Re: [PATCH v5 00/21] Virtual Swap Space

[Kairui Song]

Yosry Ahmed <yosry@xxxxxxxxxx> 于 2026年4月24日周五 04:48写道：
> > Using a swapfile does have its benefits, though. For example, the
> > virtual layer could act as an ordinary tier following YoungJun's
> > design:
> > https://lore.kernel.org/linux-mm/20260421055323.940344-1-youngjun.park@xxxxxxx/
>
> Hmm I didn't look too closely at this but I don't understand how
> making it a swapfile helps with tiering? If anything, I think it makes
> tiering more difficult. For tiering to work, we need an
> abstraction/redirection layer, such that we don't need to update the
> page tables (or shmem pagecache) if we demote/promote pages. That is
> exactly the use case for a virtual swap layer. The page tables point
> at a virtual swap ID and the backend could change transparently (e.g.
> for zswap writeback, or tiering).
>
> If we make the virtual layer a swapfile, how do we demote/promote
> without updating page tables?
>
> IOW, I think the whole reason we want a virtual layer is to separate
> the backends, which would facilitate tiering. If the virtual layer is
> itself a swapfile, wouldn't it become one of the tiers?

That's exactly what I hoped, virtual layer being part of the tier.
Tier could be set up per task / cgroup. So is the virtual tier.

A standalone implementation of the virtual layer is more heavy than
being a swapfile. Actually I think at this point, it is the word
"swapfile" is misleading now. We may rename it to "swap mapping" or
something. A swap mapping could be physical or virtual. Virtual
mapping can realloc from physical ones (redirect), and swapoff of
physical ones just read its data into virtual mapping's swap cache.

I think it's actually functionally very similar to Nhat's design
already from a high level, the only difference is we don't need
standalone infra for virtual parts.

For swapoff or migration you don't need to touch the page table, same
as in this series, just update the virtual swap mapping to be cached
or update the entry, it's identical to what this series is doing.