linux-kernel - Re: [PATCH]vmscan: handle underflow for get_scan

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Message-Id: <20100330153750.8EA2.A69D9226@jp.fujitsu.com>
Date:	Tue, 30 Mar 2010 15:40:07 +0900 (JST)
From:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
To:	Shaohua Li <shaohua.li@...el.com>
Cc:	kosaki.motohiro@...fujitsu.com,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
	"Wu, Fengguang" <fengguang.wu@...el.com>
Subject: Re: [PATCH]vmscan: handle underflow for get_scan_ratio

> On Tue, 2010-03-30 at 14:08 +0800, KOSAKI Motohiro wrote:
> > Hi
> > 
> > > Commit 84b18490d1f1bc7ed5095c929f78bc002eb70f26 introduces a regression.
> > > With it, our tmpfs test always oom. The test has a lot of rotated anon
> > > pages and cause percent[0] zero. Actually the percent[0] is a very small
> > > value, but our calculation round it to zero. The commit makes vmscan
> > > completely skip anon pages and cause oops.
> > > An option is if percent[x] is zero in get_scan_ratio(), forces it
> > > to 1. See below patch.
> > > But the offending commit still changes behavior. Without the commit, we scan
> > > all pages if priority is zero, below patch doesn't fix this. Don't know if
> > > It's required to fix this too.
> > 
> > Can you please post your /proc/meminfo 
> attached.
> > and reproduce program? I'll digg it.
> our test is quite sample. mount tmpfs with double memory size and store several
> copies (memory size * 2/G) of kernel in tmpfs, and then do kernel build.
> for example, there is 3G memory and then tmpfs size is 6G and there is 6
> kernel copy.

Wow, tmpfs size > memsize!


> > Very unfortunately, this patch isn't acceptable. In past time, vmscan 
> > had similar logic, but 1% swap-out made lots bug reports. 
> can you elaborate this?
> Completely restore previous behavior (do full scan with priority 0) is
> ok too.

This is a option. but we need to know the root cause anyway. if not,
we might reintroduce this issue again in the future.



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