[PATCH v4 9/9] mm/swap: Skip readahead only when swap slot cache is enabled

[Tim Chen <tim.c.chen@linux.intel.com>]

From: Huang Ying <ying.huang@intel.com>

Because during swap off, a swap entry may have swap_map[] ==
SWAP_HAS_CACHE (for example, just allocated).  If we return NULL in
__read_swap_cache_async(), the swap off will abort.  So when swap slot
cache is disabled, (for swap off), we will wait for page to be put
into swap cache in such race condition.  This should not be a problem
for swap slot cache, because swap slot cache should be drained after
clearing swap_slot_cache_enabled.

Signed-off-by: "Huang, Ying" <ying.huang@intel.com>
Signed-off-by: Tim Chen <tim.c.chen@linux.intel.com>
---
 include/linux/swap_slots.h |  2 ++
 mm/swap_slots.c            |  2 +-
 mm/swap_state.c            | 11 +++++++++--
 3 files changed, 12 insertions(+), 3 deletions(-)

diff --git a/include/linux/swap_slots.h b/include/linux/swap_slots.h
index a59e6e2..fb90734 100644
--- a/include/linux/swap_slots.h
+++ b/include/linux/swap_slots.h
@@ -25,4 +25,6 @@ void reenable_swap_slots_cache_unlock(void);
 int enable_swap_slots_cache(void);
 int free_swap_slot(swp_entry_t entry);
 
+extern bool swap_slot_cache_enabled;
+
 #endif /* _LINUX_SWAP_SLOTS_H */
diff --git a/mm/swap_slots.c b/mm/swap_slots.c
index 8da25df..b53c149 100644
--- a/mm/swap_slots.c
+++ b/mm/swap_slots.c
@@ -36,7 +36,7 @@
 
 static DEFINE_PER_CPU(struct swap_slots_cache, swp_slots);
 static bool	swap_slot_cache_active;
-static bool	swap_slot_cache_enabled;
+bool	swap_slot_cache_enabled;
 static bool	swap_slot_cache_initialized;
 DEFINE_MUTEX(swap_slots_cache_mutex);
 /* Serialize swap slots cache enable/disable operations */
diff --git a/mm/swap_state.c b/mm/swap_state.c
index e1f07ca..ef14f42 100644
--- a/mm/swap_state.c
+++ b/mm/swap_state.c
@@ -324,8 +324,15 @@ struct page *__read_swap_cache_async(swp_entry_t entry, gfp_t gfp_mask,
 		if (found_page)
 			break;
 
-		/* Just skip read ahead for unused swap slot */
-		if (!__swp_swapcount(entry))
+		/*
+		 * Just skip read ahead for unused swap slot.
+		 * During swap_off when swap_slot_cache is disabled,
+		 * we have to handle the race between putting
+		 * swap entry in swap cache and marking swap slot
+		 * as SWAP_HAS_CACHE.  That's done in later part of code or
+		 * else swap_off will be aborted if we return NULL.
+		*/
+		if (!__swp_swapcount(entry) && swap_slot_cache_enabled)
 			return NULL;
 
 		/*
-- 
2.5.5

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