linux-kernel - Re: [PATCH][1/2]page_fault retry with NOPAGE

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Date:	Fri, 10 Apr 2009 15:30:42 +0800
From:	Wu Fengguang <fengguang.wu@...el.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Ying Han <yinghan@...gle.com>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	"torvalds@...ux-foundation.org" <torvalds@...ux-foundation.org>,
	Ingo Molnar <mingo@...e.hu>, Mike Waychison <mikew@...gle.com>,
	Rohit Seth <rohitseth@...gle.com>,
	Hugh Dickins <hugh@...itas.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	"H. Peter Anvin" <hpa@...or.com>,
	Török Edwin <edwintorok@...il.com>,
	Lee Schermerhorn <lee.schermerhorn@...com>,
	Nick Piggin <npiggin@...e.de>
Subject: Re: [PATCH][1/2]page_fault retry with NOPAGE_RETRY

On Fri, Apr 10, 2009 at 02:02:05PM +0800, Andrew Morton wrote:
[snip]
> >  	if (unlikely(ret & (VM_FAULT_ERROR | VM_FAULT_NOPAGE)))
> >  		return ret;
> > 
> > @@ -2611,8 +2618,10 @@ static int do_linear_fault(struct mm_struct *mm, struct
> >  {
> >  	pgoff_t pgoff = (((address & PAGE_MASK)
> >  			- vma->vm_start) >> PAGE_SHIFT) + vma->vm_pgoff;
> > -	unsigned int flags = (write_access ? FAULT_FLAG_WRITE : 0);
> > +	int write = write_access & ~FAULT_FLAG_RETRY;
> > +	unsigned int flags = (write ? FAULT_FLAG_WRITE : 0);
> > 
> > +	flags |= (write_access & FAULT_FLAG_RETRY);
> 
> gee, I'm lost.

So did me.

> Can we please redo this as:
> 
> 
> 	int write;
> 	unsigned int flags;
> 
> 	/*
> 	 * Big fat comment explaining the next three lines goes here
> 	 */

Basically it's doing a
        (is_write_access  | FAULT_FLAG_RETRY) =>
        (FAULT_FLAG_WRITE | FAULT_FLAG_RETRY)
by extracting the bool part:
> 	write = write_access & ~FAULT_FLAG_RETRY;
convert bool to a bit flag:
> 	unsigned int flags = (write ? FAULT_FLAG_WRITE : 0);
and restore the FAULT_FLAG_RETRY:
> 	flags |= (write_access & FAULT_FLAG_RETRY);

Thanks,
Fengguang

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