We are considering removing this printk at boot time from RHEL because
it will confuse customers, encourage them to change the boot parameters
and generate extraneous support calls.  Its documented in
Documentation/kernel-parameters.txt anyway.  Any thoughts???

Larry Woodman


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OOPS, sorry to attach the patch:


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Yeah, that is a strange boot message...


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On Wed, 31 Mar 2010 13:57:46 -0400
please CC linux-mm and maintainers.

Acked-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>

It have been there for a year and I think memory usage by page_cgroup
will not surprise linux kernel users, more.

Assume x86-32.

RHEL allows amount of memory up to 16G, right?

without memcg: memmap uses 32bytes * 16G/4k = 128M.
with memcg:    memmap+page_cgroup uses (32+20) bytes * 16G/4k = 208M.

I thought this may cause OOM in ZONE_NORMAL. Then, I added it when I wrote
original patch. This kind of memory eater can cause trouble when it pops
up suddenly. But I think 'one year' can be an excuse.

Thanks,
-Kame


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I've seen this issue come up on multiple machines, I think the printk
is useful. However, we might need to change the panic() to a big fat
warning and disable the memcg controller if we fail to allocate memory
in page_cgroup_init_flatmem(). 

-- 
	Three Cheers,
	Balbir
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Probably a stupid question: but isn't it possible to allocate the huge
amounts of memory only if somebody activates memcg during runtime?
And then allocate everything using vmalloc?
But that probably doesn't work, since you need to record everything
from the boot of the system, I would guess?
Just wondering because we do everything to not even waste a single bit
in struct page and all of a sudden on the enterprise distros we allocate
(by default!) 40 additional bytes per page.
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On Wed, 7 Apr 2010 10:00:14 +0200

Activation can occur only at boot but page_cgroup allocation happens at

The story was..

1. at first, page_cgroup was allocated on demand. but we need to have
   page->page_cgroup pointer. Then, we pay 8bytes per page even if we
   disable memory cgroup.
   All page behavior was tracked since boot time.

2. Fedora maintaienr said "we never enable memcg if you contiue to use
   page->page_cgroup pointer, 8bytes per page costs!".
   Then, we decieded to allocate page_cgroup at boot time, and allocate
   all at once at boot time. This makes memcg runtime robust. And we
   got rid of page->page_cgroup pointer.

3. Then, I added warning when I wrote a patch to allocate page_cgroup at boot.
   It's easy to avoid extra 40bytes.
   For enterprise, I have no concern. Enterprise admin tend to be careful and
   check all default value when he use a new kernel.
   That message was for desktop guys using desktop distro.

   Disabling memory cgroup at default may be a choice. But no one send such kind
   of patch until now.

Thanks,
-Kame

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Thanks for explaining! Looks like whatever you do there's always somebody
who complains.
Distros _could_ ship with memcg by default off independently of the upstream
kernel. But it looks like they won't. Anyway that's a problem we can't solve
here.
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(Added related people to Cc list.)

I agree to removing this message.
We've already removed similar message about swap_cgroup in commit 627991a2.

	Acked-by: Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp>

Thanks,
Daisuke Nishimura.
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