aio: bump i_count instead of using igrab

The aio batching code is using igrab to get an extra reference on the
inode so it can safely batch.  igrab will go ahead and take the global
inode spinlock, which can be a bottleneck on large machines doing lots
of AIO.

In this case, igrab isn't required because we already have a reference
on the file handle.  It is safe to just bump the i_count directly
on the inode.

Benchmarking shows this patch brings IOP/s on tons of flash up by about
2.5X.
Signed-off-by: Chris Mason <chris.mason@oracle.com>

fs/aio.c

@@ -1543,7 +1543,20 @@ static void aio_batch_add(struct address_space *mapping,
 	}
 
 	abe = mempool_alloc(abe_pool, GFP_KERNEL);
-	BUG_ON(!igrab(mapping->host));
+
+	/*
+	 * we should be using igrab here, but
+	 * we don't want to hammer on the global
+	 * inode spinlock just to take an extra
+	 * reference on a file that we must already
+	 * have a reference to.
+	 *
+	 * When we're called, we always have a reference
+	 * on the file, so we must always have a reference
+	 * on the inode, so igrab must always just
+	 * bump the count and move on.
+	 */
+	atomic_inc(&mapping->host->i_count);
 	abe->mapping = mapping;
 	hlist_add_head(&abe->list, &batch_hash[bucket]);
 	return;