NFS: swap-out must always use STABLE writes.

The commit handling code is not safe against memory-pressure deadlocks
when writing to swap.  In particular, nfs_commitdata_alloc() blocks
indefinitely waiting for memory, and this can consume all available
workqueue threads.

swap-out most likely uses STABLE writes anyway as COND_STABLE indicates
that a stable write should be used if the write fits in a single
request, and it normally does.  However if we ever swap with a small
wsize, or gather unusually large numbers of pages for a single write,
this might change.

For safety, make it explicit in the code that direct writes used for swap
must always use FLUSH_STABLE.
Signed-off-by: NeilBrown <neilb@suse.de>
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>

fs/nfs/direct.c

@@ -794,7 +794,7 @@ static const struct nfs_pgio_completion_ops nfs_direct_write_completion_ops = {
  */
 static ssize_t nfs_direct_write_schedule_iovec(struct nfs_direct_req *dreq,
 					       struct iov_iter *iter,
-					       loff_t pos)
+					       loff_t pos, int ioflags)
 {
 	struct nfs_pageio_descriptor desc;
 	struct inode *inode = dreq->inode;
@@ -802,7 +802,7 @@ static ssize_t nfs_direct_write_schedule_iovec(struct nfs_direct_req *dreq,
 	size_t requested_bytes = 0;
 	size_t wsize = max_t(size_t, NFS_SERVER(inode)->wsize, PAGE_SIZE);
 
-	nfs_pageio_init_write(&desc, inode, FLUSH_COND_STABLE, false,
+	nfs_pageio_init_write(&desc, inode, ioflags, false,
 			      &nfs_direct_write_completion_ops);
 	desc.pg_dreq = dreq;
 	get_dreq(dreq);
@@ -948,11 +948,13 @@ ssize_t nfs_file_direct_write(struct kiocb *iocb, struct iov_iter *iter,
 	pnfs_init_ds_commit_info_ops(&dreq->ds_cinfo, inode);
 
 	if (swap) {
-		requested = nfs_direct_write_schedule_iovec(dreq, iter, pos);
+		requested = nfs_direct_write_schedule_iovec(dreq, iter, pos,
+							    FLUSH_STABLE);
 	} else {
 		nfs_start_io_direct(inode);
 
-		requested = nfs_direct_write_schedule_iovec(dreq, iter, pos);
+		requested = nfs_direct_write_schedule_iovec(dreq, iter, pos,
+							    FLUSH_COND_STABLE);
 
 		if (mapping->nrpages) {
 			invalidate_inode_pages2_range(mapping,