From mboxrd@z Thu Jan 1 00:00:00 1970 From: steve.capper@linaro.org (Steve Capper) Date: Wed, 30 Apr 2014 16:38:25 +0100 Subject: [RFC PATCH V4 6/7] arm64: mm: Enable HAVE_RCU_TABLE_FREE logic In-Reply-To: <20140430153317.GG31220@arm.com> References: <1396018892-6773-1-git-send-email-steve.capper@linaro.org> <1396018892-6773-7-git-send-email-steve.capper@linaro.org> <20140430152047.GF31220@arm.com> <20140430153317.GG31220@arm.com> Message-ID: <20140430153824.GA7166@linaro.org> To: linux-arm-kernel@lists.infradead.org List-Id: linux-arm-kernel.lists.infradead.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 > > > > 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