Re: [PATCH 15/16] mm, swap: require zswap for xswap devices

[Youngjun Park <youngjun.park@lge.com>]

On Thu, Aug 27, 2026 at 05:45:05PM +0800, Baoquan He wrote:
> xswap has no backing storage: swapped-out pages live only in zswap.
> Without zswap, swapout always bounces back, so the device would
> consume swap entry space without ever freeing memory.  Refuse to
> create a device when zswap is unavailable.
> 
> Runtime disabling of zswap after creation is safe: existing entries
> stay loadable (zswap_load() gates on zswap_never_enabled(), not the
> runtime zswap_enabled flag) and new swapouts merely bounce back to
> memory without freeing it.

Hello Boaquan.

If xswap is used alongside another swap device, wouldn't runtime
disabling of zswap cause a problem? 
User assumes other remained swap device used right afte zswap disabled.

Once a device is allocated as xswap and zswap gets runtime disabled,
xswap keeps receiving new swap entry allocations, and every swapout
to it just bounces back without freeing memory . until xswap's
entries are exhausted. Meanwhile the other swap device sits unused.

If my point is right...
A few ways to handle this come to mind.

1. Once xswap has started accepting writes, refuse to runtime disable zswap.
2. If zswap is runtime disabled, stop handing out new entries from
   xswap (drop it from the available list, or gate on an XSWAP flag).
   This would also need nr_swap_pages, the visible swap count to be
   reduced accordingly, and unused cluster memory reclaimed.
   behave as if xswap had been swapoff'd.
3. Support falling back to another swap device once zswap becomes
   unavailable. 

Thanks!
Youngjun