linux-kernel - Re: [PATCH] devmem: handle partial kmem write/read

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Message-ID: <20090915020518.GB15159@localhost>
Date:	Tue, 15 Sep 2009 10:05:18 +0800
From:	Wu Fengguang <fengguang.wu@...el.com>
To:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	"mtosatti@...hat.com" <mtosatti@...hat.com>,
	"gregkh@...e.de" <gregkh@...e.de>,
	"broonie@...nsource.wolfsonmicro.com" 
	<broonie@...nsource.wolfsonmicro.com>,
	"johannes@...solutions.net" <johannes@...solutions.net>,
	"avi@...ranet.com" <avi@...ranet.com>,
	"andi@...stfloor.org" <andi@...stfloor.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	WANG Cong <xiyou.wangcong@...il.com>,
	Mike Smith <scgtrp@...il.com>,
	Nick Piggin <nickpiggin@...oo.com.au>
Subject: Re: [PATCH] devmem: handle partial kmem write/read

On Tue, Sep 15, 2009 at 09:52:55AM +0800, KAMEZAWA Hiroyuki wrote:
> On Tue, 15 Sep 2009 09:24:48 +0900
> KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com> wrote:
> 
> > On Mon, 14 Sep 2009 12:34:44 +0800
> > Wu Fengguang <fengguang.wu@...el.com> wrote:
> > 
> > > Hi Kame,
> > > 
> > > This patch needs more work. I first intent to fix a bug:
> > > 
> > >                         sz = vwrite(kbuf, (char *)p, sz);
> > >                         p += sz;
> > >                 }
> > > 
> > > So if the returned len is 0, the kbuf/p pointers will mismatch. 
> > > 
> > > Then I realize it changed the write behavior. The current vwrite()
> > > behavior is strange, it returns 0 if _whole range_ is hole, otherwise
> > > ignores the hole silently. So holes can be treated differently even in
> > > the original code.
> > > 
> > Ah, ok...
> > 
> 
> I'd like to post a fix...but it seems you are woriking.
> Hmm, I'll post a patch including your fix (and your sign.)
> Is it ok ?

Sure OK. You can work based on my patches. Many of them are in -mm now
and there are three more. I'll show you in a series.

Thanks,
Fengguang

> > > I'm not really sure about the right behavior. KAME-san, do you have
> > > any suggestions?
> > > 
> > maybe following make sense.
> > =
> > written = vwrite(kbuf, (char *p), sz);
> > if (!written) // whole vmem was a hole
> > 	written = sz;
> > ==
> > needs fix.
> > 
> > Anyway, I wonder kmem is broken. It's should be totally rewritten.
> > 
> > For example, this doesn't check anything.
> > ==
> >         if (p < (unsigned long) high_memory) {
> > 
> > ==
> > 
> > But....are there users ?
> > If necessary, I'll write some...
> > 
> > Thanks,
> > -Kame
> > 
> > 
> > > Thanks,
> > > Fengguang
> > > 
> > > On Mon, Sep 14, 2009 at 11:29:01AM +0800, Wu Fengguang wrote:
> > > > Current vwrite silently ignores vwrite() to vmap holes.
> > > > Better behavior should be stop the write and return
> > > > on such holes.
> > > > 
> > > > Also return on partial read, which may happen in future
> > > > (eg. hit hwpoison pages).
> > > > 
> > > > CC: Andi Kleen <andi@...stfloor.org>
> > > > Signed-off-by: Wu Fengguang <fengguang.wu@...el.com>
> > > > ---
> > > >  drivers/char/mem.c |   30 ++++++++++++++++++------------
> > > >  1 file changed, 18 insertions(+), 12 deletions(-)
> > > > 
> > > > --- linux-mm.orig/drivers/char/mem.c	2009-09-14 10:59:50.000000000 +0800
> > > > +++ linux-mm/drivers/char/mem.c	2009-09-14 11:06:25.000000000 +0800
> > > > @@ -444,18 +444,22 @@ static ssize_t read_kmem(struct file *fi
> > > >  		if (!kbuf)
> > > >  			return -ENOMEM;
> > > >  		while (count > 0) {
> > > > +			unsigned long n;
> > > > +
> > > >  			sz = size_inside_page(p, count);
> > > > -			sz = vread(kbuf, (char *)p, sz);
> > > > -			if (!sz)
> > > > +			n = vread(kbuf, (char *)p, sz);
> > > > +			if (!n)
> > > >  				break;
> > > > -			if (copy_to_user(buf, kbuf, sz)) {
> > > > +			if (copy_to_user(buf, kbuf, n)) {
> > > >  				free_page((unsigned long)kbuf);
> > > >  				return -EFAULT;
> > > >  			}
> > > > -			count -= sz;
> > > > -			buf += sz;
> > > > -			read += sz;
> > > > -			p += sz;
> > > > +			count -= n;
> > > > +			buf += n;
> > > > +			read += n;
> > > > +			p += n;
> > > > +			if (n < sz)
> > > > +				break;
> > > >  		}
> > > >  		free_page((unsigned long)kbuf);
> > > >  	}
> > > > @@ -551,11 +555,13 @@ static ssize_t write_kmem(struct file * 
> > > >  				free_page((unsigned long)kbuf);
> > > >  				return -EFAULT;
> > > >  			}
> > > > -			sz = vwrite(kbuf, (char *)p, sz);
> > > > -			count -= sz;
> > > > -			buf += sz;
> > > > -			virtr += sz;
> > > > -			p += sz;
> > > > +			n = vwrite(kbuf, (char *)p, sz);
> > > > +			count -= n;
> > > > +			buf += n;
> > > > +			virtr += n;
> > > > +			p += n;
> > > > +			if (n < sz)
> > > > +				break;
> > > >  		}
> > > >  		free_page((unsigned long)kbuf);
> > > >  	}
> > > --
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> > > 
> > 
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