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

[Minchan Kim]

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

> 
> -- 
> Mel Gorman
> SUSE Labs
> 
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-- 
Kind regards,
Minchan Kim
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