Re: [LSF/MM/BPF TOPIC] Swap status and roadmap discussion

[Yosry Ahmed <yosry@kernel.org>]

> > - Is 64 bits really needed for reverse mapping? For the context, reverse
> >   mapping here is a swap entry recorded in a lower / physical device
> >   pointing to the ghost / virtual device.
>
> I think you can compact this a bit. Swap space itself is not fully 64
> bits right?
>
> Just not sure if the juice is worth the squeeze to save a couple of
> bits here and there, especially if the reverse mapping is already
> dynamic :)

I think we should actually revisit the need for a reverse mapping to
begin with. For swapoff, we can probably scan the virtual swap space
looking for entries that belong to the backend being swapped off. Not
as efficient as a reverse map, but still better than the status quo of
scanning page tables. I don't think optimizing for swapoff is worth
the consistent overhead.

The other use cases are probably cluster readahead and swapcache-only
reclaim, and I think both of these can also be revisited.