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

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Date:	Wed, 03 Dec 2008 14:16:07 -0800
From:	Dave Hansen <dave@...ux.vnet.ibm.com>
To:	gerald.schaefer@...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 22:25 +0100, Gerald Schaefer wrote:
> offline_pages() calls lru_add_drain_all() followed by drain_all_pages().
> While drain_all_pages() works on each cpu, lru_add_drain_all() only runs
> on the current cpu for architectures w/o CONFIG_NUMA.

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.

> This let us run
> into the BUG_ON(!PageBuddy(page)) in __offline_isolated_pages() during
> memory hotplug stress test on s390. The page in question was still on the
> pcp list, because of a race with lru_add_drain_all() and drain_all_pages()
> on different cpus.
> 
> This is fixed with this patch by adding CONFIG_MEMORY_HOTREMOVE to the
> lru_add_drain_all() #ifdef, to let it run on each 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?

-- Dave

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