From mboxrd@z Thu Jan  1 00:00:00 1970
Date: Tue, 9 Nov 2004 13:33:43 -0800
From: Andrew Morton <akpm@osdl.org>
Subject: Re: [PATCH] kswapd shall not sleep during page shortage
Message-Id: <20041109133343.0b34896d.akpm@osdl.org>
In-Reply-To: <20041109174125.GF7632@logos.cnet>
References: <20041109164642.GE7632@logos.cnet>
	<20041109121945.7f35d104.akpm@osdl.org>
	<20041109174125.GF7632@logos.cnet>
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To: Marcelo Tosatti <marcelo.tosatti@cyclades.com>
Cc: linux-mm@kvack.org, piggin@cyberone.com.au
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Marcelo Tosatti <marcelo.tosatti@cyclades.com> wrote:
>
> On Tue, Nov 09, 2004 at 12:19:45PM -0800, Andrew Morton wrote:
> > Marcelo Tosatti <marcelo.tosatti@cyclades.com> wrote:
> > >
> > > 
> > > Andrew,
> > > 
> > > I was wrong last time I read balance_pgdat() when I thought kswapd
> > > couldnt sleep under page shortage. 
> > > 
> > > It can, because all_zones_ok is set to "1" inside the 
> > > "priority=DEF_PRIORITY; priority >= 0; priority--" loop.
> > > 
> > > So this patch sets "all_zones_ok" to zero even if all_unreclaimable 
> > > is set, avoiding it from sleeping when zones are under page short.
> > > 
> > 
> > Does this solve any observed problem?  What testing was done, and what were
> > the results??
> 
> 
> The observed problem are the page allocation failures!

But the patch doesn't have any effect on that, which I can see.

> No testing has been done, but it is an obvious problem if you read the
> code. 

Not really.  The move of the all_unreclaimable test doesn't seem to do
anything, because we'll just skip that zone anyway in the next loop.

Maybe you moved the all_unreclaimable test just so that there's an
opportunity to clear all_zones_ok?  I dunno.

AFAICT, the early clearing of all_zones_ok will have no effect on kswapd
throttling because the total_scanned logic is disabled.

What I think your patch will do is to cause kswapd to do the `goto
loop_again' thing if all zones are unreclaimable.  Which appears to risk
putting kswapd into a busy loop when we're out of memory.

So I'm all confused and concerned.  It would help if you were to explain
your thinking more completely...
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