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." Thanks, -Bob >> >>> * 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.
