linux-kernel - Re: [patch 2/5] Reinstate ZERO_PAGE optimization in get_user

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Date:	Mon, 23 Jun 2008 11:32:54 -0700 (PDT)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Jeff Chua <jeff.chua.linux@...il.com>
cc:	Hugh Dickins <hugh@...itas.com>, Greg KH <gregkh@...e.de>,
	linux-kernel@...r.kernel.org, stable@...nel.org,
	Justin Forbes <jmforbes@...uxtx.org>,
	Zwane Mwaikambo <zwane@....linux.org.uk>,
	"Theodore Ts'o" <tytso@....edu>,
	Randy Dunlap <rdunlap@...otime.net>,
	Dave Jones <davej@...hat.com>,
	Chuck Wolber <chuckw@...ntumlinux.com>,
	Chris Wedgwood <reviews@...cw.f00f.org>,
	Michael Krufky <mkrufky@...uxtv.org>,
	Chuck Ebbert <cebbert@...hat.com>,
	Domenico Andreoli <cavokz@...il.com>, Willy Tarreau <w@....eu>,
	Rodrigo Rubira Branco <rbranco@...checkpoint.com>,
	akpm@...ux-foundation.org, alan@...rguk.ukuu.org.uk,
	Oleg Nesterov <oleg@...sign.ru>, Nick Piggin <npiggin@...e.de>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Ingo Molnar <mingo@...e.hu>, Roland McGrath <roland@...hat.com>
Subject: Re: [patch 2/5] Reinstate ZERO_PAGE optimization in get_user_pages()
 and fix XIP



On Tue, 24 Jun 2008, Jeff Chua wrote:

> On Tue, Jun 24, 2008 at 1:27 AM, Linus Torvalds
> <torvalds@...ux-foundation.org> wrote:
> 
> > On the other hand, if you add a trace to the "use_zero_page()" function to
> > print out the vm_flags and other details, that probably would help.
> 
> Let me know if you still want me to test this.

No, it's fine. It really was a bug, and a long-standing one, just one that 
was probably practically impossible to hit before (because we used to only 
do the FOLL_ANON logic on missing whole page tables, and just about any 
access to any mapping even nearby the one you care about will fill in the 
page tables - so you would have had to be really unlucky to trigger the 
case before).

The patch clearly fixes an issue, and makes the code more readable and 
maintainable too, so I don't care what the exact mapping flags etc were.

> Yeh, it works great! Thank you.

Thanks for bisecting, reporting and testing. 

			Linus
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