linux-kernel - Re: [RFC v2 0/5] introduce gcma

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Message-ID: <20150225161158.GI26680@dhcp22.suse.cz>
Date:	Wed, 25 Feb 2015 17:11:58 +0100
From:	Michal Hocko <mhocko@...e.cz>
To:	SeongJae Park <sj38.park@...il.com>
Cc:	akpm@...ux-foundation.org, lauraa@...eaurora.org,
	minchan@...nel.org, sergey.senozhatsky@...il.com,
	linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC v2 0/5] introduce gcma

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.

Thanks!
-- 
Michal Hocko
SUSE Labs
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