Re: [patch 3/5] x86: finish fault error path with fatal signal

[KOSAKI Motohiro <kosaki.motohiro@gmail.com>]

(7/24/13 4:32 PM), Johannes Weiner wrote:
> On Fri, Jul 19, 2013 at 12:25:02AM -0400, Johannes Weiner wrote:
>> The x86 fault handler bails in the middle of error handling when the
>> task has been killed.  For the next patch this is a problem, because
>> it relies on pagefault_out_of_memory() being called even when the task
>> has been killed, to perform proper OOM state unwinding.
>>
>> This is a rather minor optimization, just remove it.
>>
>> Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
>> ---
>>   arch/x86/mm/fault.c | 11 -----------
>>   1 file changed, 11 deletions(-)
>>
>> diff --git a/arch/x86/mm/fault.c b/arch/x86/mm/fault.c
>> index 1cebabe..90248c9 100644
>> --- a/arch/x86/mm/fault.c
>> +++ b/arch/x86/mm/fault.c
>> @@ -846,17 +846,6 @@ static noinline int
>>   mm_fault_error(struct pt_regs *regs, unsigned long error_code,
>>   	       unsigned long address, unsigned int fault)
>>   {
>> -	/*
>> -	 * Pagefault was interrupted by SIGKILL. We have no reason to
>> -	 * continue pagefault.
>> -	 */
>> -	if (fatal_signal_pending(current)) {
>> -		if (!(fault & VM_FAULT_RETRY))
>> -			up_read(&current->mm->mmap_sem);
>> -		if (!(error_code & PF_USER))
>> -			no_context(regs, error_code, address);
>> -		return 1;
>
> This is broken but I only hit it now after testing for a while.
>
> The patch has the right idea: in case of an OOM kill, we should
> continue the fault and not abort.  What I missed is that in case of a
> kill during lock_page, i.e. VM_FAULT_RETRY && fatal_signal, we have to
> exit the fault and not do up_read() etc.  This introduced a locking
> imbalance that would get everybody hung on mmap_sem.
>
> I moved the retry handling outside of mm_fault_error() (come on...)
> and stole some documentation from arm.  It's now a little bit more
> explicit and comparable to other architectures.
>
> I'll send an updated series, patch for reference:
>
> ---
> From: Johannes Weiner <hannes@cmpxchg.org>
> Subject: [patch] x86: finish fault error path with fatal signal
>
> The x86 fault handler bails in the middle of error handling when the
> task has been killed.  For the next patch this is a problem, because
> it relies on pagefault_out_of_memory() being called even when the task
> has been killed, to perform proper OOM state unwinding.
>
> This is a rather minor optimization that cuts short the fault handling
> by a few instructions in rare cases.  Just remove it.
>
> Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
> ---
>   arch/x86/mm/fault.c | 33 +++++++++++++--------------------
>   1 file changed, 13 insertions(+), 20 deletions(-)
>
> diff --git a/arch/x86/mm/fault.c b/arch/x86/mm/fault.c
> index 6d77c38..0c18beb 100644
> --- a/arch/x86/mm/fault.c
> +++ b/arch/x86/mm/fault.c
> @@ -842,31 +842,17 @@ do_sigbus(struct pt_regs *regs, unsigned long error_code, unsigned long address,
>   	force_sig_info_fault(SIGBUS, code, address, tsk, fault);
>   }
>
> -static noinline int
> +static noinline void
>   mm_fault_error(struct pt_regs *regs, unsigned long error_code,
>   	       unsigned long address, unsigned int fault)
>   {
> -	/*
> -	 * Pagefault was interrupted by SIGKILL. We have no reason to
> -	 * continue pagefault.
> -	 */
> -	if (fatal_signal_pending(current)) {
> -		if (!(fault & VM_FAULT_RETRY))
> -			up_read(&current->mm->mmap_sem);
> -		if (!(error_code & PF_USER))
> -			no_context(regs, error_code, address, 0, 0);
> -		return 1;
> -	}
> -	if (!(fault & VM_FAULT_ERROR))
> -		return 0;
> -
>   	if (fault & VM_FAULT_OOM) {
>   		/* Kernel mode? Handle exceptions or die: */
>   		if (!(error_code & PF_USER)) {
>   			up_read(&current->mm->mmap_sem);
>   			no_context(regs, error_code, address,
>   				   SIGSEGV, SEGV_MAPERR);
> -			return 1;
> +			return;
>   		}
>
>   		up_read(&current->mm->mmap_sem);
> @@ -884,7 +870,6 @@ mm_fault_error(struct pt_regs *regs, unsigned long error_code,
>   		else
>   			BUG();
>   	}
> -	return 1;
>   }
>
>   static int spurious_fault_check(unsigned long error_code, pte_t *pte)
> @@ -1189,9 +1174,17 @@ good_area:
>   	 */
>   	fault = handle_mm_fault(mm, vma, address, flags);
>
> -	if (unlikely(fault & (VM_FAULT_RETRY|VM_FAULT_ERROR))) {
> -		if (mm_fault_error(regs, error_code, address, fault))
> -			return;
> +	/*
> +	 * If we need to retry but a fatal signal is pending, handle the
> +	 * signal first. We do not need to release the mmap_sem because it
> +	 * would already be released in __lock_page_or_retry in mm/filemap.c.
> +	 */
> +	if ((fault & VM_FAULT_RETRY) && fatal_signal_pending(current))
> +		return;
> +
> +	if (unlikely(fault & VM_FAULT_ERROR)) {
> +		mm_fault_error(regs, error_code, address, fault);
> +		return;
>   	}

When I made the patch you removed code, Ingo suggested we need put all rare case code
into if(unlikely()) block. Yes, this is purely micro optimization. But it is not costly
to maintain.




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