Re: [PATCH] mm: zswap: set zswap_ever_enabled when enabling zswap at runtime

[Yosry Ahmed]

On Tue, Oct 20, 2026 at 5:30 AM Wenwu Hou <hwenwur@xxxxxxxxx> wrote:
>
> zswap can end up enabled without zswap_ever_enabled being set, which
> leads to data corruption. With CONFIG_ZSWAP_DEFAULT_ON=n:
>
> 1. zswap_setup() fails to create the initial pool. Setup still counts
>    as successful, but zswap_has_pool stays false.
> 2. The user writes a compressor name to
>    /sys/module/zswap/parameters/compressor.
>    zswap_compressor_param_set() creates a pool and sets
>    zswap_has_pool = true, but never enables the zswap_ever_enabled
>    static key.
> 3. The user runs "echo Y > /sys/module/zswap/parameters/enabled".
> 4. zswap_store() now stores pages in zswap, and swap_writeout() skips
>    writing them to the swap device.
> 5. On swap-in, zswap_load() first checks zswap_never_enabled(). It
>    returns true, so zswap_load() reports that the page is not in zswap.
>    The page is then read from the swap device, which holds stale data.
>
> Fix this by enabling zswap_ever_enabled in zswap_enabled_param_set()
> before zswap_enabled becomes true.

Wasn't this fixed by
https://lore.kernel.org/linux-mm/20260906133601.3563324-1-xialonglong2025@xxxxxxx/?

>
> Fixes: 2d4d2b1cfb85 ("mm: zswap: add zswap_never_enabled()")
> Cc: stable@xxxxxxxxxxxxxxx
> Signed-off-by: Wenwu Hou <hwenwur@xxxxxxxxx>
> ---
>  mm/zswap.c | 25 +++++++++++++++++++++----
>  1 file changed, 21 insertions(+), 4 deletions(-)
>
> diff --git a/mm/zswap.c b/mm/zswap.c
> index 37f34e406c8e3..d2b4c248baa67 100644
> --- a/mm/zswap.c
> +++ b/mm/zswap.c
> @@ -629,12 +629,29 @@ static int zswap_enabled_param_set(const char *val,
>                 if (zswap_setup())
>                         break;
>                 fallthrough;
> -       case ZSWAP_INIT_SUCCEED:
> -               if (!zswap_has_pool)
> +       case ZSWAP_INIT_SUCCEED: {
> +               bool enable;
> +
> +               if (!zswap_has_pool) {
>                         pr_err("can't enable, no pool configured\n");
> -               else
> -                       ret = param_set_bool(val, kp);
> +                       break;
> +               }
> +
> +               /*
> +                * The initial pool may have been created later by the
> +                * compressor param rather than by zswap_setup(), in which
> +                * case zswap_ever_enabled is still off. Flip it before
> +                * zswap_enabled becomes visible to zswap_store(), otherwise
> +                * zswap_load() would skip stored entries and read stale data
> +                * from the backing swap device. param_set_bool() treats a
> +                * NULL value as true.
> +                */
> +               if (!val || (!kstrtobool(val, &enable) && enable))
> +                       static_branch_enable(&zswap_ever_enabled);
> +
> +               ret = param_set_bool(val, kp);
>                 break;
> +       }
>         case ZSWAP_INIT_FAILED:
>                 pr_err("can't enable, initialization failed\n");
>         }
> --
> 2.43.0
>