linux-kernel - Re: [patch 5/7] mm: vmscan: clean up get_scan

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Message-ID: <20121221024957.GE7147@cmpxchg.org>
Date:	Thu, 20 Dec 2012 21:49:57 -0500
From:	Johannes Weiner <hannes@...xchg.org>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Rik van Riel <riel@...hat.com>, Michal Hocko <mhocko@...e.cz>,
	Mel Gorman <mgorman@...e.de>, Hugh Dickins <hughd@...gle.com>,
	Satoru Moriya <satoru.moriya@....com>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [patch 5/7] mm: vmscan: clean up get_scan_count()

On Wed, Dec 19, 2012 at 04:08:05PM -0800, Andrew Morton wrote:
> On Mon, 17 Dec 2012 13:12:35 -0500
> Johannes Weiner <hannes@...xchg.org> wrote:
> 
> > Reclaim pressure balance between anon and file pages is calculated
> > through a tuple of numerators and a shared denominator.
> > 
> > Exceptional cases that want to force-scan anon or file pages configure
> > the numerators and denominator such that one list is preferred, which
> > is not necessarily the most obvious way:
> > 
> >     fraction[0] = 1;
> >     fraction[1] = 0;
> >     denominator = 1;
> >     goto out;
> > 
> > Make this easier by making the force-scan cases explicit and use the
> > fractionals only in case they are calculated from reclaim history.
> > 
> > And bring the variable declarations/definitions in order.
> > 
> > ...
> >
> > +	u64 fraction[2], uninitialized_var(denominator);
> 
> Using uninitialized_var() puts Linus into rant mode.  Unkindly, IMO:
> uninitialized_var() is documentarily useful and reduces bloat.  There is
> a move afoot to replace it with
> 
> 	int foo = 0;	/* gcc */
> 
> To avoid getting ranted at we can do
> 
> --- a/mm/vmscan.c~mm-vmscan-clean-up-get_scan_count-fix
> +++ a/mm/vmscan.c
> @@ -1658,7 +1658,8 @@ static void get_scan_count(struct lruvec
>  			   unsigned long *nr)
>  {
>  	struct zone_reclaim_stat *reclaim_stat = &lruvec->reclaim_stat;
> -	u64 fraction[2], uninitialized_var(denominator);
> +	u64 fraction[2];
> +	u64 denominator = 0;
>  	struct zone *zone = lruvec_zone(lruvec);
>  	unsigned long anon_prio, file_prio;
>  	enum scan_balance scan_balance;

Makes sense, I guess, but then you have to delete this line from the
changelog:

"And bring the variable declarations/definitions in order."

Or change it to "partial" order or something... :-)

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