linux-kernel - Re: [PATCH] mm: hugetlb: bail out unmapping after serving reference page

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Message-Id: <20120222130659.d75b6f69.akpm@linux-foundation.org>
Date:	Wed, 22 Feb 2012 13:06:59 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Hillf Danton <dhillf@...il.com>
Cc:	Linux-MM <linux-mm@...ck.org>, LKML <linux-kernel@...r.kernel.org>,
	Michal Hocko <mhocko@...e.cz>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Hugh Dickins <hughd@...gle.com>
Subject: Re: [PATCH] mm: hugetlb: bail out unmapping after serving reference
 page

On Wed, 22 Feb 2012 20:35:34 +0800
Hillf Danton <dhillf@...il.com> wrote:

> When unmapping given VM range, we could bail out if a reference page is
> supplied and it is unmapped, which is a minor optimization.
> 
> Signed-off-by: Hillf Danton <dhillf@...il.com>
> ---
> 
> --- a/mm/hugetlb.c	Wed Feb 22 19:34:12 2012
> +++ b/mm/hugetlb.c	Wed Feb 22 19:50:26 2012
> @@ -2280,6 +2280,9 @@ void __unmap_hugepage_range(struct vm_ar
>  		if (pte_dirty(pte))
>  			set_page_dirty(page);
>  		list_add(&page->lru, &page_list);
> +
> +		if (page == ref_page)
> +			break;
>  	}
>  	spin_unlock(&mm->page_table_lock);
>  	flush_tlb_range(vma, start, end);

Perhaps add a little comment to this explaining what's going on?


It would be sufficient to do

	if (ref_page)
		break;

This is more efficient, and doesn't make people worry about whether
this value of `page' is the same as the one which
pte_page(huge_ptep_get()) earlier returned.

Why do we evaluate `page' twice inside that loop anyway?  And why do we
check for huge_pte_none() twice?  It looks all messed up.



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