linux-kernel - Re: [PATCH] memory hotplug: run lru_add_drain

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Date:	Fri, 05 Dec 2008 14:08:20 +0100
From:	Gerald Schaefer <gerald.schaefer@...ibm.com>
To:	Dave Hansen <dave@...ux.vnet.ibm.com>
Cc:	akpm@...ux-foundation.org, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org, schwidefsky@...ibm.com,
	heiko.carstens@...ibm.com, kamezawa.hiroyu@...fujitsu.com,
	y-goto@...fujitsu.com, npiggin@...e.de
Subject: Re: [PATCH] memory hotplug: run lru_add_drain_all() on each cpu

On Wed, 2008-12-03 at 14:16 -0800, Dave Hansen wrote:
> I'm a bit confused why this is.  Is this because the LRUs are per-zone
> and we expected !CONFIG_NUMA systems to only have LRUs sitting on the
> same (only) node as the current CPU?
> 
> This doesn't make any sense, though.  The pagevecs that
> drain_cpu_pagevecs() actually empties out are per-cpu.

Right, the pagevecs are per-cpu, independent from any CONFIG_NUMA
settings, and this is exactly why I would expect that lru_add_drain_all()
works on all cpus, as opposed to lru_add_drain() which works only on
the current cpu.

> This doesn't seem right to me.  CONFIG_MEMORY_HOTREMOVE doesn't change
> the layout of the LRUs, unlike NUMA or UNEVICTABLE_LRU.  So, I think
> this bug is more due to the hotremove code mis-expecting behavior out of
> lru_add_drain_all().
> 
> Why does this not affect the other lru_add_drain_all() users?

Good question, there are only a few other users and most of them were
added just recently with the unevictable lru patches. The only exception
is migrate_prep(), but this is only called from sys_move_pages(), which
is not implemented w/o CONFIG_NUMA afaik.

As explained above, the per-cpu pagevec layout should be independent
from NUMA or UNEVICTABLE_LRU, so I guess the right thing to do here
is completely remove the #ifdef as in the patch from Kosaki Motohiro
(or at least replace it with a CONFIG_SMP as suggested by Kamezawa
Hiroyuki).

Thanks,
Gerald

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