linux-kernel - Re: [PATCH 1/1] arch/arm/mm/fault.c: Porting OOM changes into __do_page

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Date:	Sat, 12 Nov 2011 23:20:39 +0000
From:	Russell King - ARM Linux <linux@....linux.org.uk>
To:	Kautuk Consul <consul.kautuk@...il.com>
Cc:	Catalin Marinas <catalin.marinas@....com>,
	Will Deacon <will.deacon@....com>, Ingo Molnar <mingo@...e.hu>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/1] arch/arm/mm/fault.c: Porting OOM changes into
	__do_page_fault

On Sat, Nov 12, 2011 at 06:08:03PM -0500, Kautuk Consul wrote:
> diff --git a/arch/arm/mm/fault.c b/arch/arm/mm/fault.c
> index aa33949..2f89dba 100644
> --- a/arch/arm/mm/fault.c
> +++ b/arch/arm/mm/fault.c
> @@ -231,11 +231,15 @@ static inline bool access_error(unsigned int fsr, struct vm_area_struct *vma)
>  
>  static int __kprobes
>  __do_page_fault(struct mm_struct *mm, unsigned long addr, unsigned int fsr,
> -		struct task_struct *tsk)
> +		struct pt_regs *regs, struct task_struct *tsk)
>  {
>  	struct vm_area_struct *vma;
>  	int fault;
> +	int write = fsr & FSR_WRITE;
> +	unsigned int flags = FAULT_FLAG_ALLOW_RETRY | FAULT_FLAG_KILLABLE |
> +					(write ? FAULT_FLAG_WRITE : 0);
>  
> +retry:
>  	vma = find_vma(mm, addr);
>  	fault = VM_FAULT_BADMAP;
>  	if (unlikely(!vma))
> @@ -257,13 +261,44 @@ good_area:
>  	 * If for any reason at all we couldn't handle the fault, make
>  	 * sure we exit gracefully rather than endlessly redo the fault.
>  	 */
> -	fault = handle_mm_fault(mm, vma, addr & PAGE_MASK, (fsr & FSR_WRITE) ? FAULT_FLAG_WRITE : 0);
> -	if (unlikely(fault & VM_FAULT_ERROR))
> +	fault = handle_mm_fault(mm, vma, addr & PAGE_MASK, flags);
> +
> +	if (unlikely((fault & VM_FAULT_ERROR)))
>  		return fault;
> -	if (fault & VM_FAULT_MAJOR)
> -		tsk->maj_flt++;
> -	else
> -		tsk->min_flt++;
> +
> +	if ((fault & VM_FAULT_RETRY) && fatal_signal_pending(current))
> +		return fault;
> +
> +	/*
> +	 * 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.
> +	 */
> +	perf_sw_event(PERF_COUNT_SW_PAGE_FAULTS, 1, regs, addr);
> +	if (flags & FAULT_FLAG_ALLOW_RETRY) {
> +		if (fault & VM_FAULT_MAJOR) {
> +			tsk->maj_flt++;
> +			perf_sw_event(PERF_COUNT_SW_PAGE_FAULTS_MAJ, 1,
> +				      regs, addr);
> +		} else {
> +			tsk->min_flt++;
> +			perf_sw_event(PERF_COUNT_SW_PAGE_FAULTS_MIN, 1,
> +				      regs, addr);
> +		}
> +		if (fault & VM_FAULT_RETRY) {
> +			/* Clear FAULT_FLAG_ALLOW_RETRY to avoid any risk
> +			 * of starvation. */
> +			flags &= ~FAULT_FLAG_ALLOW_RETRY;
> +
> +			/* Acquire the mmap_sem again before retrying this
> +			 * pagefault. This would have been released by
> +			 * __lock_page_or_retry() in mm/filemap.c. */
> +			down_read(&mm->mmap_sem);
> +
> +			goto retry;
> +		}
> +	}
> +
>  	return fault;
>  
>  check_stack:
> @@ -320,14 +355,9 @@ do_page_fault(unsigned long addr, unsigned int fsr, struct pt_regs *regs)
>  #endif
>  	}
>  
> -	fault = __do_page_fault(mm, addr, fsr, tsk);
> -	up_read(&mm->mmap_sem);
> -
> -	perf_sw_event(PERF_COUNT_SW_PAGE_FAULTS, 1, regs, addr);
> -	if (fault & VM_FAULT_MAJOR)
> -		perf_sw_event(PERF_COUNT_SW_PAGE_FAULTS_MAJ, 1, regs, addr);
> -	else if (fault & VM_FAULT_MINOR)
> -		perf_sw_event(PERF_COUNT_SW_PAGE_FAULTS_MIN, 1, regs, addr);
> +	fault = __do_page_fault(mm, addr, fsr, regs, tsk);
> +	if (likely(!(fault & VM_FAULT_RETRY)))
> +		up_read(&mm->mmap_sem);

I really don't like this.  I crafted this handling in such a way that
the locking was plainly obvious - with all locking handled in
do_page_fault and not inside __do_page_fault.  That's how I want things
to stay, so please rework this patch to maintain that.
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