linux-kernel - Re: RFC: Transparent Hugepage support

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Date:	Fri, 30 Oct 2009 09:40:37 +0900
From:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
To:	Andrea Arcangeli <aarcange@...hat.com>
Cc:	Ingo Molnar <mingo@...e.hu>, Andi Kleen <andi@...stfloor.org>,
	linux-mm@...ck.org, Marcelo Tosatti <mtosatti@...hat.com>,
	Adam Litke <agl@...ibm.com>, Avi Kivity <avi@...hat.com>,
	Izik Eidus <ieidus@...hat.com>,
	Hugh Dickins <hugh.dickins@...cali.co.uk>,
	Nick Piggin <npiggin@...e.de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org
Subject: Re: RFC: Transparent Hugepage support

On Thu, 29 Oct 2009 11:36:58 +0100
Andrea Arcangeli <aarcange@...hat.com> wrote:

> > A small comment regarding the patch itself: i think it could be 
> > simplified further by eliminating CONFIG_TRANSPARENT_HUGEPAGE and by 
> > making it a natural feature of hugepage support. If the code is correct 
> > i cannot see any scenario under which i wouldnt want a hugepage enabled 
> > kernel i'm booting to not have transparent hugepage support as well.
> 
> The two reasons why I added a config option are:
> 
> 1) because it was easy enough, gcc is smart enough to eliminate the
> external calls so I didn't need to add ifdefs with the exception of
> returning 0 from pmd_trans_huge and pmd_trans_frozen. I only had to
> make the exports of huge_memory.c visible unconditionally so it doesn't
> warn, after that I don't need to build and link huge_memory.o.
> 
> 2) to avoid breaking build of archs not implementing pmd_trans_huge
> and that may never be able to take advantage of it
> 
> But we could move CONFIG_TRANSPARENT_HUGEPAGE to an arch define forced
> to Y on x86-64 and N on power.

Ah, please keep CONFIG_TRANSPARENT_HUGEPAGE for a while.
Now, memcg don't handle hugetlbfs because it's special and cannot be freed by
the kernel, only users can free it. But this new transparent-hugepage seems to
be designed as that the kernel can free it for memory reclaiming.
So, I'd like to handle this in memcg transparently.

But it seems I need several changes to support this new rule.
I'm glad if this new huge page depends on !CONFIG_CGROUP_MEM_RES_CTRL for a
while.

Thanks,
-Kame

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