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

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Date:	Tue, 15 Sep 2009 09:24:48 +0900
From:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
To:	Wu Fengguang <fengguang.wu@...el.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 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'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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