Re: [PATCH V3 0/4] Define coherent device memory node

From: Bob Liu
Date: Thu Feb 23 2017 - 20:12:28 EST

On 2017/2/21 21:39, Anshuman Khandual wrote:
> On 02/21/2017 04:41 PM, Michal Hocko wrote:
>> On Fri 17-02-17 17:11:57, Anshuman Khandual wrote:
>> [...]
>>> * User space using mbind() to get CDM memory is an additional benefit
>>> we get by making the CDM plug in as a node and be part of the buddy
>>> allocator. But the over all idea from the user space point of view
>>> is that the application can allocate any generic buffer and try to
>>> use the buffer either from the CPU side or from the device without
>>> knowing about where the buffer is really mapped physically. That
>>> gives a seamless and transparent view to the user space where CPU
>>> compute and possible device based compute can work together. This
>>> is not possible through a driver allocated buffer.
>> But how are you going to define any policy around that. Who is allowed
> The user space VMA can define the policy with a mbind(MPOL_BIND) call
> with CDM/CDMs in the nodemask.
>> to allocate and how much of this "special memory". Is it possible that
> Any user space application with mbind(MPOL_BIND) call with CDM/CDMs in
> the nodemask can allocate from the CDM memory. "How much" gets controlled
> by how we fault from CPU and the default behavior of the buddy allocator.
>> we will eventually need some access control mechanism? If yes then mbind
> No access control mechanism is needed. If an application wants to use
> CDM memory by specifying in the mbind() it can. Nothing prevents it
> from using the CDM memory.
>> is really not suitable interface to (ab)use. Also what should happen if
>> the mbind mentions only CDM memory and that is depleted?
> IIUC *only CDM* cannot be requested from user space as there are no user
> visible interface which can translate to __GFP_THISNODE. MPOL_BIND with
> CDM in the nodemask will eventually pick a FALLBACK zonelist which will
> have zones of the system including CDM ones. If the resultant CDM zones
> run out of memory, we fail the allocation request as usual.
>> Could you also explain why the transparent view is really better than
>> using a device specific mmap (aka CDM awareness)?
> Okay with a transparent view, we can achieve a control flow of application
> like the following.
> (1) Allocate a buffer: alloc_buffer(buf, size)
> (2) CPU compute on buffer: cpu_compute(buf, size)
> (3) Device compute on buffer: device_compute(buf, size)
> (4) CPU compute on buffer: cpu_compute(buf, size)
> (5) Release the buffer: release_buffer(buf, size)
> With assistance from a device specific driver, the actual page mapping of
> the buffer can change between system RAM and device memory depending on
> which side is accessing at a given point. This will be achieved through
> driver initiated migrations.

Sorry, I'm a bit confused here.
What's the difference with the Heterogeneous memory management?
Which also "allows to use device memory transparently inside any process
without any modifications to process program code."


>>> * The placement of the memory on the buffer can happen on system memory
>>> when the CPU faults while accessing it. But a driver can manage the
>>> migration between system RAM and CDM memory once the buffer is being
>>> used from CPU and the device interchangeably. As you have mentioned
>>> driver will have more information about where which part of the buffer
>>> should be placed at any point of time and it can make it happen with
>>> migration. So both allocation and placement are decided by the driver
>>> during runtime. CDM provides the framework for this can kind device
>>> assisted compute and driver managed memory placements.
>>> * If any application is not using CDM memory for along time placed on
>>> its buffer and another application is forced to fallback on system
>>> RAM when it really wanted is CDM, the driver can detect these kind
>>> of situations through memory access patterns on the device HW and
>>> take necessary migration decisions.