Re: [PATCH 02/33] mm: overload get_user_pages() functions

* Dave Hansen [off-list ref] wrote:

From: Dave Hansen <dave.hansen@linux.intel.com>

The concept here was a suggestion from Ingo.  The implementation
horrors are all mine.

This allows get_user_pages(), get_user_pages_unlocked(), and
get_user_pages_locked() to be called with or without the
leading tsk/mm arguments.  We will give a compile-time warning
about the old style being __deprecated and we will also
WARN_ON() if the non-remote version is used for a remote-style
access.

So at minimum this should be WARN_ON_ONCE(), to make it easier to recover some 
meaningful kernel log from such incidents.

But:

Doing this, folks will get nice warnings and will not break the
build.  This should be nice for -next and will hopefully let
developers fix up their own code instead of maintainers needing
to do it at merge time.

The way we do this is hideous.  It uses the __VA_ARGS__ macro
functionality to call different functions based on the number
of arguments passed to the macro.

There's an additional hack to ensure that our EXPORT_SYMBOL()
of the deprecated symbols doesn't trigger a warning.

We should be able to remove this mess as soon as -rc1 hits in
the release after this is merged.

So when I suggested this then it looked a _lot_ cleanear to me, in my head!

OTOH this, if factored out a bit perhaps, could be the basis for a useful 
technical model to do 'phased in, -next invariant' prototype migrations in the 
future, especially when it involves lots of subsystems.

Strictly only in cases where -rc1 will truly get rid of the __VA_ARGS__ hackery - 
which we'd do in this case.

Nevertheless I'd love to have a high level buy-in from either Linus or Andrew that 
we can do it this way, as the hackery looks very hideous...

The alternative would be to allow the -next churn and to allow the occasional 
(fairly trivial but tester-disruptive) build breakage.

Thanks,

	Ingo

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