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

[Catalin Marinas]

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?

Thanks.

-- 
Catalin