linux-kernel - Re: [PATCH] ZERO

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Message-Id: <20090810091458.1e889cdc.kamezawa.hiroyu@jp.fujitsu.com>
Date:	Mon, 10 Aug 2009 09:14:58 +0900
From:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
To:	Hugh Dickins <hugh.dickins@...cali.co.uk>
Cc:	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Nick Piggin <npiggin@...e.de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH] ZERO_PAGE again v5.

On Sun, 9 Aug 2009 18:28:48 +0100 (BST)
Hugh Dickins <hugh.dickins@...cali.co.uk> wrote:

> On Wed, 5 Aug 2009, KAMEZAWA Hiroyuki wrote:
> > Updated from v4 as
> >   - avoid to add new arguments to vm_normal_page().
> >     vm_normal_page() always returns NULL if ZERO_PAGE is found.
> >   - follow_page() directly handles pte_special and ANON_ZERO_PAGE.
> > 
> > Then, amount of changes are reduced. Thanks for advices.
> > 
> > Concerns pointed out:
> >   - Does use_zero_page() cover all cases ?
> >     I think yes..
> >   - All get_user_pages() callers, which may find ZERO_PAGE is safe ?
> >     need tests.
> >   - All follow_pages() callers, which may find ZERO_PAGE is safe ?
> >     I think yes.
> 
> Sorry, KAMEZAWA-san, I'm afraid this is still some way off being right.
> 
> Certainly the extent of the v5 patch is much more to my taste than v4
> was, thank you.
> 
At first, thank you for review.

> Something that's missing, which we can get away with but probably
> need to reinstate, is the shortcut when COWing: not to copy the
> ZERO_PAGE, but just do a memset.
> 
> But just try mlock'ing a private readonly anon area into which you've
> faulted a zero page, and the "BUG: Bad page map" message tells us
> it's quite wrong to be trying use_zero_page() there.
> 
> Actually, I don't understand ignore_zero at all: it's used solely by
> the mlock case, yet its effect seems to be precisely not to fault in
> pages if they're missing - I wonder if you've got in a muddle between
> the two very different awkward cases, mlocking and coredumps of
> sparsely populated areas.
> 
Ah, then, you say mlock() should allocate 'real' page if zero page
is mapped. Right ?

"How to handle mlock" is a concern for me, too. But I selected this
to allow the same behavior to old kernels.

> And I don't at all like the way you flush_dcache_page(page) on a
> page which may now be NULL: okay, you're only encouraging x86 to
> say Yes to the Kconfig option, but that's a landmine for the first
> arch with a real flush_dcache_page(page) which says Yes to it.
> 
do_wp_page()
	-> cow_user_page()
		-> (src is NULL)
Ah....ok, it's bug. I added ....Sorry, I didn't see this in older version
and missed this.

> Actually, the Kconfig stuff seems silly to me (who's going to know
> how to choose on or off?): the only architecture which wanted more
> than one ZERO_PAGE was MIPS, and it doesn't __HAVE_ARCH_PTE_SPECIAL
> yet, so I think I'm going to drop all the Kconfig end of it.
> 
ok, I have no strong demands on it.

> Because I hate reviewing things and trying to direct other people
> by remote control: what usually happens is I send them off in some
> direction which, once I try to do it myself, turns out to have been
> the wrong direction.  I do need to try to do this myself, instead of
> standing on the sidelines criticizing.
> 
> In fairness, I think Linus himself was a little confused when he
> separated off use_zero_page(): I think we've all got confused around
> there (as we noticed a month or so ago when discussing its hugetlb
> equivalent), and I need to think it through again at last.
> 
> I'll get on to it now.
> 

Thank you for comments. I'll go to a trip until Aug/17, programming-camp,
I'll be able to consider this patch and the whole things aroung paging in calm
enviroment. I'll try to restart from scratch.

Thanks,
-Kame

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