linux-kernel - Re: [PATCH 2/3] powerpc/mm: handle VM_FAULT

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Date:   Tue, 21 Mar 2017 10:57:26 +0100
From:   Laurent Dufour <ldufour@...ux.vnet.ibm.com>
To:     "Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>,
        mpe@...erman.id.au, benh@...nel.crashing.org, paulus@...ba.org,
        bsingharora@...il.com, npiggin@...il.com
Cc:     linuxppc-dev@...ts.ozlabs.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/3] powerpc/mm: handle VM_FAULT_RETRY earlier

On 21/03/2017 10:12, Aneesh Kumar K.V wrote:
> Laurent Dufour <ldufour@...ux.vnet.ibm.com> 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 <ldufour@...ux.vnet.ibm.com>
>> ---
>>  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 <aneesh.kumar@...ux.vnet.ibm.com>
> 
> 
>>
>>  	/*
>> -	 * 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(&mm->mmap_sem);
>> -- 
>> 2.7.4