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

From: Tomasz Stanislawski
Date: Wed Sep 25 2013 - 13:10:00 EST

> 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.
> This might actually do away with the explicit pool size too as the compressed
> pool pages wouldn't be outside the control of the MM anymore.
> I'm just starting to explore this but I think it has promise.
> Seth

Hi Seth,
There is a problem with the proposed idea.
The radix tree used 'struct address_space' is a part of
a bigger data structure.
The radix tree is used to translate an offset to a page.
That is ok for zswap. But struct page has a field named 'index'.
The MM assumes that this index is an offset in radix tree
where one can find the page. A lot is done by MM to sustain
this consistency.

In case of zbud, there are two swap offset pointing to
the same page. There might be more if zsmalloc is used.
What is worse it is possible that one swap entry could
point to data that cross a page boundary.

Of course, one could try to modify MM to support
multiple mapping of a page in the radix tree.
But I think that MM guys will consider this as a hack
and they will not accept it.

Tomasz Stanislawski

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