SMLPAR */
>> -} else {
>> -current->min_flt++;
>> -perf_sw_event(PERF_COUNT_SW_PAGE_FAULTS_MIN, 1,
>> -  regs, address);
>> -}
>> -if (fault & VM_FAULT_RETRY) {
>> -/* Clear FAULT_FLAG_ALLOW_RETRY to avoid any risk
>> - * of starvation. */
>> -flags &= ~FAULT_FLAG_ALLOW_RETRY;
>> -flags |= FAULT_FLAG_TRIED;
>> -goto retry;
>> +if (firmware_has_feature(FW_FEATURE_CMO)) {
>> +u32 page_ins;
>> +
>> +preempt_disable();
>> +page_ins = be32_to_cpu(get_lppaca()->page_ins);
>> +page_ins += 1 << PAGE_FACTOR;
>> +get_lppaca()->page_ins = cpu_to_be32(page_ins);
>> +preempt_enable();
>>  }
>> +#endif /* CONFIG_PPC_SMLPAR */
>> +} else {
>> +current->min_flt++;
>> +perf_sw_event(PERF_COUNT_SW_PAGE_FAULTS_MIN, 1,
>> +  regs, address);
>>  }
>>
>>  up_read(>mmap_sem);
>> -- 
>> 2.7.4

Re: [PATCH 2/3] powerpc/mm: handle VM_FAULT_RETRY earlier

[Aneesh Kumar K.V]

Laurent Dufour  writes:

> In do_page_fault() if handle_mm_fault() returns VM_FAULT_RETRY, retry
> the page fault handling before anything else.
>
> This would simplify the handling of the mmap_sem lock in this part of
> the code.
>
> Signed-off-by: Laurent Dufour 
> ---
>  arch/powerpc/mm/fault.c | 67 
> -
>  1 file changed, 38 insertions(+), 29 deletions(-)
>
> diff --git a/arch/powerpc/mm/fault.c b/arch/powerpc/mm/fault.c
> index ee09604bbe12..2a6bc7e6e69a 100644
> --- a/arch/powerpc/mm/fault.c
> +++ b/arch/powerpc/mm/fault.c
> @@ -434,6 +434,26 @@ int do_page_fault(struct pt_regs *regs, unsigned long 
> address,
>* the fault.
>*/
>   fault = handle_mm_fault(vma, address, flags);
> +
> + /*
> +  * Handle the retry right now, the mmap_sem has been released in that
> +  * case.
> +  */
> + if (unlikely(fault & VM_FAULT_RETRY)) {
> + /* We retry only once */
> + if (flags & FAULT_FLAG_ALLOW_RETRY) {
> + /*
> +  * Clear FAULT_FLAG_ALLOW_RETRY to avoid any risk
> +  * of starvation.
> +  */
> + flags &= ~FAULT_FLAG_ALLOW_RETRY;
> + flags |= FAULT_FLAG_TRIED;
> + if (!fatal_signal_pending(current))
> + goto retry;
> + }
> + /* We will enter mm_fault_error() below */
> + }
> +
>   if (unlikely(fault & (VM_FAULT_RETRY|VM_FAULT_ERROR))) {
>   if (fault & VM_FAULT_SIGSEGV)
>   goto bad_area;
> @@ -445,38 +465,27 @@ int do_page_fault(struct pt_regs *regs, unsigned long 
> address,
>   }

We could make it further simpler, by handling the FAULT_RETRY without
FLAG_ALLOW_RETRY set earlier. But i guess that can be done later ?

Reviewed-by: Aneesh Kumar K.V 


>
>   /*
> -  * Major/minor page fault accounting is only done on the
> -  * initial attempt. If we go through a retry, it is extremely
> -  * likely that the page will be found in page cache at that point.
> +  * Major/minor page fault accounting.
>*/
> - if (flags & FAULT_FLAG_ALLOW_RETRY) {
> - if (fault & VM_FAULT_MAJOR) {
> - current->maj_flt++;
> - perf_sw_event(PERF_COUNT_SW_PAGE_FAULTS_MAJ, 1,
> -   regs, address);
> + if (fault & VM_FAULT_MAJOR) {
> + current->maj_flt++;
> + perf_sw_event(PERF_COUNT_SW_PAGE_FAULTS_MAJ, 1,
> +   regs, address);
>  #ifdef CONFIG_PPC_SMLPAR
> - if (firmware_has_feature(FW_FEATURE_CMO)) {
> - u32 page_ins;
> -
> - preempt_disable();
> - page_ins = be32_to_cpu(get_lppaca()->page_ins);
> - page_ins += 1 << PAGE_FACTOR;
> - get_lppaca()->page_ins = cpu_to_be32(page_ins);
> - preempt_enable();
> - }
> -#endif /* CONFIG_PPC_SMLPAR */
> - } else {
> - current->min_flt++;
> - perf_sw_event(PERF_COUNT_SW_PAGE_FAULTS_MIN, 1,
> -   regs, address);
> - }
> - if (fault & VM_FAULT_RETRY) {
> - /* Clear FAULT_FLAG_ALLOW_RETRY to avoid any risk
> -  * of starvation. */
> - flags &= ~FAULT_FLAG_ALLOW_RETRY;
> - flags |= FAULT_FLAG_TRIED;
> - goto retry;
> + if (firmware_has_feature(FW_FEATURE_CMO)) {
> + u32 page_ins;
> +
> + preempt_disable();
> + page_ins = be32_to_cpu(get_lppaca()->page_ins);
> + page_ins += 1 << PAGE_FACTOR;
> + get_lppaca()->page_ins = cpu_to_be32(page_ins);
> + preempt_enable();
>   }
> +#endif /* CONFIG_PPC_SMLPAR */
> + } else {
> + current->min_flt++;
> + perf_sw_event(PERF_COUNT_SW_PAGE_FAULTS_MIN, 1,
> +   regs, address);
>   }
>
>   up_read(>mmap_sem);
> -- 
> 2.7.4

[PATCH 2/3] powerpc/mm: handle VM_FAULT_RETRY earlier

[Laurent Dufour]

In do_page_fault() if handle_mm_fault() returns VM_FAULT_RETRY, retry
the page fault handling before anything else.

This would simplify the handling of the mmap_sem lock in this part of
the code.

Signed-off-by: Laurent Dufour 
---
 arch/powerpc/mm/fault.c | 67 -
 1 file changed, 38 insertions(+), 29 deletions(-)

diff --git a/arch/powerpc/mm/fault.c b/arch/powerpc/mm/fault.c
index ee09604bbe12..2a6bc7e6e69a 100644
--- a/arch/powerpc/mm/fault.c
+++ b/arch/powerpc/mm/fault.c
@@ -434,6 +434,26 @@ int do_page_fault(struct pt_regs *regs, unsigned long 
address,
 * the fault.
 */
fault = handle_mm_fault(vma, address, flags);
+
+   /*
+* Handle the retry right now, the mmap_sem has been released in that
+* case.
+*/
+   if (unlikely(fault & VM_FAULT_RETRY)) {
+   /* We retry only once */
+   if (flags & FAULT_FLAG_ALLOW_RETRY) {
+   /*
+* Clear FAULT_FLAG_ALLOW_RETRY to avoid any risk
+* of starvation.
+*/
+   flags &= ~FAULT_FLAG_ALLOW_RETRY;
+   flags |= FAULT_FLAG_TRIED;
+   if (!fatal_signal_pending(current))
+   goto retry;
+   }
+   /* We will enter mm_fault_error() below */
+   }
+
if (unlikely(fault & (VM_FAULT_RETRY|VM_FAULT_ERROR))) {
if (fault & VM_FAULT_SIGSEGV)
goto bad_area;
@@ -445,38 +465,27 @@ int do_page_fault(struct pt_regs *regs, unsigned long 
address,
}
 
/*
-* Major/minor page fault accounting is only done on the
-* initial attempt. If we go through a retry, it is extremely
-* likely that the page will be found in page cache at that point.
+* Major/minor page fault accounting.
 */
-   if (flags & FAULT_FLAG_ALLOW_RETRY) {
-   if (fault & VM_FAULT_MAJOR) {
-   current->maj_flt++;
-   perf_sw_event(PERF_COUNT_SW_PAGE_FAULTS_MAJ, 1,
- regs, address);
+   if (fault & VM_FAULT_MAJOR) {
+   current->maj_flt++;
+   perf_sw_event(PERF_COUNT_SW_PAGE_FAULTS_MAJ, 1,
+ regs, address);
 #ifdef CONFIG_PPC_SMLPAR
-   if (firmware_has_feature(FW_FEATURE_CMO)) {
-   u32 page_ins;
-
-   preempt_disable();
-   page_ins = be32_to_cpu(get_lppaca()->page_ins);
-   page_ins += 1 << PAGE_FACTOR;
-   get_lppaca()->page_ins = cpu_to_be32(page_ins);
-   preempt_enable();
-   }
-#endif /* CONFIG_PPC_SMLPAR */
-   } else {
-   current->min_flt++;
-   perf_sw_event(PERF_COUNT_SW_PAGE_FAULTS_MIN, 1,
- regs, address);
-   }
-   if (fault & VM_FAULT_RETRY) {
-   /* Clear FAULT_FLAG_ALLOW_RETRY to avoid any risk
-* of starvation. */
-   flags &= ~FAULT_FLAG_ALLOW_RETRY;
-   flags |= FAULT_FLAG_TRIED;
-   goto retry;
+   if (firmware_has_feature(FW_FEATURE_CMO)) {
+   u32 page_ins;
+
+   preempt_disable();
+   page_ins = be32_to_cpu(get_lppaca()->page_ins);
+   page_ins += 1 << PAGE_FACTOR;
+   get_lppaca()->page_ins = cpu_to_be32(page_ins);
+   preempt_enable();
}
+#endif /* CONFIG_PPC_SMLPAR */
+   } else {
+   current->min_flt++;
+   perf_sw_event(PERF_COUNT_SW_PAGE_FAULTS_MIN, 1,
+ regs, address);
}
 
up_read(>mmap_sem);
-- 
2.7.4