Date: Sun, 21 Feb 2010 20:45:37 +0900 (JST)
From: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
To: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Subject: Re: [PATCH] mm: Make copy_from_user() in migrate.c statically predictable
Cc: kosaki.motohiro@jp.fujitsu.com, "H. Peter Anvin" <hpa@linux.intel.com>,
       Linus Torvalds <torvalds@linux-foundation.org>,
       linux-kernel@vger.kernel.org, "H. Peter Anvin" <hpa@zytor.com>,
       Arjan van de Ven <arjan@linux.kernel.org>,
       Andrew Morton <akpm@linux-foundation.org>,
       Christoph Lameter <cl@linux-foundation.org>,
       Hugh Dickins <hugh.dickins@tiscali.co.uk>,
       Rik van Riel <riel@redhat.com>, Ingo Molnar <mingo@elte.hu>,
       Thomas Gleixner <tglx@linutronix.de>
In-Reply-To: <20100219102247.B939.A69D9226@jp.fujitsu.com>
References: <4B7DD89F.4050003@linux.intel.com> <20100219102247.B939.A69D9226@jp.fujitsu.com>
Message-Id: <20100221203219.B80D.A69D9226@jp.fujitsu.com>
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> > 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 :-)

sorry for the delay. I had little machine trouble.
	Reviewed-and-Tested-by: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>


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