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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