Re: [PATCH 1/1] mm/zswap: enable static key after runtime pool recovery

[Yosry Ahmed]

On Sat, Sep 5, 2026 at 5:51 AM Longlong Xia <xialonglong2025@xxxxxxx> wrote:
>
> From: Longlong Xia <xialonglong@xxxxxxxxxx>
>
> When CONFIG_ZSWAP_DEFAULT_ON is disabled, zswap_setup() can complete
> without a pool after a failed initial pool creation. A later compressor
> parameter update can create and publish a pool, but does not enable
> zswap_ever_enabled.
>
> If users then enable zswap, zswap_store() intercepts swapout while
> zswap_load() still returns -ENOENT without consulting the xarray. The
> swapin path therefore reads a stale backing swap slot because the store
> skipped writing it.
>
> Enable the static key after a successful compressor and pool update. Do
> this outside zswap_pools_lock because static key updates may sleep.
>
> Verified with fault injection on a stock kernel (compressor builtin,
> CONFIG_ZSWAP_DEFAULT_ON=n):
>
>   1. Boot with zswap.enabled=1; pool creation fails, init completes
>      pool-less (static key off).
>   2. Echo an available compressor name to zswap.compressor; a pool is
>      recovered but the key stays off.
>   3. Enable zswap.
>   4. madvise(MADV_PAGEOUT) a pattern-verified 512 MiB region, then
>      fault it back in and verify.
>
> Step 4 reads back 131072/131072 zeroed pages (zswpin=0, zswpout=131072)
> without this patch; all pages intact (zswpin=131072) with it.
>
> Fixes: 2d4d2b1cfb85 ("mm: zswap: add zswap_never_enabled()")
> Cc: stable@xxxxxxxxxxxxxxx
> Assisted-by: Zcode:GLM-5.3
> Signed-off-by: Longlong Xia <xialonglong@xxxxxxxxxx>
> ---
>  mm/zswap.c | 3 +++
>  1 file changed, 3 insertions(+)
>
> diff --git a/mm/zswap.c b/mm/zswap.c
> index 37f34e406c8e3..c48c4df63f188 100644
> --- a/mm/zswap.c
> +++ b/mm/zswap.c
> @@ -586,6 +586,9 @@ static int zswap_compressor_param_set(const char *val, const struct kernel_param
>         else
>                 ret = -EINVAL;
>
> +       if (!ret)
> +               static_branch_enable(&zswap_ever_enabled);

What if we move it from zswap_setup() to zswap_pool_create()? IIUC
this would cover both cases?

> +
>         spin_lock_bh(&zswap_pools_lock);
>
>         if (!ret) {
> --
> 2.43.0
>