Re: [LSF/MM/BPF TOPIC] Swap Abstraction / Native Zswap

[Chris Li <chrisl@kernel.org>]

On Mon, Mar 20, 2023 at 10:55:03AM +0800, Huang, Ying wrote:
> >
> > How so? With the indirection enabled, the page tables & page cache
> > have the swap id (or swap_desc index), which can point to a swap entry
> > or a zswap entry -- which can change when the page is moved between
> > zswap & swapfiles. How is xarray (a) indexed by the swap entry in this
> > case? Shouldn't be indexed by the abstract swap id so that the
> > writeback from zswap is transparent?
> 
> In my mind,
> 
> - swap core will define a abstract interface to swap implementations
>   (zswap, swap device/file, maybe more in the future), like VFS.

I like your idea very much.

> 
> - zswap will be a special swap implementation (compressing instead of
>   writing to disk).

Agree.

> 
> - swap core will manage the indirection layer and swap cache.

Agree, those are very good points.

> 
> - swap core can move swap pages between swap implementations (e.g., from
>   zswap to a swap device, or from one swap device to another swap
>   device) with the help of the indirection layer.
 
We need to carefully design the swap cache that, when moving between
swap implementaions, there will be one shared swap cache. The current
swap cache belongs to swap devices, so two devices will have the same
page in two swap caches.

> In this design, the writeback from zswap becomes moving swapped pages
> from zswap to a swap device.

Ack.

> 
> If my understanding were correct, your suggestion is kind of moving
> zswap logic to the swap core?  And zswap will be always at a higher
> layer on top of swap device/file?

It seems that way to me. I will let Yosry confirm that.

> > I am not sure how this works with zswap. Currently swap_map[]
> > implementation is specific for swapfiles, it does not work for zswap
> > unless we implement separate swap counting logic for zswap &
> > swapfiles. Same for the swapcache, it currently supports being indexed
> > by a swap entry, it would need to support being indexed by a swap id,
> > or have a separate swap cache for zswap. Having separate
> > implementation would add complexity, and we would need to perform
> > handoffs of the swap count/cache when a page is moved from zswap to a
> > swapfile.
> 
> We can allocate a swap entry for each swapped page in zswap.

One thing to consider when moving page from zswap to swap file, is the
zswap swap entry the same entry as the swap file entry.

> > I think for this proposal, there are only 2 hardcoded tiers. Zswap is
> > fast, swapfile is slow. In the future, we can support more dynamic
> > tiering if the need arises.
> 
> We can start from a simple implementation.  And I think that it's better
> to consider the general design too.  Try not to make it impossible now.

In my mind there are a few usage cases:
1) using only swap file.
2) using only zswap, no swap file.
3) Using zswap + swap file (SSD).

The swap core should handle both 3 cases well with minial memory waste.

Chris