Hi Jerome,
On 02/08/2013 11:21 PM, Jerome Glisse wrote:
On Fri, Feb 8, 2013 at 6:18 AM, Shachar Raindel <raindel@xxxxxxxxxxxx> 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.
get_user_pages_fast just get page reference count instead of populate
the pte to page table, correct? Then how can GPU driver use iommu to
access the page?
-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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