NFS: Ensure NFS writeback allocations don't recurse back into NFS.

All the allocations that we can hit in the NFS layer and sunrpc layers
themselves are already marked as GFP_NOFS, but we need to ensure that
any calls to generic kernel functionality do the right thing as well.
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>

fs/nfs/write.c

@@ -26,6 +26,7 @@
 #include <linux/iversion.h>
 
 #include <linux/uaccess.h>
+#include <linux/sched/mm.h>
 
 #include "delegation.h"
 #include "internal.h"
@@ -712,11 +713,13 @@ int nfs_writepages(struct address_space *mapping, struct writeback_control *wbc)
 {
 	struct inode *inode = mapping->host;
 	struct nfs_pageio_descriptor pgio;
-	struct nfs_io_completion *ioc = nfs_io_completion_alloc(GFP_NOFS);
+	struct nfs_io_completion *ioc;
+	unsigned int pflags = memalloc_nofs_save();
 	int err;
 
 	nfs_inc_stats(inode, NFSIOS_VFSWRITEPAGES);
 
+	ioc = nfs_io_completion_alloc(GFP_NOFS);
 	if (ioc)
 		nfs_io_completion_init(ioc, nfs_io_completion_commit, inode);
 
@@ -727,6 +730,8 @@ int nfs_writepages(struct address_space *mapping, struct writeback_control *wbc)
 	nfs_pageio_complete(&pgio);
 	nfs_io_completion_put(ioc);
 
+	memalloc_nofs_restore(pflags);
+
 	if (err < 0)
 		goto out_err;
 	err = pgio.pg_error;