Re: [PATCHv4 0/3] new APIs to allocate buffer-cache with user specific flag

[Joonsoo Kim <iamjoonsoo.kim@lge.com>]

On Fri, Sep 05, 2014 at 10:14:16AM -0400, Theodore Ts'o wrote:
> On Fri, Sep 05, 2014 at 04:32:48PM +0900, Joonsoo Kim wrote:
> > I also test another approach, such as allocate freepage in CMA
> > reserved region as late as possible, which is also similar to your
> > suggestion and this doesn't works well. When reclaim is started,
> > too many pages reclaim at once, because lru list has successive pages
> > in CMA region and these doesn't help kswapd's reclaim. kswapd stop
> > reclaiming when freepage count is recovered. But, CMA pages isn't
> > counted for freepage for kswapd because they can't be usable for
> > unmovable, reclaimable allocation. So kswap reclaim too many pages
> > at once unnecessarilly.
> 
> Have you considered putting the pages in a CMA region in a separate
> zone?  After all, that's what we originally did with brain-damaged
> hardware that could only DMA into the low 16M of memory.  We just
> reserved a separate zone for that?  That way, we could do
> zone-directed reclaim and free pages in that zone, if that was what
> was actually needed.

Sorry for long delay. It was long holidays.

No, I haven't consider it. It sounds good idea to place the pages in a
CMA region into a separate zone. Perhaps we can remove one of
migratetype, MIGRATE_CMA, with this way and it would be a good long-term
architecture for CMA.

I don't know exact history and reason why CMA is implemented in current
form. Ccing some experts in this area.

Thanks.