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Date: Tue, 26 Jan 2010 11:12:16 -0500
From: Rik van Riel <riel@redhat.com>
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Subject: Re: [PATCH 04 of 31] update futex compound knowledge
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To: Andrea Arcangeli <aarcange@redhat.com>
Cc: linux-mm@kvack.org, Marcelo Tosatti <mtosatti@redhat.com>, Adam Litke <agl@us.ibm.com>, Avi Kivity <avi@redhat.com>, Izik Eidus <ieidus@redhat.com>, Hugh Dickins <hugh.dickins@tiscali.co.uk>, Nick Piggin <npiggin@suse.de>, Mel Gorman <mel@csn.ul.ie>, Andi Kleen <andi@firstfloor.org>, Dave Hansen <dave@linux.vnet.ibm.com>, Benjamin Herrenschmidt <benh@kernel.crashing.org>, Ingo Molnar <mingo@elte.hu>, Mike Travis <travis@sgi.com>, KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>, Christoph Lameter <cl@linux-foundation.org>, Chris Wright <chrisw@sous-sol.org>, Andrew Morton <akpm@linux-foundation.org>, bpicco@redhat.com, Christoph Hellwig <chellwig@redhat.com>, KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
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On 01/26/2010 08:51 AM, Andrea Arcangeli wrote:
> From: Andrea Arcangeli<aarcange@redhat.com>
>
> Futex code is smarter than most other gup_fast O_DIRECT code and knows about
> the compound internals. However now doing a put_page(head_page) will not
> release the pin on the tail page taken by gup-fast, leading to all sort of
> refcounting bugchecks. Getting a stable head_page is a little tricky.
>
> Signed-off-by: Andrea Arcangeli<aarcange@redhat.com>
> ---
>
> diff --git a/kernel/futex.c b/kernel/futex.c
> --- a/kernel/futex.c
> +++ b/kernel/futex.c
> @@ -218,7 +218,7 @@ get_futex_key(u32 __user *uaddr, int fsh
>   {
>   	unsigned long address = (unsigned long)uaddr;
>   	struct mm_struct *mm = current->mm;
> -	struct page *page;
> +	struct page *page, *page_head;
>   	int err;
>
>   	/*
> @@ -250,10 +250,32 @@ again:
>   	if (err<  0)
>   		return err;
>
> -	page = compound_head(page);
> -	lock_page(page);
> -	if (!page->mapping) {
> -		unlock_page(page);
> +	page_head = page;

...

> +	if (unlikely(page_head != page)) {

Should the line above be "page_head = compound_head(page);" or
am I missing something?

If I am missing something, the changelog message could be a
little more verbose :)

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