Subject: Re: memory hotplug: hot-add to ZONE_MOVABLE vs. min_free_kbytes
From: Gerald Schaefer <gerald.schaefer@de.ibm.com>
To: Yasunori Goto <y-goto@jp.fujitsu.com>
Cc: Mel Gorman <mel@csn.ul.ie>, linux-kernel@vger.kernel.org,
       linux-mm@kvack.org, schwidefsky@de.ibm.com, heiko.carstens@de.ibm.com,
       KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>,
       Dave Hansen <haveblue@us.ibm.com>, Andy Whitcroft <apw@shadowen.org>,
       Christoph Lameter <cl@linux-foundation.org>,
       Nick Piggin <npiggin@suse.de>, Peter Zijlstra <peterz@infradead.org>,
       Andrew Morton <akpm@linux-foundation.org>
In-Reply-To: <20080801192646.EC99.E1E9C6FF@jp.fujitsu.com>
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	 <1217526327.4643.35.camel@localhost.localdomain>
	 <20080801192646.EC99.E1E9C6FF@jp.fujitsu.com>
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Date: Fri, 01 Aug 2008 18:04:55 +0200
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On Fri, 2008-08-01 at 20:16 +0900, Yasunori Goto wrote:
> > My assumption is now, that the reserved 3 MB in ZONE_MOVABLE won't be
> > usable by the kernel anymore, e.g. for PF_MEMALLOC, because it is in
> > ZONE_MOVABLE now.
> 
> I don't make sense here. I suppose there is no relationship between
> ZONE_MOVABLE, PF_MEMALLOC and MIGRATE_RESERVE pages.
> Could you tell me more?

Ok, I thought that PF_MEMALLOC allocations work on the MIGRATE_RESERVE
pageblocks, and that only kernel allocations can use PF_MEMALLOC. I also
thought that kernel allocations cannot use ZONE_MOVABLE, e.g. for page
cache memory, because such pages would not be migratable. So I assumed
that MIGRATE_RESERVE pageblocks in ZONE_MOVABLE would not be available
for PF_MEMALLOC allocations.

With this assumption, which can be totally wrong, the redistribution
of MIGRATE_RESERVE pageblocks in setup_per_zone_pages_min() looks like
it will take away reserved pageblocks that should be available to the
kernel in emergency situations.

Maybe I should have explained this assumption earlier, because my whole
min_free_kbytes issue depends on it. If I'm wrong, I apologize for
confusing you all with this "issue", and I will go back to the original
problem with removing the lowest memory chunk in ZONE_MOVABLE...

Thanks,
Gerald


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