nfs: fix congestion control: use atomic_longs

Change the atomic_t in struct nfs_server to atomic_long_t in anticipation
of machines that can handle 8+TB of (4K) pages under writeback.

However I suspect other things in NFS will start going *bang* by then.
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Trond Myklebust <trond.myklebust@fys.uio.no>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/nfs/write.c

@@ -224,7 +224,7 @@ static int nfs_set_page_writeback(struct page *page)
 		struct inode *inode = page->mapping->host;
 		struct nfs_server *nfss = NFS_SERVER(inode);
 
-		if (atomic_inc_return(&nfss->writeback) >
+		if (atomic_long_inc_return(&nfss->writeback) >
 				NFS_CONGESTION_ON_THRESH)
 			set_bdi_congested(&nfss->backing_dev_info, WRITE);
 	}
@@ -237,7 +237,7 @@ static void nfs_end_page_writeback(struct page *page)
 	struct nfs_server *nfss = NFS_SERVER(inode);
 
 	end_page_writeback(page);
-	if (atomic_dec_return(&nfss->writeback) < NFS_CONGESTION_OFF_THRESH) {
+	if (atomic_long_dec_return(&nfss->writeback) < NFS_CONGESTION_OFF_THRESH) {
 		clear_bdi_congested(&nfss->backing_dev_info, WRITE);
 		congestion_end(WRITE);
 	}

include/linux/nfs_fs_sb.h

@@ -82,7 +82,7 @@ struct nfs_server {
 	struct rpc_clnt *	client_acl;	/* ACL RPC client handle */
 	struct nfs_iostats *	io_stats;	/* I/O statistics */
 	struct backing_dev_info	backing_dev_info;
-	atomic_t		writeback;	/* number of writeback pages */
+	atomic_long_t		writeback;	/* number of writeback pages */
 	int			flags;		/* various flags */
 	unsigned int		caps;		/* server capabilities */
 	unsigned int		rsize;		/* read size */