linux-kernel - Re: [patch 20/22] vmscan: avoid multiplication overflow in shrink

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Date:	Fri, 1 May 2009 09:22:12 +0800
From:	Wu Fengguang <fengguang.wu@...el.com>
To:	"akpm@...ux-foundation.org" <akpm@...ux-foundation.org>
Cc:	"torvalds@...ux-foundation.org" <torvalds@...ux-foundation.org>,
	"kosaki.motohiro@...fujitsu.com" <kosaki.motohiro@...fujitsu.com>,
	"lee.schermerhorn@...com" <lee.schermerhorn@...com>,
	"peterz@...radead.org" <peterz@...radead.org>,
	"riel@...hat.com" <riel@...hat.com>, linux-mm@...ck.org,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [patch 20/22] vmscan: avoid multiplication overflow in
	shrink_zone()

On Fri, May 01, 2009 at 06:08:55AM +0800, Andrew Morton wrote:
> 
> Local variable `scan' can overflow on zones which are larger than
> 
> 	(2G * 4k) / 100 = 80GB.
> 
> Making it 64-bit on 64-bit will fix that up.

A side note about the "one HUGE scan inside shrink_zone":

Isn't this low level scan granularity way tooooo large?

It makes things a lot worse on memory pressure:
- the over reclaim, somehow workarounded by Rik's early bail out patch
- the throttle_vm_writeout()/congestion_wait() guards could work in a
  very sparse manner and hence is useless: imagine to stop and wait
  after shooting away every 1GB memory.

The long term fix could be to move the granularity control up to the
shrink_zones() level: there it can bail out early without hurting the
balanced zone aging.

Thanks,
Fengguang

> --- a/mm/vmscan.c~vmscan-avoid-multiplication-overflow-in-shrink_zone
> +++ a/mm/vmscan.c
> @@ -1471,7 +1471,7 @@ static void shrink_zone(int priority, st
>  
>  	for_each_evictable_lru(l) {
>  		int file = is_file_lru(l);
> -		int scan;
> +		unsigned long scan;
>  
>  		scan = zone_nr_pages(zone, sc, l);
>  		if (priority) {
> _
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