NFS: add comments clarifying the use of nfs_post_op_update()

Comments-only change to clarify a detail of the NFS protocol and how it is
implemented in Linux.

Test plan:
None.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>

1 files changed, 6 insertions, 0 deletions

diff --git a/fs/nfs/inode.c b/fs/nfs/inode.c
index a56add0bc5b7..e8c143d182c4 100644
--- a/fs/nfs/inode.c
+++ b/fs/nfs/inode.c
@@ -840,6 +840,12 @@ int nfs_refresh_inode(struct inode *inode, struct nfs_fattr *fattr)
  *
  * After an operation that has changed the inode metadata, mark the
  * attribute cache as being invalid, then try to update it.
+ *
+ * NB: if the server didn't return any post op attributes, this
+ * function will force the retrieval of attributes before the next
+ * NFS request.  Thus it should be used only for operations that
+ * are expected to change one or more attributes, to avoid
+ * unnecessary NFS requests and trips through nfs_update_inode().
  */
 int nfs_post_op_update_inode(struct inode *inode, struct nfs_fattr *fattr)
 {