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

[Minchan Kim <minchan@kernel.org>]

On Mon, Sep 15, 2014 at 10:10:18AM +0900, Joonsoo Kim wrote:
> 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.

IIRC, Mel suggested two options, ZONE_MOVABLE zone and MIGRATE_ISOLATE.
Absolutely, movable zone option is better solution if we consider
interacting with reclaim but one problem was CMA had a specific
requirement for memory in the middle of an existing zone.
And his concern comes true.
Look at https://lkml.org/lkml/2014/5/28/64.
It starts to add more stuff in allocator's fast path to overcome the
problem. :(

Let's rethink. We already have a logic to handle overlapping nodes/zones
in compaction.c so isn't it possible to have discrete address ranges
in a movable zone? If so, movable zone can include specific ranges horrible
devices want and it could make allocation/reclaim logic simple than now and
add overheads to slow path(ie, linear pfn scanning logic of zone like
compaction).

> 
> I don't know exact history and reason why CMA is implemented in current
> form. Ccing some experts in this area.
> 
> Thanks.
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-- 
Kind regards,
Minchan Kim