Re: [PATCH 1/2] Export get_user_pages_fast

[Andrew Morton <akpm@linux-foundation.org>]

On Wed, 30 Jul 2008 20:09:39 +0300 Adrian Bunk <bunk@kernel.org> wrote:

> On Wed, Jul 30, 2008 at 10:03:55AM -0700, Andrew Morton wrote:
> > On Wed, 30 Jul 2008 18:47:36 +0300 Adrian Bunk <bunk@kernel.org> wrote:
> > 
> > > On Wed, Jul 30, 2008 at 03:35:23AM -0700, Andrew Morton wrote:
> > > > On Wed, 30 Jul 2008 14:38:55 +1000 Rusty Russell <rusty@rustcorp.com.au> wrote:
> > > > 
> > > > > After all, it was made for lguest wasn't it?  Still think it should be
> > > > > called get_current_pages() though.
> > > > > 
> > > > > Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
> > > > > ---
> > > > >  arch/x86/mm/gup.c |    2 ++
> > > > >  1 file changed, 2 insertions(+)
> > > > > 
> > > > > diff -r cb465381f6d5 arch/x86/mm/gup.c
> > > > > --- a/arch/x86/mm/gup.c	Wed Jul 30 10:18:44 2008 +1000
> > > > > +++ b/arch/x86/mm/gup.c	Wed Jul 30 14:22:53 2008 +1000
> > > > > @@ -8,6 +8,7 @@
> > > > >  #include <linux/mm.h>
> > > > >  #include <linux/vmstat.h>
> > > > >  #include <linux/highmem.h>
> > > > > +#include <linux/module.h>
> > > > >  
> > > > >  #include <asm/pgtable.h>
> > > > >  
> > > > > @@ -296,3 +303,4 @@ slow_irqon:
> > > > >  		return ret;
> > > > >  	}
> > > > >  }
> > > > > +EXPORT_SYMBOL_GPL(get_user_pages_fast);
> > > > 
> > > > A regularly-occurring problem with an export like this is that someone
> > > > writes a driver and tests it on x86, then the driver explodes on an
> > > > architecture which didn't export the same symbol.
> > > > 
> > > > So a better implementation might be to put
> > > > 
> > > > #ifdef CONFIG_HAVE_GET_USER_PAGES_FAST
> > > > EXPORT_SYMBOL_GPL(get_user_pages_fast);
> > > > #endif
> > > > 
> > > > into generic code somewhere.
> > > 
> > > How would that help?
> > 
> > It would fix the problem which I described.
> 
> The export would still only be available on x86 since no other arch 
> would define CONFIG_HAVE_GET_USER_PAGES_FAST.
> 

That's presently true in Linus mainline.  The powerpc implementation is
in -mm, but apprarently needs a bit more work.