Re: [patch 1/6] mempolicy: convert MPOL constants to enum

[David Rientjes]

On Mon, 25 Feb 2008, Paul Jackson wrote:

> +enum {
> +	MPOL_DEFAULT,
> +	MPOL_PREFERRED,
> +	MPOL_BIND,
> +	MPOL_INTERLEAVE,
> +	MPOL_MAX,	/* always last member of enum */
> 
> Aren't the values that these constants take part of the
> user visible kernel API?
> 
> In other words, if someone added another MPOL_* in the middle
> of this enum, it would break mbind/set_mempolicy/get_mempolicy
> users, right:
> 
> +enum {
> +	MPOL_DEFAULT,
> +	MPOL_PREFERRED,
> +	MPOL_YET_ANOTHER_FLAG,		/* <== added flag ... oops */
> +	MPOL_BIND,
> +	MPOL_INTERLEAVE,
> +	MPOL_MAX,	/* always last member of enum */
> 

I don't suspect that a kernel developer is going to make such an egregious 
error.  The user would need to be using a new linux/mempolicy.h with an 
old kernel to get the wrong behavior.

> I'm thinking that we should still specify the specific value
> of each of these flags, by way of documenting these necessary
> values, as in:
> 
> +enum {
> +	MPOL_DEFAULT = 0,
> +	MPOL_PREFERRED = 1,
> +	MPOL_BIND = 2,
> +	MPOL_INTERLEAVE = 3,
> +	MPOL_MAX,	/* always last member of enum */
> 

That looks overly redundant to me and doesn't protect against adding 
MPOL_YET_ANOTHER_FLAG in the middle of preferred and bind to get two mode 
values with the int value of 1.

		David
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/