linux-kernel - Re: [git pull] cpus4096 tree, part 3

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Date:	Mon, 26 Jan 2009 21:09:57 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Nick Piggin <npiggin@...e.de>, linux-arch@...r.kernel.org,
	torvalds@...ux-foundation.org, rusty@...tcorp.com.au,
	travis@....com, linux-kernel@...r.kernel.org,
	venkatesh.pallipadi@...el.com, suresh.b.siddha@...el.com,
	arjan@...radead.org, hpa@...or.com, tglx@...utronix.de
Subject: Re: [git pull] cpus4096 tree, part 3

* Andrew Morton <akpm@...ux-foundation.org> wrote:

> but why did the current code pass testing at all??

i queued it up a week ago and beyond a same-day breakage i reported to 
Nick (and which he fixed) this commit was problem-free and passed all 
testing here.

Does it cause problems for you? If yes then please describe the kind of 
problems.

Note: i see that -mm modifies a few other details of the x86 pagefault 
handling path (there a pagefault-retry patch in there) - so there might be 
contextual interactions there. But this particular cleanup/improvement 
from Nick is working fine on a wide range of systems here.

Btw., regarding pagefault retry. The bits that are in -mm currently i 
find a bit ugly:

> +++ a/arch/x86/mm/fault.c
> @@ -799,7 +799,7 @@ void __kprobes do_page_fault(struct pt_r
>  	struct vm_area_struct *vma;
>  	int write;
>  	int fault;
> -	unsigned int retry_flag = FAULT_FLAG_RETRY;
> +	int retry_flag = 1;
>  
>  	tsk = current;
>  	mm = tsk->mm;
> @@ -951,6 +951,7 @@ good_area:
>  	}
>  
>  	write |= retry_flag;
> +
>  	/*
>  	 * If for any reason at all we couldn't handle the fault,
>  	 * make sure we exit gracefully rather than endlessly redo
> @@ -969,8 +970,8 @@ good_area:
>  	 * be removed or changed after the retry.
>  	 */
>  	if (fault & VM_FAULT_RETRY) {
> -		if (write & FAULT_FLAG_RETRY) {
> -			retry_flag &= ~FAULT_FLAG_RETRY;
> +		if (retry_flag) {
> +			retry_flag = 0;
>  			goto retry;
>  		}
>  		BUG();

as this complicates every architecture with a 'can the fault be retried' 
logic and open-coded retry loop.

But that logic is rather repetitive and once an architecture filters out 
all its special in-kernel sources of faults and the hw quirks it has, the 
handling of pte faults is rather generic and largely offloaded into 
handle_pte_fault() already.

So when this patch was submitted a few weeks ago i suggested that retry 
should be done purely in mm/memory.c instead, and the low level code 
should at most be refactored to suit this method, but not complicated any 
further.

Any deep reasons for why such a more generic approach is not desirable?

	Ingo
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