linux-kernel - Re: [patch 3/4] mempolicy: add MPOL_F_STATIC

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Date:	Thu, 14 Feb 2008 13:38:57 -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 3/4] mempolicy: add MPOL_F_STATIC_NODES flag

On Thu, 14 Feb 2008, Paul Jackson wrote:

> In mempolicy.h, the lines:
> 
>     /*
>      * The lower MPOL_FLAG_SHIFT bits of the policy mode represent the MPOL_*
>      * constants defined in enum mempolicy_mode.  The upper bits represent
>      * optional set_mempolicy() MPOL_F_* mode flags.
>      */
>     #define MPOL_FLAG_SHIFT (8)
>     #define MPOL_MODE_MASK  ((1 << MPOL_FLAG_SHIFT) - 1)
> 
>     /* Flags for set_mempolicy */
>     #define MPOL_F_STATIC_NODES     (1 << MPOL_FLAG_SHIFT)
>     #define MPOL_MODE_FLAGS (MPOL_F_STATIC_NODES)   /* legal set_mempolicy() MPOL_* mode flags */
> 
> could be simplified, to:
> 
>     /*
>      * Optional flags that modify nodemask numbering.
>      */
>     #define MPOL_F_RELATIVE_NODES (1<<14)	/* remapped relative to cpuset */
>     #define MPOL_F_STATIC_NODES (1<<15)		/* unremapped physical masks */
>     #define MPOL_MODE_FLAGS (MPOL_F_RELATIVE_NODES|MPOL_F_STATIC_NODES)
>     						/* combined MPOL_F_* mask flags */
> 
> (really, that MPOL_FLAG_SHIFT is just unnecessary distracting detail.)
> 

It would be easy to define mpol_mode() and mpol_flags() in terms of 
MPOL_MODE_FLAGS as well, yes.  But without MPOL_FLAG_SHIFT it becomes 
impossible to determine whether a user passed an invalid flag.

I think we're all in agreement that passing an invalid flag bit should be 
rejected with -EINVAL.  So to do that we need MPOL_MODE_MASK to expicitly 
define the parts of the int *policy passed from set_mempolicy() that 
represent the mode.

		David
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