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

[Steve Capper <steve.capper@linaro.org>]

On Wed, Apr 30, 2014 at 04:33:17PM +0100, Catalin Marinas wrote:
> 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@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.

Hi Catalin,
Yes that is roughly the case.
Essentially we want to RCU free the page table backing pages at a
later time when we aren't walking on them.

Other arches use IPI, some others have their own RCU logic. I opted to
activate some existing logic to reduce code duplication.

Cheers,
-- 
Steve

> 
> -- 
> Catalin

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