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

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Date:	Mon, 25 Feb 2008 19:35:46 -0800 (PST)
From:	David Rientjes <rientjes@...gle.com>
To:	Paul Jackson <pj@....com>
cc:	akpm@...ux-foundation.org, clameter@....com,
	Lee.Schermerhorn@...com, ak@...e.de, linux-kernel@...r.kernel.org
Subject: Re: [patch 1/6] mempolicy: convert MPOL constants to enum

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
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