Re: [LSF/MM TOPIC] Hardware initiated paging of user process pages, hardware access to the CPU page tables of user processes

[Jerome Glisse <j.glisse@gmail.com>]

On Fri, Feb 8, 2013 at 6:18 AM, Shachar Raindel <raindel@mellanox.com> wrote:
> Hi,
>
> We would like to present a reference implementation for safely sharing
> memory pages from user space with the hardware, without pinning.
>
> We will be happy to hear the community feedback on our prototype
> implementation, and suggestions for future improvements.
>
> We would also like to discuss adding features to the core MM subsystem to
> assist hardware access to user memory without pinning.
>
> Following is a longer motivation and explanation on the technology
> presented:
>
> Many application developers would like to be able to be able to communicate
> directly with the hardware from the userspace.
>
> Use cases for that includes high performance networking API such as
> InfiniBand, RoCE and iWarp and interfacing with GPUs.
>
> Currently, if the user space application wants to share system memory with
> the hardware device, the kernel component must pin the memory pages in RAM,
> using get_user_pages.
>
> This is a hurdle, as it usually makes large portions the application memory
> unmovable. This pinning also makes the user space development model very
> complicated – one needs to register memory before using it for communication
> with the hardware.
>
> We use the mmu-notifiers [1] mechanism to inform the hardware when the
> mapping of a page is changed. If the hardware tries to access a page which
> is not yet mapped for the hardware, it requests a resolution for the page
> address from the kernel.
>
> This mechanism allows the hardware to access the entire address space of the
> user application, without pinning even a single page.
>
> We would like to use the LSF/MM forum opportunity to discuss open issues we
> have for further development, such as:
>
> -Allowing the hardware to perform page table walk, similar to
> get_user_pages_fast to resolve user pages that are already in RAM.
>
> -Batching page eviction by various kernel subsystems (swapper, page-cache)
> to reduce the amount of communication needed with the hardware in such
> events
>
> -Hinting from the hardware to the MM regarding page fetches which are
> speculative, similarly to prefetching done by the page-cache
>
> -Page-in notifications from the kernel to the driver, such that we can keep
> our secondary TLB in sync with the kernel page table without incurring page
> faults.
>
> -Allowed and banned actions while in an MMU notifier callback. We have
> already done some work on making the MMU notifiers sleepable [2], but there
> might be additional limitations, which we would like to discuss.
>
> -Hinting from the MMU notifiers as for the reason for the notification - for
> example we would like to react differently if a page was moved by NUMA
> migration vs. page being swapped out.
>
> [1] http://lwn.net/Articles/266320/
>
> [2] http://comments.gmane.org/gmane.linux.kernel.mm/85002
>
> Thanks,
>
> --Shachar

As a GPU driver developer i can say that this is something we want to
do in a very near future. Also i think we would like another
capabilities :

- hint to mm on memory range that are best not to evict (easier for
driver to know what is hot and gonna see activities)

Dunno how big the change to the page eviction path would need to be.

Cheers,
Jerome

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