Re: [PATCH 2/2] mm: support MIGRATE_DISCARD

[Mel Gorman]

On Thu, Sep 06, 2012 at 02:31:12PM +0900, Minchan Kim wrote:
> Hi Mel,
> 
> On Wed, Sep 05, 2012 at 11:56:11AM +0100, Mel Gorman wrote:
> > On Wed, Sep 05, 2012 at 05:11:13PM +0900, Minchan Kim wrote:
> > > This patch introudes MIGRATE_DISCARD mode in migration.
> > > It drops *clean cache pages* instead of migration so that
> > > migration latency could be reduced by avoiding (memcpy + page remapping).
> > > It's useful for CMA because latency of migration is very important rather
> > > than eviction of background processes's workingset. In addition, it needs
> > > less free pages for migration targets so it could avoid memory reclaiming
> > > to get free pages, which is another factor increase latency.
> > > 
> > 
> > Bah, this was released while I was reviewing the older version. I did
> > not read this one as closely but I see the enum problems have gone away
> > at least. I'd still prefer if CMA had an additional helper to discard
> > some pages with shrink_page_list() and migrate the remaining pages with
> > migrate_pages(). That would remove the need to add a MIGRATE_DISCARD
> > migrate mode at all.
> 
> I am not convinced with your point. What's the benefit on separating
> reclaim and migration? For just removing MIGRATE_DISCARD mode?

Maintainability. There are reclaim functions and there are migration
functions. Your patch takes migrate_pages() and makes it partially a
reclaim function mixing up the responsibilities of migrate.c and vmscan.c.

> I don't think it's not bad because my implementation is very simple(maybe
> it's much simpler than separating reclaim and migration) and
> could be used by others like memory-hotplug in future.

They could also have used the helper function from CMA that takes a list
of pages, reclaims some and migrates other.

> If you're not strong against with me, I would like to insist on my implementation.
> 

I'm not very strongly against it but I'm also very unhappy.

-- 
Mel Gorman
SUSE Labs
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