linux-kernel - Re: [PATCH -v2] mm: frontswap: fix a wrong if condition in frontswap

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Date:	Fri, 28 Sep 2012 16:54:31 +0200
From:	Paul Bolle <pebolle@...cali.nl>
To:	zhenzhong.duan@...cle.com
Cc:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	linux-mm@...ck.org, Dan Magenheimer <dan.magenheimer@...cle.com>,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	levinsasha928@...il.com, Feng Jin <joe.jin@...cle.com>,
	dan.carpenter@...cle.com
Subject: Re: [PATCH -v2] mm: frontswap: fix a wrong if condition in
 frontswap_shrink

On Fri, 2012-09-28 at 11:43 +0800, Zhenzhong Duan wrote:
> On 2012-09-27 19:35, Paul Bolle wrote:
> > I think setting pages_to_unuse to zero here is not needed. It is
> > initiated to zero in frontswap_shrink() and hasn't been touched since.
> > See my patch at https://lkml.org/lkml/2012/9/27/250.
> Yes, it's unneeded. But I didn't see warning as you said in above link 
> when run 'make V=1 mm/frontswap.o'.

Not even before applying your patch? Anyhow, after applying your patch
the warnings gone here too.

> >> -		return 0;
> >> +		return 1;
> >>   	}
> >>   	total_pages_to_unuse = total_pages - target_pages;
> >>   	return __frontswap_unuse_pages(total_pages_to_unuse, pages_to_unuse, type);
> >> @@ -302,7 +307,7 @@ void frontswap_shrink(unsigned long target_pages)
> >>   	spin_lock(&swap_lock);
> >>   	ret = __frontswap_shrink(target_pages,&pages_to_unuse,&type);
> >>   	spin_unlock(&swap_lock);
> >> -	if (ret == 0&&  pages_to_unuse)
> >> +	if (ret == 0)
> >>   		try_to_unuse(type, true, pages_to_unuse);
> >>   	return;
> >>   }
> >
> > Are you sure pages_to_unuse won't be zero here? I've stared quite a bit
> > at __frontswap_unuse_pages() and it's not obvious pages_to_unuse (there
> > also called unused) will never be zero when that function returns zero.
> pages_to_unuse==0 means all pages need to be unused.

Ah, now I see. I was focusing on changing the code as little as possible
and didn't realize that you actually wanted to change behavior here.
Looking at it again this change makes sense (though I hardly understand
frontswap, so I can't properly evaluate it). Anyhow, as I said, your
patch also does what I care about - silence a warning - so we might as
well forget about my patch.
 
Thanks,


Paul Bolle

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