Re: [HMM 12/15] mm/migrate: new memory migration helper for use with device memory v4

[Jerome Glisse]

On Wed, May 31, 2017 at 01:59:54PM +1000, Balbir Singh wrote:
> On Wed, 24 May 2017 13:20:21 -0400
> Jérôme Glisse <jglisse@xxxxxxxxxx> wrote:
> 
> > This patch add a new memory migration helpers, which migrate memory
> > backing a range of virtual address of a process to different memory
> > (which can be allocated through special allocator). It differs from
> > numa migration by working on a range of virtual address and thus by
> > doing migration in chunk that can be large enough to use DMA engine
> > or special copy offloading engine.
> > 
> > Expected users are any one with heterogeneous memory where different
> > memory have different characteristics (latency, bandwidth, ...). As
> > an example IBM platform with CAPI bus can make use of this feature
> > to migrate between regular memory and CAPI device memory. New CPU
> > architecture with a pool of high performance memory not manage as
> > cache but presented as regular memory (while being faster and with
> > lower latency than DDR) will also be prime user of this patch.
> > 
> > Migration to private device memory will be useful for device that
> > have large pool of such like GPU, NVidia plans to use HMM for that.
> > 
> 
> It is helpful, for HMM-CDM however we would like to avoid the downsides
> of MIGRATE_SYNC_NOCOPY

What are the downside you are referring too ?

Cheers,
Jérôme