linux-kernel - Re: [PATCH 3/4] mm: vmscan: Evaluate the watermarks against the correct classzone

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Date:	Tue, 28 Jun 2011 13:52:49 +0100
From:	Mel Gorman <mgorman@...e.de>
To:	Minchan Kim <minchan.kim@...il.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	P?draig Brady <P@...igbrady.com>,
	James Bottomley <James.Bottomley@...senpartnership.com>,
	Colin King <colin.king@...onical.com>,
	Andrew Lutomirski <luto@....edu>,
	Rik van Riel <riel@...hat.com>,
	Johannes Weiner <hannes@...xchg.org>,
	linux-mm <linux-mm@...ck.org>,
	linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 3/4] mm: vmscan: Evaluate the watermarks against the
 correct classzone

On Mon, Jun 27, 2011 at 03:53:04PM +0900, Minchan Kim wrote:
> On Fri, Jun 24, 2011 at 11:44 PM, Mel Gorman <mgorman@...e.de> wrote:
> > When deciding if kswapd is sleeping prematurely, the classzone is
> > taken into account but this is different to what balance_pgdat() and
> > the allocator are doing. Specifically, the DMA zone will be checked
> > based on the classzone used when waking kswapd which could be for a
> > GFP_KERNEL or GFP_HIGHMEM request. The lowmem reserve limit kicks in,
> > the watermark is not met and kswapd thinks its sleeping prematurely
> > keeping kswapd awake in error.
> 
> 
> I thought it was intentional when you submitted a patch firstly.

It was, it also wasn't right.

> "Kswapd makes sure zones include enough free pages(ie, include reserve
> limit of above zones).
> But you seem to see DMA zone can't meet above requirement forever in
> some situation so that kswapd doesn't sleep.
> Right?
> 

Right.

> >
> > Reported-and-tested-by: Pádraig Brady <P@...igBrady.com>
> > Signed-off-by: Mel Gorman <mgorman@...e.de>
> > ---
> >  mm/vmscan.c |    2 +-
> >  1 files changed, 1 insertions(+), 1 deletions(-)
> >
> > diff --git a/mm/vmscan.c b/mm/vmscan.c
> > index 9cebed1..a76b6cc2 100644
> > --- a/mm/vmscan.c
> > +++ b/mm/vmscan.c
> > @@ -2341,7 +2341,7 @@ static bool sleeping_prematurely(pg_data_t *pgdat, int order, long remaining,
> >                }
> >
> >                if (!zone_watermark_ok_safe(zone, order, high_wmark_pages(zone),
> > -                                                       classzone_idx, 0))
> > +                                                       i, 0))
> 
> Isn't it  better to use 0 instead of i?
> 

I considered it but went with i to compromise between making sure zones
included enough free pages without requiring that ZONE_DMA meet an
almost impossible requirement when under continual memory pressure.

-- 
Mel Gorman
SUSE Labs
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