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.

[[email protected]: expose zswap_never_enabled() in the header]
  Link: https://lkml.kernel.org/r/[email protected]
Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Yosry Ahmed <[email protected]>
Reviewed-by: Nhat Pham <[email protected]>
Cc: Barry Song <[email protected]>
Cc: Chengming Zhou <[email protected]>
Cc: Chris Li <[email protected]>
Cc: David Hildenbrand <[email protected]>
Cc: Johannes Weiner <[email protected]>
Cc: Matthew Wilcox (Oracle) <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

1 files changed, 6 insertions, 0 deletions

diff --git a/include/linux/zswap.h b/include/linux/zswap.h
index ce5e7bfe8f1e..bf83ae5e285d 100644
--- a/include/linux/zswap.h
+++ b/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 */