I have reported this upstreams (http://debbugs.gnu.org/cgi/bugreport.cgi?bug=11245), but I submit this report too in case you wish to apply a patch until a fix from upstreams is available.  (Or as a tracker in case you wish to wait for an upstreams fix.)

Description of problem:
After upgrading to the Fedora 17 version of emacs, I started to get messages about emacs failing to write backup files.  It says it is writing a %backup%~ file instead.  Checking in the file system, emacs has successfully made the backup file.  After a bit of investigation, I believe it has to do with emacs trying to set SELinux context on a file system which doesn't support it.  In my case, my home directory is mounted via NFS.

Version-Release number of selected component (if applicable):
emacs-24.0.95-1.fc17.x86_64

How reproducible:
Every time

Steps to Reproduce:
1. Go to a writeable directory in an NFS-mounted file system
2. touch apa
3. emacs
4. In emacs: set backup-by-copying to true (M-x s e t - v a r <tab> <return> b a c k u <tab> b y <tab> <return> t <return>)
5. In emacs: visit the file touched (C-x C-f a p a <return>)
6. In emacs: modify the file: (a p a)
7. In emacs: save the file: (C-x C-s)

Actual results:
I get the error message: Cannot write backup file; backing up in ~/.emacs.d/%backup%~

Expected results:
No error message

Additional info:
My understanding is that the root cause is the function set-file-selinux-context in fileio.c failing with a "file error" when the lsetfilecon() call fails.  But it is not really an error if it fails with errno set to ENOTSUP.  A system which does support SELinux may very well have some parts of the file tree where you can't modify the context.  Such as an NFS home directory.

It seems to me that this case should be handled in the same way as if when SELinux isn't enabled.  I.e. just return without any error.  Only if lsetfilecon() fails with errno set to something ELSE than ENOTSUP, only then should set-file-selinux-context report an error.