Re: [LSF/MM/BPF TOPIC] Swap Abstraction "the pony"

[Matthew Wilcox <willy@infradead.org>]

On Fri, Mar 01, 2024 at 04:53:43PM +0700, Nhat Pham wrote:
> IMHO, one thing this new abstraction should support is seamless
> transfer/migration of pages from one backend to another (perhaps from
> high to low priority backends, i.e writeback).
> 
> I think this will require some careful redesigns. The closest thing we
> have right now is zswap -> backing swapfile. But it is currently
> handled in a rather peculiar manner - the underlying swap slot has
> already been reserved for the zswap entry. But there's a couple of
> problems with this:
> 
> a) This is wasteful. We're essentially having the same piece of data
> occupying spaces in two levels in the hierarchies.
> b) How do we generalize to a multi-tier hierarchy?
> c) This is a bit too backend-specific. It'd be nice if we can make
> this as backend-agnostic as possible (if possible).
> 
> Motivation: I'm currently working/thinking about decoupling zswap and
> swap, and this is one of the more challenging aspects (as I can't seem
> to find a precedent in the swap world for inter-swap backends pages
> migration), and especially with respect to concurrent loads (and
> swapcache interactions).

Have you considered (and already rejected?) the opposite approach --
coupling zswap and swap more tightly?  That is, we always write out
the original pages today.  Why don't we write out the compressed pages
instead?  For the same amount of I/O, we'd free up more memory!  That
sounds like a win to me.

I'm sure it'd be a big redesign, but that seems to be what we're talking
about anyway.