mm: zswap: add zswap_never_enabled()

Add zswap_never_enabled() to skip the xarray lookup in zswap_load() if
zswap was never enabled on the system.  It is implemented using static
branches for efficiency, as enabling zswap should be a rare event.  This
could shave some cycles off zswap_load() when CONFIG_ZSWAP is used but
zswap is never enabled.

However, the real motivation behind this patch is two-fold:
- Incoming large folio swapin work will need to fallback to order-0
  folios if zswap was ever enabled, because any part of the folio could be
  in zswap, until proper handling of large folios with zswap is added.

- A warning and recovery attempt will be added in a following change in
  case the above was not done incorrectly.  Zswap will fail the read if
  the folio is large and it was ever enabled.

Expose zswap_never_enabled() in the header for the swapin work to use
it later.

[yosryahmed@google.com: expose zswap_never_enabled() in the header]
  Link: https://lkml.kernel.org/r/Zmjf0Dr8s9xSW41X@google.com
Link: https://lkml.kernel.org/r/20240611024516.1375191-2-yosryahmed@google.comSigned-off-by: Yosry Ahmed <yosryahmed@google.com>
Reviewed-by: Nhat Pham <nphamcs@gmail.com>
Cc: Barry Song <baohua@kernel.org>
Cc: Chengming Zhou <chengming.zhou@linux.dev>
Cc: Chris Li <chrisl@kernel.org>
Cc: David Hildenbrand <david@redhat.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

include/linux/zswap.h

@@ -36,6 +36,7 @@ void zswap_memcg_offline_cleanup(struct mem_cgroup *memcg);
 void zswap_lruvec_state_init(struct lruvec *lruvec);
 void zswap_folio_swapin(struct folio *folio);
 bool zswap_is_enabled(void);
+bool zswap_never_enabled(void);
 #else
 
 struct zswap_lruvec_state {};
@@ -65,6 +66,11 @@ static inline bool zswap_is_enabled(void)
 	return false;
 }
 
+static inline bool zswap_never_enabled(void)
+{
+	return false;
+}
+
 #endif
 
 #endif /* _LINUX_ZSWAP_H */

mm/zswap.c

@@ -83,6 +83,7 @@ static bool zswap_pool_reached_full;
 static int zswap_setup(void);
 
 /* Enable/disable zswap */
+static DEFINE_STATIC_KEY_MAYBE(CONFIG_ZSWAP_DEFAULT_ON, zswap_ever_enabled);
 static bool zswap_enabled = IS_ENABLED(CONFIG_ZSWAP_DEFAULT_ON);
 static int zswap_enabled_param_set(const char *,
 				   const struct kernel_param *);
@@ -136,6 +137,11 @@ bool zswap_is_enabled(void)
 	return zswap_enabled;
 }
 
+bool zswap_never_enabled(void)
+{
+	return !static_branch_maybe(CONFIG_ZSWAP_DEFAULT_ON, &zswap_ever_enabled);
+}
+
 /*********************************
 * data structures
 **********************************/
@@ -1557,6 +1563,9 @@ bool zswap_load(struct folio *folio)
 
 	VM_WARN_ON_ONCE(!folio_test_locked(folio));
 
+	if (zswap_never_enabled())
+		return false;
+
 	/*
 	 * When reading into the swapcache, invalidate our entry. The
 	 * swapcache can be the authoritative owner of the page and
@@ -1735,6 +1744,7 @@ static int zswap_setup(void)
 			zpool_get_type(pool->zpools[0]));
 		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;