linux-kernel - Re: [PATCH 1/3] mm: Minor clean-up of page flags in mm/page

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Date:	Wed, 14 May 2008 16:30:02 -0500
From:	Russ Anderson <rja@....com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	linux-kernel@...r.kernel.org, linux-ia64@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Tony Luck <tony.luck@...el.com>,
	Christoph Lameter <clameter@....com>
Subject: Re: [PATCH 1/3] mm: Minor clean-up of page flags in mm/page_alloc.c v4

On Wed, May 14, 2008 at 01:28:53PM -0700, Linus Torvalds wrote:
> On Tue, 13 May 2008, Russ Anderson wrote:
> >
> > Minor source code cleanup of page flags in mm/page_alloc.c.  
> 
> Could we (a) make the naming reflect the *use* rather than the flags 
> involved and (b) perhaps add a comment about that use at the point of 
> definition?

Yes, will do.  The reason for the cleanup is the following patch
defines PG_memerror, but only on configs with extended page flags.
So checking them in page_alloc.c would require adding #ifdef, which
would be rather ugly.  Moving the definitions into page-flags.h
allows the #ifdef to be in the header file rather than the C code.

>From page.discard.v4:
+#ifdef CONFIG_PAGEFLAGS_EXTENDED
+PAGEFLAG(MemError, memerror)
+#define        PAGE_FLAGS      (PAGE_FLAGS_BASE | 1UL << PG_memerror)
+#else
+PAGEFLAG_FALSE(MemError)
+#define        PAGE_FLAGS      (PAGE_FLAGS_BASE)
+#endif

 
> > @@ -237,16 +237,7 @@ static void bad_page(struct page *page)
> >  	printk(KERN_EMERG "Trying to fix it up, but a reboot is needed\n"
> >  		KERN_EMERG "Backtrace:\n");
> >  	dump_stack();
> > -	page->flags &= ~(1 << PG_lru	|
> > -			1 << PG_private |
> > -			1 << PG_locked	|
> > -			1 << PG_active	|
> > -			1 << PG_dirty	|
> > -			1 << PG_reclaim |
> > -			1 << PG_slab    |
> > -			1 << PG_swapcache |
> > -			1 << PG_writeback |
> > -			1 << PG_buddy );
> > +	page->flags &= ~(PAGE_FLAGS_RECLAIM);
> 
> Because here I'm now otherwise looking at the code, and I wonder
>  - why the extra odd parenthesis?
>  - what does PAGE_FLAG_RECLAIM mean?
> 
> and it would be much nicer (I think) if the mask was instead called 
> something that reflected what it was all about, ie something along the 
> lines of PAGE_FLAG_CLEAR_WHEN_BAD instead.

I'll make those changes.

-- 
Russ Anderson, OS RAS/Partitioning Project Lead  
SGI - Silicon Graphics Inc          rja@....com
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