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

[Johannes Weiner <hannes@cmpxchg.org>]

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.

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.

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