[RFC PATCH V4 6/7] arm64: mm: Enable HAVE_RCU_TABLE_FREE logic

[Catalin Marinas]

On Wed, Apr 30, 2014 at 04:20:47PM +0100, Catalin Marinas wrote:
> On Fri, Mar 28, 2014 at 03:01:31PM +0000, Steve Capper wrote:
> > In order to implement fast_get_user_pages we need to ensure that the
> > page table walker is protected from page table pages being freed from
> > under it.
> > 
> > This patch enables HAVE_RCU_TABLE_FREE, any page table pages belonging
> > to address spaces with multiple users will be call_rcu_sched freed.
> > Meaning that disabling interrupts will block the free and protect the
> > fast gup page walker.
> > 
> > Signed-off-by: Steve Capper <steve.capper at linaro.org>
> 
> While this patch is simple, I'd like to better understand the reason for
> it. Currently HAVE_RCU_TABLE_FREE is enabled for powerpc and sparc while
> __get_user_pages_fast() is supported by a few other architectures that
> don't select HAVE_RCU_TABLE_FREE. So why do we need it for fast gup on
> arm/arm64 while not all the other archs need it?

OK, replying to myself. I assume the other architectures that don't need
HAVE_RCU_TABLE_FREE use IPI for TLB shootdown, hence they gup_fast
synchronisation for free.

-- 
Catalin