Re: [RFC] cpuset relative memory policies - second choice

[David Rientjes <rientjes@google.com>]

On Wed, 31 Oct 2007, Paul Jackson wrote:

> The basic reason that I went with an additional per-task modal
> state, rather than a modal flag for each mbind, set_mempolicy and
> get_mempolicy call was to reduce the likely rate of bugs in user
> level C code using this API.
> 

I think it may be more error prone to accidently leave off the 
get_mempolicy() system call to use the system-wide numbering or for 
user-level code to forget that the mode was already set (and now stored in 
their task_struct) without a subsequent get_mempolicy() to revert back to 
the default behavior.  Both of these problems can be addressed by checking 
the *policy returned by get_mempolicy(), as you've coded it, but many 
applications will probably ignore that overhead.

Allowing the mode to be passed on each set_mempolicy() system call seems 
better, this is where the nodemask is passed anyway so it's legitimate for 
the caller to specify how that nodemask should be interpreted (either 
system-wide or cpuset-wide).  This keeps all semantics of the nodemask to 
a single invocation of a system call instead of setting policy modes with 
get_mempolicy() and confusing the matter later.

I think get_mempolicy() can remain unchanged because it will simply return 
the contextualized nodemask in either case and would not require a mode to 
be passed.

		David
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