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

[Wu Fengguang <fengguang.wu@intel.com>]

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.

I'm not really sure about the right behavior. KAME-san, do you have
any suggestions?

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@firstfloor.org>
> Signed-off-by: Wu Fengguang <fengguang.wu@intel.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);
>  	}