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

[Andrew Morton]

On Sat,  5 Sep 2026 20:50:28 +0800 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.

That sounds bad.  I'll leave it to reviewers to suggest whether this is
a sufficient description of the runtime effects, and to decide whether
a backport is appropriate.  Please.

> 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.

And thanks.  Sashiko might have found another issue in this zswap code:
	https://sashiko.dev/#/patchset/20260905125101.2970456-1-xialonglong2025@xxxxxxx