Re: [PATCH 2/3] devmem: introduce size_inside_page()

[Andrew Morton <akpm@linux-foundation.org>]

On Fri, 11 Sep 2009 10:23:35 +0800
Wu Fengguang <fengguang.wu@intel.com> wrote:

> Introduce size_inside_page() to replace duplicate /dev/mem code.
> 
> Also apply it to /dev/kmem, whose alignment logic was buggy.
> 
> 
> CC: Marcelo Tosatti <mtosatti@redhat.com>
> CC: Greg Kroah-Hartman <gregkh@suse.de>
> CC: Mark Brown <broonie@opensource.wolfsonmicro.com>
> CC: Johannes Berg <johannes@sipsolutions.net>
> CC: Avi Kivity <avi@qumranet.com>
> Signed-off-by: Wu Fengguang <fengguang.wu@intel.com>
> ---
>  drivers/char/mem.c |   60 +++++++++++++------------------------------
>  1 file changed, 19 insertions(+), 41 deletions(-)
> 
> --- linux.orig/drivers/char/mem.c
> +++ linux/drivers/char/mem.c
> @@ -35,6 +35,19 @@
>  # include <linux/efi.h>
>  #endif
>  
> +static inline unsigned long size_inside_page(unsigned long start,
> +					     unsigned long size)
> +{
> +	unsigned long sz;
> +
> +	if (-start & (PAGE_SIZE - 1))
> +		sz = -start & (PAGE_SIZE - 1);

What on earth is this doing?  Negating an unsigned number?

Can we get rid of these party tricks and use something more
conventional here?  In a separate patch I guess.

> +	else
> +		sz = PAGE_SIZE;
> +
> +	return min_t(unsigned long, sz, size);

Can use min() here.

> +}

Please have a think about the types.  Should we be using unsigned long,
or size_t?  Which makes more sense?  Which maps better onto reality?

I suspect that the min_t which you inherited was added somewhere
because someone didn't get the types right: int-vs-size_t or something.
If we actually get the types right, this sort of thing goes away.


> @@ -462,10 +451,8 @@ static ssize_t read_kmem(struct file *fi
>  		if (!kbuf)
>  			return -ENOMEM;
>  		while (count > 0) {
> -			int len = count;
> +			int len = size_inside_page(p, count);

int?