Re: [PATCH]vmscan: handle underflow for get_scan_ratio

[Shaohua Li <shaohua.li@intel.com>]

On Tue, Mar 30, 2010 at 02:40:07PM +0800, KOSAKI Motohiro wrote:
> > 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.
I thought I mentioned the root cause in first mail. My debug shows
recent_rotated[0] is big, but recent_rotated[1] is almost zero, which makes
percent[0] 0. But you can double check too.

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