mm,ksm: add __GFP_HIGH to the allocation in alloc_stable_node()

According to Hugh's suggestion, alloc_stable_node() with GFP_KERNEL can
in rare cases cause a hung task warning.

At present, if alloc_stable_node() allocation fails, two break_cows may
want to allocate a couple of pages, and the issue will come up when free
memory is under pressure.

We fix it by adding __GFP_HIGH to GFP, to grant access to memory
reserves, increasing the likelihood of allocation success.

[[email protected]: tweak comment]
Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: zhong jiang <[email protected]>
Suggested-by: Hugh Dickins <[email protected]>
Acked-by: Hugh Dickins <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

mm/ksm.c

@@ -299,7 +299,12 @@ static inline void free_rmap_item(struct rmap_item *rmap_item)
 
 static inline struct stable_node *alloc_stable_node(void)
 {
-	return kmem_cache_alloc(stable_node_cache, GFP_KERNEL);
+	/*
+	 * The allocation can take too long with GFP_KERNEL when memory is under
+	 * pressure, which may lead to hung task warnings.  Adding __GFP_HIGH
+	 * grants access to memory reserves, helping to avoid this problem.
+	 */
+	return kmem_cache_alloc(stable_node_cache, GFP_KERNEL | __GFP_HIGH);
 }
 
 static inline void free_stable_node(struct stable_node *stable_node)