Re: [RFC PATCH v4 2/3] mm/zswap: replace the zswap_pools list with a fixed pools array

[Yosry Ahmed]

On Sun, Aug 30, 2026 at 4:47 AM Jianyue Wu <wujianyue000@xxxxxxxxx> wrote:
>
> Originally zswap holds its pools on an RCU list whose head also serves
> as the "current pool". Only a handful of pools are ever live at once,
> since a new pool is only created when the compressor is (re)set and
> pools are reused across compressor switches.
>
> Hold the pools in a fixed ZSWAP_MAX_POOLS-element array so each pool
> has a stable slot number, and track the current pool with a separate
> rcu-protected pointer.
>
> Slot 0 is intentionally left unused (always NULL): a zeroed or
> incorrectly initialized pool index then resolves to NULL and trips a
> WARN rather than silently aliasing a live pool in another slot.
>
> The array keeps the same RCU publish/retire discipline the list had,
> so lookup and teardown stay equivalent. A fully-constructed pool is
> stored into its slot as the last step of zswap_pool_create(), so array
> walkers only ever observe a NULL slot or a ready pool. Pool creation
> is serialized by the module-wide kernel param mutex (all built-in
> params share one lock) and otherwise only happens during
> single-threaded init, so no two creators race for a slot.
> zswap_pools_lock still serializes the store against a retiring pool
> clearing its slot in __zswap_pool_empty().
>
> Behavior change: the fixed array bounds the number of simultaneously
> live pools at ZSWAP_MAX_POOLS - 1 (15, since slot 0 is reserved),
> whereas the old list was unbounded. A pool is only live while it is
> the current pool or still has stored pages referencing it, and pools
> are reused across compressor switches, so 15 is far more than any real
> configuration needs. Once all slots are occupied, creating a pool for
> a 16th distinct compressor fails: zswap_pool_create() errors and
> returns NULL, and the compressor switch is rejected with -EINVAL
> rather than silently succeeding. The cap can be raised by increasing
> ZSWAP_MAX_POOLS (bounded by the u8 slot index, so up to 256).
>
> Suggested-by: Nhat Pham <nphamcs@xxxxxxxxx>
> Suggested-by: Yosry Ahmed <yosry@xxxxxxxxxx>
> Signed-off-by: Jianyue Wu <wujianyue000@xxxxxxxxx>

Acked-by: Yosry Ahmed <yosry@xxxxxxxxxx>