If you have NFS mounts on a client using a sufficiently recent kernel, such as that shipped with Ubuntu 26.04 LTS, and you inspect /proc/mounts (or 'mount -t nfs' ), you'll probably discover a new NFS mount option that is listed for your mounts. For example:
[...]:/h/281 /h/281 nfs4 rw,[...],fatal_neterrors=none,[...]
You might be curious what this option is, especially if you have code that parses NFS (v4) options and sorts options into ones you care about and ones you don't ( as we do ). Unfortunately, as I write this about a year after the option was added to the Linux kernel, 'fatal_neterrors' isn't documented in any manual page that I can readily find. As far as I know, the only accessible documentation for it is in the commit that added it , which I'm just going to quote:
NFS: Add a mount option to make ENETUNREACH errors fatal
If the NFS client was initially created in a container, and that container is torn down, there is usually no possibility to go back and destroy any NFS clients that are hung because their virtual network devices have been unlinked.
Add a flag that tells the NFS client that in these circumstances, it should treat ENETDOWN and ENETUNREACH errors as fatal to the NFS client.
The option defaults to being on when the mount happens from inside a net namespace that is not "init_net".
As you can see in that commit, currently the only two values you can see for this are '
fatal_neterrors=none
' or '
fatal_neterrors=ENETDOWN:ENETUNREACH
'. As a mount option that you provide manually, you can also ask for 'default' or 'ENETUNREACH:ENETDOWN'.
If I'm reading the tea leaves correctly, this commit first appeared in Linux 6.15.0 (and so a 'fatal_neterrors' option for NFS mounts first started being reported in /proc/mounts then). This specific commit seems to be merely one of a whole series of net/sunrpc and fs/nfs commits that make ENETDOWN and ENETUNREACH errors as 'fatal' under the right magic circumstances, and I think that by 'fatal' what it means is that any client NFS request fails immediately and isn't ever retried. I don't think it makes the NFS mount itself disappear. Hopefully it will allow you to unmount the now permanently non-functional NFS mount.
(The current state of affairs is that all NFS mounts will report some value for the 'fatal_neterrors' option, although the value of 'none' is effectively the pre-6.15 state of affairs.)
PS: Probably the place where documentation will appear at some point is the upstream source for nfs(5) , which will then wind up on man7.org as nfs(5) .