NFS: Fall back to synchronous writes when a background write errors...

This helps prevent huge queues of background writes from building up
whenever the server runs out of disk or quota space, or if someone changes
the file access modes behind our backs.
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>

fs/nfs/file.c

@@ -176,6 +176,31 @@ static loff_t nfs_file_llseek(struct file *filp, loff_t offset, int origin)
 	return remote_llseek(filp, offset, origin);
 }
 
+/*
+ * Helper for nfs_file_flush() and nfs_fsync()
+ *
+ * Notice that it clears the NFS_CONTEXT_ERROR_WRITE before synching to
+ * disk, but it retrieves and clears ctx->error after synching, despite
+ * the two being set at the same time in nfs_context_set_write_error().
+ * This is because the former is used to notify the _next_ call to
+ * nfs_file_write() that a write error occured, and hence cause it to
+ * fall back to doing a synchronous write.
+ */
+static int nfs_do_fsync(struct nfs_open_context *ctx, struct inode *inode)
+{
+	int have_error, status;
+	int ret = 0;
+
+	have_error = test_and_clear_bit(NFS_CONTEXT_ERROR_WRITE, &ctx->flags);
+	status = nfs_wb_all(inode);
+	have_error |= test_bit(NFS_CONTEXT_ERROR_WRITE, &ctx->flags);
+	if (have_error)
+		ret = xchg(&ctx->error, 0);
+	if (!ret)
+		ret = status;
+	return ret;
+}
+
 /*
  * Flush all dirty pages, and check for write errors.
  *
@@ -192,16 +217,11 @@ nfs_file_flush(struct file *file, fl_owner_t id)
 	if ((file->f_mode & FMODE_WRITE) == 0)
 		return 0;
 	nfs_inc_stats(inode, NFSIOS_VFSFLUSH);
-	lock_kernel();
+
 	/* Ensure that data+attribute caches are up to date after close() */
-	status = nfs_wb_all(inode);
-	if (!status) {
-		status = ctx->error;
-		ctx->error = 0;
+	status = nfs_do_fsync(ctx, inode);
 	if (!status)
 		nfs_revalidate_inode(NFS_SERVER(inode), inode);
-	}
-	unlock_kernel();
 	return status;
 }
 
@@ -278,19 +298,11 @@ nfs_fsync(struct file *file, struct dentry *dentry, int datasync)
 {
 	struct nfs_open_context *ctx = (struct nfs_open_context *)file->private_data;
 	struct inode *inode = dentry->d_inode;
-	int status;
 
 	dfprintk(VFS, "nfs: fsync(%s/%ld)\n", inode->i_sb->s_id, inode->i_ino);
 
 	nfs_inc_stats(inode, NFSIOS_VFSFSYNC);
-	lock_kernel();
-	status = nfs_wb_all(inode);
-	if (!status) {
-		status = ctx->error;
-		ctx->error = 0;
-	}
-	unlock_kernel();
-	return status;
+	return nfs_do_fsync(ctx, inode);
 }
 
 /*
@@ -377,6 +389,18 @@ static struct vm_operations_struct nfs_file_vm_ops = {
 	.page_mkwrite = nfs_vm_page_mkwrite,
 };
 
+static int nfs_need_sync_write(struct file *filp, struct inode *inode)
+{
+	struct nfs_open_context *ctx;
+
+	if (IS_SYNC(inode) || (filp->f_flags & O_SYNC))
+		return 1;
+	ctx = filp->private_data;
+	if (test_bit(NFS_CONTEXT_ERROR_WRITE, &ctx->flags))
+		return 1;
+	return 0;
+}
+
 static ssize_t nfs_file_write(struct kiocb *iocb, const struct iovec *iov,
 				unsigned long nr_segs, loff_t pos)
 {
@@ -413,8 +437,8 @@ static ssize_t nfs_file_write(struct kiocb *iocb, const struct iovec *iov,
 	nfs_add_stats(inode, NFSIOS_NORMALWRITTENBYTES, count);
 	result = generic_file_aio_write(iocb, iov, nr_segs, pos);
 	/* Return error values for O_SYNC and IS_SYNC() */
-	if (result >= 0 && (IS_SYNC(inode) || (iocb->ki_filp->f_flags & O_SYNC))) {
-		int err = nfs_fsync(iocb->ki_filp, dentry, 1);
+	if (result >= 0 && nfs_need_sync_write(iocb->ki_filp, inode)) {
+		int err = nfs_do_fsync(iocb->ki_filp->private_data, inode);
 		if (err < 0)
 			result = err;
 	}

fs/nfs/write.c

@@ -110,6 +110,13 @@ void nfs_writedata_release(void *wdata)
 	nfs_writedata_free(wdata);
 }
 
+static void nfs_context_set_write_error(struct nfs_open_context *ctx, int error)
+{
+	ctx->error = error;
+	smp_wmb();
+	set_bit(NFS_CONTEXT_ERROR_WRITE, &ctx->flags);
+}
+
 static struct nfs_page *nfs_page_find_request_locked(struct page *page)
 {
 	struct nfs_page *req = NULL;
@@ -945,7 +952,7 @@ static void nfs_writeback_done_partial(struct rpc_task *task, void *calldata)
 
 	if (task->tk_status < 0) {
 		nfs_set_pageerror(page);
-		req->wb_context->error = task->tk_status;
+		nfs_context_set_write_error(req->wb_context, task->tk_status);
 		dprintk(", error = %d\n", task->tk_status);
 		goto out;
 	}
@@ -1008,7 +1015,7 @@ static void nfs_writeback_done_full(struct rpc_task *task, void *calldata)
 
 		if (task->tk_status < 0) {
 			nfs_set_pageerror(page);
-			req->wb_context->error = task->tk_status;
+			nfs_context_set_write_error(req->wb_context, task->tk_status);
 			dprintk(", error = %d\n", task->tk_status);
 			goto remove_request;
 		}
@@ -1222,7 +1229,7 @@ static void nfs_commit_done(struct rpc_task *task, void *calldata)
 			req->wb_bytes,
 			(long long)req_offset(req));
 		if (task->tk_status < 0) {
-			req->wb_context->error = task->tk_status;
+			nfs_context_set_write_error(req->wb_context, task->tk_status);
 			nfs_inode_remove_request(req);
 			dprintk(", error = %d\n", task->tk_status);
 			goto next;

include/linux/nfs_fs.h

@@ -77,6 +77,9 @@ struct nfs_open_context {
 	struct nfs4_state *state;
 	fl_owner_t lockowner;
 	int mode;
+
+	unsigned long flags;
+#define NFS_CONTEXT_ERROR_WRITE		(0)
 	int error;
 
 	struct list_head list;