Re: [RFC PATCH 02/14] mm/hms: heterogenenous memory system (HMS) documentation

[Dan Williams]

On Tue, Dec 4, 2018 at 10:58 AM Jerome Glisse <jglisse@xxxxxxxxxx> wrote:
>
> On Tue, Dec 04, 2018 at 10:31:17AM -0800, Dan Williams wrote:
> > On Tue, Dec 4, 2018 at 10:24 AM Jerome Glisse <jglisse@xxxxxxxxxx> wrote:
> > >
> > > On Tue, Dec 04, 2018 at 09:06:59AM -0800, Andi Kleen wrote:
> > > > jglisse@xxxxxxxxxx writes:
> > > >
> > > > > +
> > > > > +To help with forward compatibility each object as a version value and
> > > > > +it is mandatory for user space to only use target or initiator with
> > > > > +version supported by the user space. For instance if user space only
> > > > > +knows about what version 1 means and sees a target with version 2 then
> > > > > +the user space must ignore that target as if it does not exist.
> > > >
> > > > So once v2 is introduced all applications that only support v1 break.
> > > >
> > > > That seems very un-Linux and will break Linus' "do not break existing
> > > > applications" rule.
> > > >
> > > > The standard approach that if you add something incompatible is to
> > > > add new field, but keep the old ones.
> > >
> > > No that's not how it is suppose to work. So let says it is 2018 and you
> > > have v1 memory (like your regular main DDR memory for instance) then it
> > > will always be expose a v1 memory.
> > >
> > > Fast forward 2020 and you have this new type of memory that is not cache
> > > coherent and you want to expose this to userspace through HMS. What you
> > > do is a kernel patch that introduce the v2 type for target and define a
> > > set of new sysfs file to describe what v2 is. On this new computer you
> > > report your usual main memory as v1 and your new memory as v2.
> > >
> > > So the application that only knew about v1 will keep using any v1 memory
> > > on your new platform but it will not use any of the new memory v2 which
> > > is what you want to happen. You do not have to break existing application
> > > while allowing to add new type of memory.
> >
> > That sounds needlessly restrictive. Let the kernel arbitrate what
> > memory an application gets, don't design a system where applications
> > are hard coded to a memory type. Applications can hint, or optionally
> > specify an override and the kernel can react accordingly.
>
> You do not want to randomly use non cache coherent memory inside your
> application :)

The kernel arbitrates memory, it's a bug if it hands out something
that exotic to an unaware application.