linux-kernel - Re: [PATCH 7/8] mm: reinstate ZERO

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Date:	Wed, 9 Sep 2009 10:44:23 +0900
From:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
To:	Hugh Dickins <hugh.dickins@...cali.co.uk>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Nick Piggin <npiggin@...e.de>, Rik van Riel <riel@...hat.com>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH 7/8] mm: reinstate ZERO_PAGE

On Tue, 8 Sep 2009 12:56:57 +0100 (BST)
Hugh Dickins <hugh.dickins@...cali.co.uk> wrote:

> On Tue, 8 Sep 2009, KAMEZAWA Hiroyuki wrote:
> > 
> > A nitpick but this was a concern you shown, IIUC.
> > 
> > == __get_user_pages()..
> > 
> >                         if (pages) {
> >                                 pages[i] = page;
> > 
> >                                 flush_anon_page(vma, page, start);
> >                                 flush_dcache_page(page);
> >                         }
> > ==
> > 
> > This part will call flush_dcache_page() even when ZERO_PAGE is found.
> > 
> > Don't we need to mask this ?
> 
> No, it's okay to flush_dcache_page() on ZERO_PAGE: we always used to
> do that there, and the arches I remember offhand won't do anything
> with it anyway, once they see page->mapping NULL.
> 
> What you're remembering, that I did object to, was the way your
> FOLL_NOZERO ended up doing
> 				pages[i] = NULL;
> 				flush_anon_page(vma, NULL, start);
> 				flush_dcache_page(NULL);
> 
> which would cause an oops when those arches look at page->mapping.
> 
> I should take another look at your FOLL_NOZERO: I may have dismissed
> it too quickly, after seeing that bug, and oopsing on x86 when
> mlocking a readonly anonymous area.
> 
> Though I like that we don't _need_ to change mlock.c for reinstated
> ZERO_PAGE, this morning I'm having trouble persuading myself that
> mlocking a readonly anonymous area is too silly to optimize for.
> 
> Maybe the very people who persuaded you to bring back the anonymous
> use of ZERO_PAGE, are also doing a huge mlock of the area first?
No, as far as I know, they'll not do huge mlock.


Thanks,
-Kame

> So if two or more are starting up at the same time on the same box,
> more bouncing than is healthy (and more than they would have seen
> in the old days of ZERO_PAGE but no lock_page on it there).
> 
> I'd like to persuade myself not to bother,
> but may want to add a further patch for that later.
> 
> Hugh
> 
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