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

From: Krzysztof Kozlowski
Date: Tue Sep 24 2013 - 05:20:37 EST


On pon, 2013-09-23 at 17:07 -0500, Seth Jennings wrote:
> On Tue, Sep 17, 2013 at 02:59:24PM +0800, Bob Liu wrote:
> > 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."
> >
> > 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.
> I just had an idea this afternoon to potentially kill both these birds with one
> stone: Replace the rbtree in zswap with an address_space.
> Each swap type would have its own page_tree to organize the compressed objects
> by type and offset (radix tree is more suited for this anyway) and a_ops that
> could be called by shrink_page_list() (writepage) or the migration code
> (migratepage).
> Then zbud pages could be put on the normal LRU list, maybe at the beginning of
> the inactive LRU so they would live for another cycle through the list, then be
> reclaimed in the normal way with the mapping->a_ops->writepage() pointing to a
> zswap_writepage() function that would decompress the pages and call
> __swap_writepage() on them.

How exactly the address space can be used here? Do you want to point to
zbud pages in address_space.page_tree? If yes then which index should be

Best regards,

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