Re: [HMM 00/16] HMM (Heterogeneous Memory Management) v18

[Bob Liu]

On 2017/3/17 7:49, Jerome Glisse wrote:
> On Thu, Mar 16, 2017 at 01:43:21PM -0700, Andrew Morton wrote:
>> On Thu, 16 Mar 2017 12:05:19 -0400 J__r__me Glisse <jglisse@xxxxxxxxxx> wrote:
>>
>>> Cliff note:
>>
>> "Cliff's notes" isn't appropriate for a large feature such as this. 
>> Where's the long-form description?  One which permits readers to fully
>> understand the requirements, design, alternative designs, the
>> implementation, the interface(s), etc?
>>
>> Have you ever spoken about HMM at a conference?  If so, the supporting
>> presentation documents might help here.  That's the level of detail
>> which should be presented here.
> 
> Longer description of patchset rational, motivation and design choices
> were given in the first few posting of the patchset to which i included
> a link in my cover letter. Also given that i presented that for last 3
> or 4 years to mm summit and kernel summit i thought that by now peoples
> were familiar about the topic and wanted to spare them the long version.
> My bad.
> 
> I attach a patch that is a first stab at a Documentation/hmm.txt that
> explain the motivation and rational behind HMM. I can probably add a
> section about how to use HMM from device driver point of view.
> 

Please, that would be very helpful!

> +3) Share address space and migration
> +
> +HMM intends to provide two main features. First one is to share the address
> +space by duplication the CPU page table into the device page table so same
> +address point to same memory and this for any valid main memory address in
> +the process address space.

Is this an optional feature?
I mean the device don't have to duplicate the CPU page table.
But only make use of the second(migration) feature.

> +The second mechanism HMM provide is a new kind of ZONE_DEVICE memory that does
> +allow to allocate a struct page for each page of the device memory. Those page
> +are special because the CPU can not map them. They however allow to migrate
> +main memory to device memory using exhisting migration mechanism and everything
> +looks like if page was swap out to disk from CPU point of view. Using a struct
> +page gives the easiest and cleanest integration with existing mm mechanisms.
> +Again here HMM only provide helpers, first to hotplug new ZONE_DEVICE memory
> +for the device memory and second to perform migration. Policy decision of what
> +and when to migrate things is left to the device driver.
> +
> +Note that any CPU acess to a device page trigger a page fault which initiate a
> +migration back to system memory so that CPU can access it.

A bit confused here, do you mean CPU access to a main memory page but that page has been migrated to device memory?
Then a page fault will be triggered and initiate a migration back.

Thanks,
Bob