On Thu, Apr 11, 2013 at 9:54 PM, Simon Jeons wrote: > Hi Jerome, > > On 04/12/2013 02:38 AM, Jerome Glisse wrote: > >> On Thu, Apr 11, 2013 at 11:42:05AM +0800, Simon Jeons wrote: >> >>> Hi Jerome, >>> On 04/11/2013 04:45 AM, Jerome Glisse wrote: >>> >>>> On Wed, Apr 10, 2013 at 09:41:57AM +0800, Simon Jeons wrote: >>>> >>>>> Hi Jerome, >>>>> On 04/09/2013 10:21 PM, Jerome Glisse wrote: >>>>> >>>>>> On Tue, Apr 09, 2013 at 04:28:09PM +0800, Simon Jeons wrote: >>>>>> >>>>>>> Hi Jerome, >>>>>>> On 02/10/2013 12:29 AM, Jerome Glisse wrote: >>>>>>> >>>>>>>> On Sat, Feb 9, 2013 at 1:05 AM, Michel Lespinasse < >>>>>>>> walken@google.com> wrote: >>>>>>>> >>>>>>>>> On Fri, Feb 8, 2013 at 3:18 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. >>>>>>>>>> >>>>>>>>> This sounds kinda scary TBH; however I do understand the need for >>>>>>>>> such >>>>>>>>> technology. >>>>>>>>> >>>>>>>>> I think one issue is that many MM developers are insufficiently >>>>>>>>> aware >>>>>>>>> of such developments; having a technology presentation would >>>>>>>>> probably >>>>>>>>> help there; but traditionally LSF/MM sessions are more interactive >>>>>>>>> between developers who are already quite familiar with the >>>>>>>>> technology. >>>>>>>>> I think it would help if you could send in advance a detailed >>>>>>>>> presentation of the problem and the proposed solutions (and then >>>>>>>>> what >>>>>>>>> they require of the MM layer) so people can be better prepared. >>>>>>>>> >>>>>>>>> And first I'd like to ask, aren't IOMMUs supposed to already >>>>>>>>> largely >>>>>>>>> solve this problem ? (probably a dumb question, but that just tells >>>>>>>>> you how much you need to explain :) >>>>>>>>> >>>>>>>> For GPU the motivation is three fold. With the advance of GPU >>>>>>>> compute >>>>>>>> and also with newer graphic program we see a massive increase in GPU >>>>>>>> memory consumption. We easily can reach buffer that are bigger than >>>>>>>> 1gbytes. So the first motivation is to directly use the memory the >>>>>>>> user allocated through malloc in the GPU this avoid copying 1gbytes >>>>>>>> of >>>>>>>> data with the cpu to the gpu buffer. The second and mostly important >>>>>>>> to GPU compute is the use of GPU seamlessly with the CPU, in order >>>>>>>> to >>>>>>>> achieve this you want the programmer to have a single address space >>>>>>>> on >>>>>>>> the CPU and GPU. So that the same address point to the same object >>>>>>>> on >>>>>>>> GPU as on the CPU. This would also be a tremendous cleaner design >>>>>>>> from >>>>>>>> driver point of view toward memory management. >>>>>>>> >>>>>>>> And last, the most important, with such big buffer (>1gbytes) the >>>>>>>> memory pinning is becoming way to expensive and also drastically >>>>>>>> reduce the freedom of the mm to free page for other process. Most of >>>>>>>> the time a small window (every thing is relative the window can be > >>>>>>>> 100mbytes not so small :)) of the object will be in use by the >>>>>>>> hardware. The hardware pagefault support would avoid the necessity >>>>>>>> to >>>>>>>> >>>>>>> What's the meaning of hardware pagefault? >>>>>>> >>>>>> It's a PCIE extension (well it's a combination of extension that allow >>>>>> that see http://www.pcisig.com/**specifications/iov/ats/). >>>>>> Idea is that the >>>>>> iommu can trigger a regular pagefault inside a process address space >>>>>> on >>>>>> behalf of the hardware. The only iommu supporting that right now is >>>>>> the >>>>>> AMD iommu v2 that you find on recent AMD platform. >>>>>> >>>>> Why need hardware page fault? regular page fault is trigger by cpu >>>>> mmu, correct? >>>>> >>>> Well here i abuse regular page fault term. Idea is that with hardware >>>> page >>>> fault you don't need to pin memory or take reference on page for >>>> hardware to >>>> use it. So that kernel can free as usual page that would otherwise have >>>> been >>>> >>> For the case when GPU need to pin memory, why GPU need grap the >>> memory of normal process instead of allocating for itself? >>> >> Pin memory is today world where gpu allocate its own memory (GB of memory) >> that disappear from kernel control ie kernel can no longer reclaim this >> memory it's lost memory (i had complain about that already from user than >> saw GB of memory vanish and couldn't understand why the GPU was using so >> much). >> >> Tomorrow world we want gpu to be able to access memory that the >> application >> allocated through a simple malloc and we want the kernel to be able to >> recycly any page at any time because of memory pressure or because kernel >> decide to do so. >> >> That's just what we want to do. To achieve so we are getting hw that can >> do >> pagefault. No change to kernel core mm code (some improvement might be >> made). >> > > The memory disappear since you have a reference(gup) against it, correct? > Tomorrow world you want the page fault trigger through iommu driver that > call get_user_pages, it also will take a reference(since gup is called), > isn't it? Anyway, assume tomorrow world doesn't take a reference, we don't > need care page which used by GPU is reclaimed? > > Right now code use gup because it's convenient but it drop the reference right after the fault. So reference is hold only for short period of time. No you don't need to care about reclaim thanks to mmu notifier, ie before page is remove mmu notifier is call and iommu register a notifier, so it get the invalidate event and invalidate the device tlb and things goes on. If gpu access the page a new pagefault happen and a new page is allocated. All this code is upstream in linux kernel just read it. There is just no device that use it yet. That being said we will want improvement so that page that are hot in the device are not reclaimed. But it can work without such improvement. Cheers, Jerome