Re: [PATCH v2 0/5] mm: migrate zbud pages

Seth Jennings
2013-09-23T17:40:45Z

On Tue, Sep 17, 2013 at 02:59:24PM +0800, Bob Liu wrote:
> Hi Krzysztof,
>
> On 09/11/2013 04:58 PM, Krzysztof Kozlowski wrote:
> > Hi,
> >
> > Currently zbud pages are not movable and they cannot be allocated from CMA
> > (Contiguous Memory Allocator) region. These patches add migration of zbud pages.
> >
>
> I agree that the migration of zbud pages is important so that system
> will not enter order-0 page fragmentation and can be helpful for page
> compaction/huge pages etc..
>
> But after I looked at the [patch 4/5], I found it will make zbud very
> complicated.
> I'd prefer to add this migration feature later until current version
> zswap/zbud becomes better enough and more stable.

I agree with this. We are also looking to add zsmalloc as an option too. It
would be nice to come up with a solution that worked for both (any) allocator
that zswap used.

>
> Mel mentioned several problems about zswap/zbud in thread "[PATCH v6
> 0/5] zram/zsmalloc promotion".
>
> Like "it's clunky as hell and the layering between zswap and zbud is
> twisty" and "I think I brought up its stalling behaviour during review
> when it was being merged. It would have been preferable if writeback
> could be initiated in batches and then waited on at the very least..
> It's worse that it uses _swap_writepage directly instead of going
> through a writepage ops. It would have been better if zbud pages
> existed on the LRU and written back with an address space ops and
> properly handled asynchonous writeback."

Yes, the laying in zswap vs zbud is wonky and should be addressed before adding
new layers.

>
> So I think it would be better if we can address those issues at first
> and it would be easier to address these issues before adding more new
> features. Welcome any ideas.

Agreed.

Seth