NFS: Ensure that we revalidate the cached file length for llseek(SEEK_END)

  This fixes a data corruption error for mail delivery applications that
  expect to be able to do posix locking and then append writes on NFS.

 Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>
---
 fs/nfs/file.c |   42 +++++++++++++++++++++++++++++++++++++++++-
 1 files changed, 41 insertions(+), 1 deletion(-)

Index: linux-2.6.12-rc1/fs/nfs/file.c
===================================================================
--- linux-2.6.12-rc1.orig/fs/nfs/file.c
+++ linux-2.6.12-rc1/fs/nfs/file.c
@@ -37,6 +37,7 @@
 
 static int nfs_file_open(struct inode *, struct file *);
 static int nfs_file_release(struct inode *, struct file *);
+static loff_t nfs_file_llseek(struct file *file, loff_t offset, int origin);
 static int  nfs_file_mmap(struct file *, struct vm_area_struct *);
 static ssize_t nfs_file_sendfile(struct file *, loff_t *, size_t, read_actor_t, void *);
 static ssize_t nfs_file_read(struct kiocb *, char __user *, size_t, loff_t);
@@ -48,7 +49,7 @@ static int nfs_lock(struct file *filp, i
 static int nfs_flock(struct file *filp, int cmd, struct file_lock *fl);
 
 struct file_operations nfs_file_operations = {
-	.llseek		= remote_llseek,
+	.llseek		= nfs_file_llseek,
 	.read		= do_sync_read,
 	.write		= do_sync_write,
 	.aio_read		= nfs_file_read,
@@ -126,6 +127,45 @@ nfs_file_release(struct inode *inode, st
 	return NFS_PROTO(inode)->file_release(inode, filp);
 }
 
+/**
+ * nfs_revalidate_size - Revalidate the file size
+ * @inode - pointer to inode struct
+ * @file - pointer to struct file
+ *
+ * Revalidates the file length. This is basically a wrapper around
+ * nfs_revalidate_inode() that takes into account the fact that we may
+ * have cached writes (in which case we don't care about the server's
+ * idea of what the file length is), or O_DIRECT (in which case we
+ * shouldn't trust the cache).
+ */
+static int nfs_revalidate_file_size(struct inode *inode, struct file *filp)
+{
+	struct nfs_server *server = NFS_SERVER(inode);
+	struct nfs_inode *nfsi = NFS_I(inode);
+
+	if (server->flags & NFS_MOUNT_NOAC)
+		goto force_reval;
+	if (filp->f_flags & O_DIRECT)
+		goto force_reval;
+	if (nfsi->npages != 0)
+		return 0;
+	return nfs_revalidate_inode(server, inode);
+force_reval:
+	return __nfs_revalidate_inode(server, inode);
+}
+
+static loff_t nfs_file_llseek(struct file *filp, loff_t offset, int origin)
+{
+	/* origin == SEEK_END => we must revalidate the cached file length */
+	if (origin == 2) {
+		struct inode *inode = filp->f_mapping->host;
+		int retval = nfs_revalidate_file_size(inode, filp);
+		if (retval < 0)
+			return (loff_t)retval;
+	}
+	return remote_llseek(filp, offset, origin);
+}
+
 /*
  * Flush all dirty pages, and check for write errors.
  *