NFS: Avoid flushing data while holding directory locks in nfs_rename()

The Linux client assumes that all filehandles are non-volatile for
renames within the same directory (otherwise sillyrename cannot work).
However, the existence of the Linux 'subtree_check' export option has
meant that nfs_rename() has always assumed it needs to flush writes
before attempting to rename.

Since NFSv4 does allow the client to query whether or not the server
exhibits this behaviour, and since knfsd does actually set the
appropriate flag when 'subtree_check' is enabled on an export, it
should be OK to optimise away the write flushing behaviour in the cases
where it is clearly not needed.

Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>
Reviewed-by: Jeff Layton <jlayton@kernel.org>

Author: Trond Myklebust

fs/nfs/client.c

@@ -1105,6 +1105,8 @@ struct nfs_server *nfs_create_server(struct fs_context *fc)
 		if (server->namelen == 0 || server->namelen > NFS2_MAXNAMLEN)
 			server->namelen = NFS2_MAXNAMLEN;
 	}
+	/* Linux 'subtree_check' borkenness mandates this setting */
+	server->fh_expire_type = NFS_FH_VOL_RENAME;
 
 	if (!(fattr->valid & NFS_ATTR_FATTR)) {
 		error = ctx->nfs_mod->rpc_ops->getattr(server, ctx->mntfh,

fs/nfs/dir.c

@@ -2676,6 +2676,18 @@ nfs_unblock_rename(struct rpc_task *task, struct nfs_renamedata *data)
 	unblock_revalidate(new_dentry);
 }
 
+static bool nfs_rename_is_unsafe_cross_dir(struct dentry *old_dentry,
+					   struct dentry *new_dentry)
+{
+	struct nfs_server *server = NFS_SB(old_dentry->d_sb);
+
+	if (old_dentry->d_parent != new_dentry->d_parent)
+		return false;
+	if (server->fh_expire_type & NFS_FH_RENAME_UNSAFE)
+		return !(server->fh_expire_type & NFS_FH_NOEXPIRE_WITH_OPEN);
+	return true;
+}
+
 /*
  * RENAME
  * FIXME: Some nfsds, like the Linux user space nfsd, may generate a
@@ -2763,7 +2775,8 @@ int nfs_rename(struct mnt_idmap *idmap, struct inode *old_dir,
 
 	}
 
-	if (S_ISREG(old_inode->i_mode))
+	if (S_ISREG(old_inode->i_mode) &&
+	    nfs_rename_is_unsafe_cross_dir(old_dentry, new_dentry))
 		nfs_sync_inode(old_inode);
 	task = nfs_async_rename(old_dir, new_dir, old_dentry, new_dentry,
 				must_unblock ? nfs_unblock_rename : NULL);

include/linux/nfs_fs_sb.h

@@ -213,6 +213,15 @@ struct nfs_server {
 	char			*fscache_uniq;	/* Uniquifier (or NULL) */
 #endif
 
+	/* The following #defines numerically match the NFSv4 equivalents */
+#define NFS_FH_NOEXPIRE_WITH_OPEN (0x1)
+#define NFS_FH_VOLATILE_ANY (0x2)
+#define NFS_FH_VOL_MIGRATION (0x4)
+#define NFS_FH_VOL_RENAME (0x8)
+#define NFS_FH_RENAME_UNSAFE (NFS_FH_VOLATILE_ANY | NFS_FH_VOL_RENAME)
+	u32			fh_expire_type;	/* V4 bitmask representing file
+						   handle volatility type for
+						   this filesystem */
 	u32			pnfs_blksize;	/* layout_blksize attr */
 #if IS_ENABLED(CONFIG_NFS_V4)
 	u32			attr_bitmask[3];/* V4 bitmask representing the set
@@ -236,9 +245,6 @@ struct nfs_server {
 	u32			acl_bitmask;	/* V4 bitmask representing the ACEs
 						   that are supported on this
 						   filesystem */
-	u32			fh_expire_type;	/* V4 bitmask representing file
-						   handle volatility type for
-						   this filesystem */
 	struct pnfs_layoutdriver_type  *pnfs_curr_ld; /* Active layout driver */
 	struct rpc_wait_queue	roc_rpcwaitq;
 	void			*pnfs_ld_data;	/* per mount point data */