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

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Date:	Wed, 19 Dec 2012 16:08:05 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Johannes Weiner <hannes@...xchg.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 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;
_

Which bloats the text by six bytes, but will force a nice div-by-zero
if we ever hit that can't-happen path.
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