Re: [RFC PATCH 0/5] mm, swap: Virtual Swap Space (Swap Table Edition)

[Yosry Ahmed <yosry@kernel.org>]

> > > I don't like that the code bifurcates for vswap vs. normal swap entries
> > > though. Not sure if this is an issue that can be fixed with proper
> > > abstractions to hide it, or if the design needs modifications. I was
> > > honestly really hoping we don't end up with this. I was hoping that the
> > > physical swap device no longer uses a full swap table and all, and
> > > everything goes through vswap.
> > >
> > > I hoping that if redirection isn't needed (e.g. zswap is disabled),
> > > vswap can directly encode the physical swap slot so that the reverse
> > > mapping isn't needed -- so we avoid the overhead without keeping the
> > > physical swap device using a fully-fledged swap table.
> >
> > Can you expand on "vswap can directly encode the physical swap slot"?
> > I'm not sure I follow here.
> >
> > >
> > > All that being said, perhaps I am too out of touch with the code to
> > > realize it's simply not possible.
> > >
> > > Honestly, if the main reason we can't have a single swap table for vswap
> > > is saving 8 bytes on the reverse mapping, it sounds like a weak-ish
> > > argument, even if we can't optimize the reverse mapping away. But maybe
> > > I am also out of touch with RAM prices :)
> >
> > In terms of the space overhead I do agree, FWIW :)
> >
> > I think the other concern is the indirection overhead with going
> > through the xarray for every swap operation, hence the per-CPU vswap
> > cluster lookup caching idea:
> >
> > https://lore.kernel.org/all/20260505153854.1612033-23-nphamcs@gmail.com/
> >
> > >
> > > I at least hope that, the current design is not painting us into a
> > > corner (e.g. through userspace interfaces), and we can still achieve a
> > > vswap-for-all implementation in the future (maybe that's what you have
> > > in mind already?).
> >
> > That's still my plan. Operationally speaking, I want to make this
> > completely transparent to users, with minimal to no performance
> > overhead.
> 
> I do want to add that, even without achieving this, the current design
> already enables a lot of use cases. I think it is a good compromise to
> maintain both virtual and directly mapped physical swap entries for
> now, and revisit the conversation of whether we can afford a mandatory
> vswap layer once all the optimizations have been done :)
> 
> We should strive to simplify the codebase, and it will naturally
> happen when the original overhead concern is no longer there. A
> swap-related example: a few years ago, everyone thought swap slot
> cache was needed. But then, Kairui optimized the swap allocator's lock
> contention issue away, and that swap slot cache is suddenly redundant.
> That finally allowed us to get rid of it. Similar thing happened (or
> is happening?) with the SWP_SYNCHRONOUS_IO swapcache-skipping
> heuristics.

I agree, I just want to make sure we have a line of sight (or at least
no blockers) to having a unified vswap layer in the future.