btrfs: fallback to vmalloc in btrfs_compare_tree

The allocation of node could fail if the memory is too fragmented for a
given node size, practically observed with 64k.

http://article.gmane.org/gmane.comp.file-systems.btrfs/54689

Reported-and-tested-by: Jean-Denis Girard <jd.girard@sysnux.pf>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/ctree.c

@@ -19,6 +19,7 @@
 #include <linux/sched.h>
 #include <linux/slab.h>
 #include <linux/rbtree.h>
+#include <linux/vmalloc.h>
 #include "ctree.h"
 #include "disk-io.h"
 #include "transaction.h"
@@ -5361,10 +5362,13 @@ int btrfs_compare_trees(struct btrfs_root *left_root,
 		goto out;
 	}
 
-	tmp_buf = kmalloc(left_root->nodesize, GFP_KERNEL);
+	tmp_buf = kmalloc(left_root->nodesize, GFP_KERNEL | __GFP_NOWARN);
 	if (!tmp_buf) {
-		ret = -ENOMEM;
-		goto out;
+		tmp_buf = vmalloc(left_root->nodesize);
+		if (!tmp_buf) {
+			ret = -ENOMEM;
+			goto out;
+		}
 	}
 
 	left_path->search_commit_root = 1;
@@ -5565,7 +5569,7 @@ int btrfs_compare_trees(struct btrfs_root *left_root,
 out:
 	btrfs_free_path(left_path);
 	btrfs_free_path(right_path);
-	kfree(tmp_buf);
+	kvfree(tmp_buf);
 	return ret;
 }