Re: [PATCH RFC] mm: ghost swapfile support for zswap

[Kairui Song <ryncsn@gmail.com>]

On Sat, Nov 22, 2025 at 10:09 AM Chris Li <chrisl@kernel.org> wrote:
>
> On Fri, Nov 21, 2025 at 3:40 AM Johannes Weiner <hannes@cmpxchg.org> wrote:
> >
> > On Fri, Nov 21, 2025 at 01:31:43AM -0800, Chris Li wrote:
> > > The current zswap requires a backing swapfile. The swap slot used
> > > by zswap is not able to be used by the swapfile. That waste swapfile
> > > space.
> > >
> > > The ghost swapfile is a swapfile that only contains the swapfile header
> > > for zswap. The swapfile header indicate the size of the swapfile. There
> > > is no swap data section in the ghost swapfile, therefore, no waste of
> > > swapfile space.  As such, any write to a ghost swapfile will fail. To
> > > prevents accidental read or write of ghost swapfile, bdev of
> > > swap_info_struct is set to NULL. Ghost swapfile will also set the SSD
> > > flag because there is no rotation disk access when using zswap.
> >
> > Zswap is primarily a compressed cache for real swap on secondary
> > storage. It's indeed quite important that entries currently in zswap
> > don't occupy disk slots; but for a solution to this to be acceptable,
> > it has to work with the primary usecase and support disk writeback.
>
> Well, my plan is to support the writeback via swap.tiers.

That sounds interesting. Have been watching YoungJun and yours
swap.tiers discussion for a while, looking forward to see how they
play together.

Using tiering to resolve the writeback issue sounds like a nice
solution, we definitely don't want to limit the writeback to
zswap/ram-block only, we will also want things like
block-block writeback.

We (and I have noticed many community users) have setups involving
hybrid tiers. We have a internal module that moves swap entry from SSD
to HDD too. To do it upstreamly we need something like the swap.tiers.

>
> > This direction is a dead-end. Please take a look at Nhat's swap
> > virtualization patches. They decouple zswap from disk geometry, while
> > still supporting writeback to an actual backend file.
>
> Yes, there are many ways to decouple zswap from disk geometry, my swap
> ...
> Solving it from the swap.tiers angle is cleaner.

Agree with the swap.tiers part, that sounds cleaner.

>
> > Nacked-by: Johannes Weiner <hannes@cmpxchg.org>

I think that's too early to justify. Let's stay open for ideas.