linux-kernel - Re: memory hotplug: hot-add to ZONE_MOVABLE vs. min_free

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Message-Id: <1217606695.5678.4.camel@localhost.localdomain>
Date:	Fri, 01 Aug 2008 18:04:55 +0200
From:	Gerald Schaefer <gerald.schaefer@...ibm.com>
To:	Yasunori Goto <y-goto@...fujitsu.com>
Cc:	Mel Gorman <mel@....ul.ie>, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org, schwidefsky@...ibm.com,
	heiko.carstens@...ibm.com,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Dave Hansen <haveblue@...ibm.com>,
	Andy Whitcroft <apw@...dowen.org>,
	Christoph Lameter <cl@...ux-foundation.org>,
	Nick Piggin <npiggin@...e.de>,
	Peter Zijlstra <peterz@...radead.org>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: memory hotplug: hot-add to ZONE_MOVABLE vs. min_free_kbytes

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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