Re: Path forward for Virtualized Swap?

[Rik van Riel <riel@surriel.com>]

On Sun, 2026-10-25 at 12:21 +0800, Kairui Song wrote:
> Rik van Riel <riel@surriel.com> 于 2026年10月25日周日 02:16写道：
> > 
> > On Sat, 2026-10-24 at 23:50 +0800, Kairui Song wrote:
> > > On Sat, Oct 24, 2026 at 4:03 AM Rik van Riel <riel@surriel.com>
> > > wrote:
> > > > 
> > > > 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  |  |       |   |       |
> > > > +-------+  +-------+   +-------+
> > > > 
> 
> > However, having swapoff remove all data from the swap
> > cache, after mapping the memory back into userspace
> > page tables, seems like it would be fairly easy to do.
> > 
> > Does that meet the needs of your use case?
> 
> Right, it's nothing hard, as long as we don't add mandatory new
> layers. For very high bandwidth swap devices, a mandatory indirection
> layer is pure overhead. 

Don't we need a swap cache lookup regardless, to
make sure the data we want to swap into the
process is not already present in memory?

At that point, it shouldn't matter whether we
get the backing swap location from the swap
cache lookup data structure, or from the
page tables?

We have the page table lookup, and the swap
cache lookup. That is two lookups, regardless
of whether there is indirection, or not.

Am I missing something?

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