linux-kernel - Re: [BUGFIX][PATCH 2/3] kcore: fix vread/vwrite to be aware of holes.

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Date:	Tue, 4 Aug 2009 18:24:57 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
Cc:	scgtrp@...il.com, bugzilla-daemon@...zilla.kernel.org,
	bugme-daemon@...zilla.kernel.org, xiyou.wangcong@...il.com,
	linux-kernel@...r.kernel.org
Subject: Re: [BUGFIX][PATCH 2/3] kcore: fix vread/vwrite to be aware of
 holes.

On Wed, 5 Aug 2009 10:08:43 +0900 KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com> wrote:

> > > +		/*
> > > +		 * To do safe access to this _mapped_ area, we need
> > > +		 * lock. But adding lock here means that we need to add
> > > +		 * overhead of vmalloc()/vfree() calles for this _debug_
> > > +		 * interface, rarely used. Instead of that, we'll use
> > > +		 * kmap() and get small overhead in this access function.
> > > +		 */
> > > +		if (p) {
> > > +			/* we can expect USR1 is not used */
> > 
> > It would be nice if the comment were to explain _why_ KM_USER1 is known
> > to be free here.
> > 
> ok, will do. Hmm, but KM_USR0 is better ?
> I'm not sure difference between KM_USR0/KM_USR1...

KM_USER0 is conventionally the first one to use.  We only use KM_USER1
if it is known that KM_USER0 is already used.

> 
> 
> > 
> > > +			void *map = kmap_atomic(p, KM_USER1);
> > > +			memcpy(buf, map + offset, length);
> > > +			kunmap_atomic(map, KM_USER1);
> > 
> > Can use clear_highpage().
> > 
> ah, "buf" is expected to be kmalloc'ed buffer. I'll explain it in Usage
> or adds comment.
> 

argh, memcpy != memset.  I knew that ;)
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