Re: [PATCH]vmscan: handle underflow for get_scan_ratio

[KOSAKI Motohiro]

> > On Tue, Mar 30, 2010 at 02:08:53PM +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 and reproduce program? I'll digg it.
> > > 
> > > Very unfortunately, this patch isn't acceptable. In past time, vmscan 
> > > had similar logic, but 1% swap-out made lots bug reports. 
> > if 1% is still big, how about below patch?
> 
> This patch makes a lot of sense than previous. however I think <1% anon ratio
> shouldn't happen anyway because file lru doesn't have reclaimable pages.
> <1% seems no good reclaim rate.

Oops, the above mention is wrong. sorry. only 1 page is still too big.
because under streaming io workload, the number of scanning anon pages should
be zero. this is very strong requirement. if not, backup operation will makes
a lot of swapping out.

Anyway, I'm digging this issue.


> 
> perhaps I'll take your patch for stable tree. but we need to attack the root
> cause. iow, I guess we need to fix scan ratio equation itself.



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