Re: [PATCH V4 1/6] mm: Introduce a general RCU get_user_pages_fast.

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

On 30 September 2014 04:51, Hugh Dickins <hughd@google.com> wrote:
> On Fri, 26 Sep 2014, Steve Capper wrote:
>
>> get_user_pages_fast attempts to pin user pages by walking the page
>> tables directly and avoids taking locks. Thus the walker needs to be
>> protected from page table pages being freed from under it, and needs
>> to block any THP splits.
>>
>> One way to achieve this is to have the walker disable interrupts, and
>> rely on IPIs from the TLB flushing code blocking before the page table
>> pages are freed.
>>
>> On some platforms we have hardware broadcast of TLB invalidations, thus
>> the TLB flushing code doesn't necessarily need to broadcast IPIs; and
>> spuriously broadcasting IPIs can hurt system performance if done too
>> often.
>>
>> This problem has been solved on PowerPC and Sparc by batching up page
>> table pages belonging to more than one mm_user, then scheduling an
>> rcu_sched callback to free the pages. This RCU page table free logic
>> has been promoted to core code and is activated when one enables
>> HAVE_RCU_TABLE_FREE. Unfortunately, these architectures implement
>> their own get_user_pages_fast routines.
>>
>> The RCU page table free logic coupled with a an IPI broadcast on THP
>> split (which is a rare event), allows one to protect a page table
>> walker by merely disabling the interrupts during the walk.
>>
>> This patch provides a general RCU implementation of get_user_pages_fast
>> that can be used by architectures that perform hardware broadcast of
>> TLB invalidations.
>>
>> It is based heavily on the PowerPC implementation by Nick Piggin.
>>
>> Signed-off-by: Steve Capper <steve.capper@linaro.org>
>> Tested-by: Dann Frazier <dann.frazier@canonical.com>
>> Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
>
> Acked-by: Hugh Dickins <hughd@google.com>
>

Thanks Hugh!

> Thanks for making all those clarifications, Steve: this looks very
> good to me now.  I'm not sure which tree you're hoping will take this
> and the arm+arm64 patches 2-6: although this one would normally go
> through akpm, I expect it's easier for you to synchronize if it goes
> in along with the arm+arm64 2-6 - would that be okay with you, Andrew?
> I see no clash with what's currently in mmotm.

I see it's gone into mmotm.

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