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

[Longlong Xia <xialonglong2025@163.com>]

Thanks for taking a look.

在 2026/9/6 7:09, Andrew Morton 写道:
> On Sat,  5 Sep 2026 20:50:28 +0800 Longlong Xia <xialonglong2025@163.com> wrote:
>
>> From: Longlong Xia <xialonglong@kylinos.cn>
>>
>> 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@163.com

I'll send a separate fix patch.


Thanks,

Longlong