Re: [RFC v2 0/5] introduce gcma

[SeongJae Park]

On Wed, 25 Feb 2015, Michal Hocko wrote:

> On Wed 25-02-15 14:31:08, SeongJae Park wrote:
> > Hello Michal,
> >
> > Thanks for your comment :)
> >
> > On Tue, 24 Feb 2015, Michal Hocko wrote:
> >
> > > On Tue 24-02-15 04:54:18, SeongJae Park wrote:
> > > [...]
> > > > include/linux/cma.h  |    4 +
> > > > include/linux/gcma.h |   64 +++
> > > > mm/Kconfig           |   24 +
> > > > mm/Makefile          |    1 +
> > > > mm/cma.c             |  113 ++++-
> > > > mm/gcma.c            | 1321 ++++++++++++++++++++++++++++++++++++++++++++++++++
> > > > 6 files changed, 1508 insertions(+), 19 deletions(-)
> > > > create mode 100644 include/linux/gcma.h
> > > > create mode 100644 mm/gcma.c
> > >
> > > Wow this is huge! And I do not see reason for it to be so big. Why
> > > cannot you simply define (per-cma area) 2-class users policy? Either via
> > > kernel command line or export areas to userspace and allow to set policy
> > > there.
> >
> > For implementation of the idea, we should develop not only policy selection,
> > but also backend for discardable memory. Most part of this patch were made
> > for the backend.
>
> What is the backend and why is it needed? I thought the discardable will
> go back to the CMA pool. I mean the cover email explained why the
> current CMA allocation policy might lead to lower success rate or
> stalls. So I would expect a new policy would be a relatively small
> change in the CMA allocation path to serve 2-class users as per policy.
> It is not clear to my why we need to pull a whole gcma layer in. I might
> be missing something obvious because I haven't looked at the patches yet
> but this should better be explained in the cover letter.

I meant backend for 2nd-class clients like cleancache and frontswap.
Because implementing backend for cleancache or frontswap is subsystem's
responsibility, gcma was needed to implement those backend. I believe
second ("gcma: utilize reserved memory as discardable memory") and
third ("gcma: adopt cleancache and frontswap as second-class
clients") could be helpful to understand about that.

And yes, I agree the explanation was not enough. My fault, sorry. My
explanation was too concentrated on policy itself. I should explained
about how the policy could be implemented and how gcma did. I will explain
about that in coverletter with next version.

Thanks for your helpful and nice comment.


Thanks,
SeongJae Park

> Thanks!
> --
> Michal Hocko
> SUSE Labs
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