[PATCH v3 1/1] mm/zswap: enable zswap_ever_enabled in zswap_pool_create()

[Longlong Xia]

From: Longlong Xia <xialonglong@xxxxxxxxxx>

If zswap is enabled by default at boot and pool creation fails, then a
pool is later created by updating the compressor, data written to
zswap is corrupted on swapin.

Enable the static key in zswap_pool_create(), covering boot-time and
runtime pool creation with a single site.

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.
  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()")
Suggested-by: Yosry Ahmed <yosry@xxxxxxxxxx>
Acked-by: Yosry Ahmed <yosry@xxxxxxxxxx>
Cc: stable@xxxxxxxxxxxxxxx
Assisted-by: Zcode:GLM-5.3
Signed-off-by: Longlong Xia <xialonglong@xxxxxxxxxx>
---
Changes in v3:
- Drop the code comment per Yosry, and add his Acked-by.

Changes in v2:
- Enable the static key in zswap_pool_create() instead of adding a
  second site next to the compressor parameter update, as suggested
  by Yosry; boot-time and runtime pool creation are now covered by a
  single site, and the zswap_setup() site is gone.
- Use the high-level description from Yosry's reply to Andrew for the
  corruption scenario, and rename the subject accordingly.

Link: https://lore.kernel.org/r/20260905125101.2970456-1-xialonglong2025@xxxxxxx

 mm/zswap.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/mm/zswap.c b/mm/zswap.c
index 37f34e406c8e3..3abcd8433425b 100644
--- a/mm/zswap.c
+++ b/mm/zswap.c
@@ -324,6 +324,8 @@ static struct zswap_pool *zswap_pool_create(char *compressor)
 
 	zswap_pool_debug("created", pool);
 
+	static_branch_enable(&zswap_ever_enabled);
+
 	return pool;
 
 ref_fail:
@@ -1805,7 +1807,6 @@ static int zswap_setup(void)
 		pr_info("loaded using pool %s\n", pool->tfm_name);
 		list_add(&pool->list, &zswap_pools);
 		zswap_has_pool = true;
-		static_branch_enable(&zswap_ever_enabled);
 	} else {
 		pr_err("pool creation failed\n");
 		zswap_enabled = false;
-- 
2.43.0