Re: Path forward for Virtualized Swap?

[Yosry Ahmed]

On Fri, Oct 23, 2026 at 1:03 PM Rik van Riel <riel@xxxxxxxxxxx> wrote:
>
> On Fri, 2026-10-23 at 10:24 -0700, Nhat Pham wrote:
> >
> > * Swapoff: this was brought up as a request, but we ran out of time
> > before we could discuss the motivations and use cases behind them.
> > Folks interested, could you explain as a follow-up?
>
> This is one that I really don't understand.
>
> In my mind, we have vswap sitting as kind
> of a swap VFS between the rest of the system
> and the swap devices:
>
> +-------+     +------------+
> | vswap |-----| swap cache |
> +---+---+     +------------+
>     |
>     +----------+-----------+
>     |          |           |
> +---+---+  +---+---+   +---+---+
> | swap  |  | zswap |   | zram  |
> | file  |  |       |   |       |
> +-------+  +-------+   +-------+
>
> When a page is unmapped from userspace by the
> reclaim code, it goes into the swap cache.
>
> The vswap layer then decides where to store it
> at eviction time. This is configurable by userspace.
>
> If the zswap pool is shrunk, the vswap layer knows
> where to put the data next. There is no need to
> change page table entries to indicate that the
> data was moved from zswap to a swap file.
>
> If we decide to disable one of the backing swap
> devices, the vswap layer can just move the data
> into the swap cache, and the page reclaim code
> can evict it into a different backing store, if
> needed.
>
> Using vswap as a swap VFS would also allow us to
> get rid of the page table and tmpfs scanning
> code we have in swapoff today.
>
> In that model, I really do not understand what
> "swapoff vswap" means.
>
> We cannot umount the VFS.
>
> What exactly do people want to accomplish here?

Today if you swapoff a swapfile, every is swapped in into the
swapcache, whether it's in zswap or on disk. With vswap, you have no
way to trigger swapin of compressed memory in zswap.

Wei, is this what you had in mind? Can you please share more details
about the use case?