NFS: remove support for multi-segment iovs in the direct write path

Eliminate the persistent use of automatic storage in all parts of the
NFS client's direct write path to pave the way for introducing support
for aio against files opened with the O_DIRECT flag.

Test plan:
Compile the kernel with CONFIG_NFS and CONFIG_NFS_DIRECTIO enabled.
Millions of fsx-odirect ops.  OraSim.
Signed-off-by: NChuck Lever <cel@netapp.com>
Signed-off-by: NTrond Myklebust <Trond.Myklebust@netapp.com>

fs/nfs/direct.c

@@ -510,6 +510,7 @@ static void nfs_direct_write_schedule(struct nfs_direct_req *dreq, struct inode
 		data->args.count = bytes;
 		data->res.fattr = &data->fattr;
 		data->res.count = bytes;
+		data->res.verf = &data->verf;
 
 		rpc_init_task(&data->task, NFS_CLIENT(inode), RPC_TASK_ASYNC,
 				&nfs_write_direct_ops, data);
@@ -538,7 +539,7 @@ static void nfs_direct_write_schedule(struct nfs_direct_req *dreq, struct inode
 	} while (count != 0);
 }
 
-static ssize_t nfs_direct_write_seg(struct inode *inode, struct nfs_open_context *ctx, unsigned long user_addr, size_t count, loff_t file_offset, struct page **pages, int nr_pages)
+static ssize_t nfs_direct_write(struct inode *inode, struct nfs_open_context *ctx, unsigned long user_addr, size_t count, loff_t file_offset, struct page **pages, int nr_pages)
 {
 	ssize_t result;
 	sigset_t oldset;
@@ -552,6 +553,8 @@ static ssize_t nfs_direct_write_seg(struct inode *inode, struct nfs_open_context
 	dreq->pages = pages;
 	dreq->npages = nr_pages;
 
+	nfs_add_stats(inode, NFSIOS_DIRECTWRITTENBYTES, count);
+
 	nfs_begin_data_update(inode);
 
 	rpc_clnt_sigmask(clnt, &oldset);
@@ -565,50 +568,6 @@ static ssize_t nfs_direct_write_seg(struct inode *inode, struct nfs_open_context
 	return result;
 }
 
-/*
- * Upon return, generic_file_direct_IO invalidates any cached pages
- * that non-direct readers might access, so they will pick up these
- * writes immediately.
- */
-static ssize_t nfs_direct_write(struct inode *inode, struct nfs_open_context *ctx, const struct iovec *iov, loff_t file_offset, unsigned long nr_segs)
-{
-	ssize_t tot_bytes = 0;
-	unsigned long seg = 0;
-
-	while ((seg < nr_segs) && (tot_bytes >= 0)) {
-		ssize_t result;
-		int page_count;
-		struct page **pages;
-		const struct iovec *vec = &iov[seg++];
-		unsigned long user_addr = (unsigned long) vec->iov_base;
-		size_t size = vec->iov_len;
-
-                page_count = nfs_get_user_pages(WRITE, user_addr, size, &pages);
-                if (page_count < 0) {
-                        nfs_free_user_pages(pages, 0, 0);
-			if (tot_bytes > 0)
-				break;
-                        return page_count;
-                }
-
-		nfs_add_stats(inode, NFSIOS_DIRECTWRITTENBYTES, size);
-		result = nfs_direct_write_seg(inode, ctx, user_addr, size,
-				file_offset, pages, page_count);
-
-		if (result <= 0) {
-			if (tot_bytes > 0)
-				break;
-			return result;
-		}
-		nfs_add_stats(inode, NFSIOS_SERVERWRITTENBYTES, result);
-		tot_bytes += result;
-		file_offset += result;
-		if (result < size)
-			break;
-	}
-	return tot_bytes;
-}
-
 /**
  * nfs_file_direct_read - file direct read operation for NFS files
  * @iocb: target I/O control block
@@ -701,14 +660,13 @@ ssize_t nfs_file_direct_read(struct kiocb *iocb, char __user *buf, size_t count,
 ssize_t nfs_file_direct_write(struct kiocb *iocb, const char __user *buf, size_t count, loff_t pos)
 {
 	ssize_t retval;
+	int page_count;
+	struct page **pages;
 	struct file *file = iocb->ki_filp;
 	struct nfs_open_context *ctx =
 			(struct nfs_open_context *) file->private_data;
 	struct address_space *mapping = file->f_mapping;
 	struct inode *inode = mapping->host;
-	struct iovec iov = {
-		.iov_base = (char __user *)buf,
-	};
 
 	dfprintk(VFS, "nfs: direct write(%s/%s, %lu@%Ld)\n",
 		file->f_dentry->d_parent->d_name.name,
@@ -729,17 +687,25 @@ ssize_t nfs_file_direct_write(struct kiocb *iocb, const char __user *buf, size_t
 	retval = 0;
 	if (!count)
 		goto out;
-	iov.iov_len = count,
 
 	retval = -EFAULT;
-	if (!access_ok(VERIFY_READ, iov.iov_base, iov.iov_len))
+	if (!access_ok(VERIFY_READ, buf, count))
 		goto out;
 
 	retval = nfs_sync_mapping(mapping);
 	if (retval)
 		goto out;
 
-	retval = nfs_direct_write(inode, ctx, &iov, pos, 1);
+	page_count = nfs_get_user_pages(WRITE, (unsigned long) buf,
+						count, &pages);
+	if (page_count < 0) {
+		nfs_free_user_pages(pages, 0, 0);
+		retval = page_count;
+		goto out;
+	}
+
+	retval = nfs_direct_write(inode, ctx, (unsigned long) buf, count,
+					pos, pages, page_count);
 	if (mapping->nrpages)
 		invalidate_inode_pages2(mapping);
 	if (retval > 0)