NFS: Add a helper to convert a struct nfs_page into an inode

Replace all the open coded calls to page_file_mapping(req->wb_page)->host.
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>

fs/nfs/pagelist.c

@@ -426,7 +426,7 @@ nfs_page_group_init(struct nfs_page *req, struct nfs_page *prev)
 		 * has extra ref from the write/commit path to handle handoff
 		 * between write and commit lists. */
 		if (test_bit(PG_INODE_REF, &prev->wb_head->wb_flags)) {
-			inode = page_file_mapping(req->wb_page)->host;
+			inode = nfs_page_to_inode(req);
 			set_bit(PG_INODE_REF, &req->wb_flags);
 			kref_get(&req->wb_kref);
 			atomic_long_inc(&NFS_I(inode)->nrequests);

fs/nfs/write.c

@@ -421,7 +421,7 @@ static void nfs_set_page_writeback(struct page *page)
 
 static void nfs_end_page_writeback(struct nfs_page *req)
 {
-	struct inode *inode = page_file_mapping(req->wb_page)->host;
+	struct inode *inode = nfs_page_to_inode(req);
 	struct nfs_server *nfss = NFS_SERVER(inode);
 	bool is_done;
 
@@ -592,8 +592,7 @@ nfs_lock_and_join_requests(struct page *page)
 
 static void nfs_write_error(struct nfs_page *req, int error)
 {
-	trace_nfs_write_error(page_file_mapping(req->wb_page)->host, req,
-			      error);
+	trace_nfs_write_error(nfs_page_to_inode(req), req, error);
 	nfs_mapping_set_error(req->wb_page, error);
 	nfs_inode_remove_request(req);
 	nfs_end_page_writeback(req);
@@ -1420,7 +1419,7 @@ static void nfs_initiate_write(struct nfs_pgio_header *hdr,
  */
 static void nfs_redirty_request(struct nfs_page *req)
 {
-	struct nfs_inode *nfsi = NFS_I(page_file_mapping(req->wb_page)->host);
+	struct nfs_inode *nfsi = NFS_I(nfs_page_to_inode(req));
 
 	/* Bump the transmission count */
 	req->wb_nio++;

include/linux/nfs_page.h

@@ -189,6 +189,19 @@ static inline struct page *nfs_page_to_page(const struct nfs_page *req,
 	return folio_page(folio, pgbase >> PAGE_SHIFT);
 }
 
+/**
+ * nfs_page_to_inode - Retrieve an inode for the request
+ * @req: pointer to a struct nfs_page
+ */
+static inline struct inode *nfs_page_to_inode(const struct nfs_page *req)
+{
+	struct folio *folio = nfs_page_to_folio(req);
+
+	if (folio == NULL)
+		return page_file_mapping(req->wb_page)->host;
+	return folio_file_mapping(folio)->host;
+}
+
 /**
  * nfs_page_max_length - Retrieve the maximum possible length for a request
  * @req: pointer to a struct nfs_page