pNFS/flexfiles: Ensure we have enough buffer for layoutreturn

The flexfiles client can piggyback both layout errors and layoutstats
as part of the layoutreturn. Both these payloads can get large, with
20 layout error entries taking up about 1.2K, and 4 layoutstats entries
taking up another 1K.
This patch allows a maximum payload of 4k by allocating a full page.
Signed-off-by: NTrond Myklebust <trond.myklebust@primarydata.com>

fs/nfs/flexfilelayout/flexfilelayout.c

@@ -2049,16 +2049,28 @@ ff_layout_encode_layoutreturn(struct xdr_stream *xdr,
 {
 	const struct nfs4_layoutreturn_args *args = voidargs;
 	struct nfs4_flexfile_layoutreturn_args *ff_args = ff_opaque->data;
+	struct xdr_buf tmp_buf = {
+		.head = {
+			[0] = {
+				.iov_base = page_address(ff_args->pages[0]),
+			},
+		},
+		.buflen = PAGE_SIZE,
+	};
+	struct xdr_stream tmp_xdr;
 	__be32 *start;
 
 	dprintk("%s: Begin\n", __func__);
-	start = xdr_reserve_space(xdr, 4);
-	BUG_ON(!start);
 
-	ff_layout_encode_ioerr(xdr, args, ff_args);
-	ff_layout_encode_iostats_array(xdr, args, ff_args);
+	xdr_init_encode(&tmp_xdr, &tmp_buf, NULL);
+
+	ff_layout_encode_ioerr(&tmp_xdr, args, ff_args);
+	ff_layout_encode_iostats_array(&tmp_xdr, args, ff_args);
+
+	start = xdr_reserve_space(xdr, 4);
+	*start = cpu_to_be32(tmp_buf.len);
+	xdr_write_pages(xdr, ff_args->pages, 0, tmp_buf.len);
 
-	*start = cpu_to_be32((xdr->p - start - 1) * 4);
 	dprintk("%s: Return\n", __func__);
 }
 
@@ -2075,6 +2087,7 @@ ff_layout_free_layoutreturn(struct nfs4_xdr_opaque_data *args)
 	ff_layout_free_ds_ioerr(&ff_args->errors);
 	ff_layout_free_iostats_array(ff_args->devinfo, ff_args->num_dev);
 
+	put_page(ff_args->pages[0]);
 	kfree(ff_args);
 }
 
@@ -2091,7 +2104,10 @@ ff_layout_prepare_layoutreturn(struct nfs4_layoutreturn_args *args)
 
 	ff_args = kmalloc(sizeof(*ff_args), GFP_KERNEL);
 	if (!ff_args)
-		return -ENOMEM;
+		goto out_nomem;
+	ff_args->pages[0] = alloc_page(GFP_KERNEL);
+	if (!ff_args->pages[0])
+		goto out_nomem_free;
 
 	INIT_LIST_HEAD(&ff_args->errors);
 	ff_args->num_errors = ff_layout_fetch_ds_ioerr(args->layout,
@@ -2106,6 +2122,10 @@ ff_layout_prepare_layoutreturn(struct nfs4_layoutreturn_args *args)
 	args->ld_private->ops = &layoutreturn_ops;
 	args->ld_private->data = ff_args;
 	return 0;
+out_nomem_free:
+	kfree(ff_args);
+out_nomem:
+	return -ENOMEM;
 }
 
 static int

fs/nfs/flexfilelayout/flexfilelayout.h

@@ -113,6 +113,7 @@ struct nfs4_flexfile_layoutreturn_args {
 	struct nfs42_layoutstat_devinfo devinfo[FF_LAYOUTSTATS_MAXDEV];
 	unsigned int num_errors;
 	unsigned int num_dev;
+	struct page *pages[1];
 };
 
 static inline struct nfs4_flexfile_layout *