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Date: Sun, 19 Feb 2012 13:21:02 -0800 (PST)
From: Hugh Dickins <hughd@google.com>
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To: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
cc: linux-mm@kvack.org, Andrew Morton <akpm@linux-foundation.org>,
        David Rientjes <rientjes@google.com>, Andi Kleen <andi@firstfloor.org>,
        Wu Fengguang <fengguang.wu@intel.com>,
        Andrea Arcangeli <aarcange@redhat.com>,
        KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>,
        KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH 2/6] thp: optimize away unnecessary page table locking
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On Wed, 8 Feb 2012, Naoya Horiguchi wrote:
> Currently when we check if we can handle thp as it is or we need to
> split it into regular sized pages, we hold page table lock prior to
> check whether a given pmd is mapping thp or not. Because of this,
> when it's not "huge pmd" we suffer from unnecessary lock/unlock overhead.
> To remove it, this patch introduces a optimized check function and
> replace several similar logics with it.
> 
> Signed-off-by: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
> Cc: David Rientjes <rientjes@google.com>
> 
> Changes since v4:
>   - Rethink returned value of __pmd_trans_huge_lock()

[snip]

> --- 3.3-rc2.orig/mm/mremap.c
> +++ 3.3-rc2/mm/mremap.c
> @@ -155,8 +155,6 @@ unsigned long move_page_tables(struct vm_area_struct *vma,
>  			if (err > 0) {
>  				need_flush = true;
>  				continue;
> -			} else if (!err) {
> -				split_huge_page_pmd(vma->vm_mm, old_pmd);
>  			}
>  			VM_BUG_ON(pmd_trans_huge(*old_pmd));
>  		}

Is that what you intended to do there?
I just hit that VM_BUG_ON on rc3-next-20120217.

Hugh