linux-kernel - Re: [PATCH] mm: Make copy_from_user() in migrate.c statically predictable

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Message-Id: <20100219102247.B939.A69D9226@jp.fujitsu.com>
Date:	Fri, 19 Feb 2010 10:25:15 +0900 (JST)
From:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
To:	"H. Peter Anvin" <hpa@...ux.intel.com>
Cc:	kosaki.motohiro@...fujitsu.com,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, "H. Peter Anvin" <hpa@...or.com>,
	Arjan van de Ven <arjan@...ux.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Christoph Lameter <cl@...ux-foundation.org>,
	Hugh Dickins <hugh.dickins@...cali.co.uk>,
	Rik van Riel <riel@...hat.com>, Ingo Molnar <mingo@...e.hu>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH] mm: Make copy_from_user() in migrate.c statically predictable

> On 02/18/2010 03:02 PM, Linus Torvalds wrote:
> > 
> > Hmm. When making simplifications like this, I would really suggest you 
> > also move the declaration of the variable itself into the block where it 
> > is now used, rather than leaving it be function-wide.
> > 
> > Yes, it's used in the final condition of the for-loop, but that whole loop 
> > is just screwy. The 'err' handling is insane. Sometimes 'err' is a return 
> > value form copy_to/from_user, and sometimes it's a errno. The two are 
> > _not_ the same thing, they don't even have the same type!
> > 
> > And 'i' is totally useless too.
> > 
> > So that whole loop should be rewritten.
> > 
> 
> OK, I was trying to make the minimal set of changes given the late -rc
> status.
> 
> > I don't even have page migration enabled, so I haven't even compile-tested 
> > this, but wouldn't something like this work? It's smaller, gets rid of two 
> > pointless variables, and looks simpler to me. Hmm?
> 
> The code definitely looks cleaner, and it's a much more standard
> "chunked data loop" form.  Weirdly enough, though, gcc 4.4.2 can't
> figure out the copy_from_user() that way... despite having the same
> min() structure as my code.
> 
> However, if I change it to:
> 
> 		chunk_nr = nr_pages;
> 		if (chunk_nr > DO_PAGES_STAT_CHUNK_NR)
> 			chunk_nr = DO_PAGES_STAT_CHUNK_NR;
> 
> ... then it works!
> 
> Overall, it looks like gcc is rather fragile with regards to its ability
> to constant-propagate.  It's probably no coincidence that chunked loops
> is the place where we really have problems with this kind of stuff.
> 
> Updated patch, which compile-tests for me, attached.

hehe, I'm ESPer. I think you hope I do runtime-test, plz wait 12 hour :-)



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