How to disable subtree checking for an NFS export

Writable NFS shares often move and rename files while clients keep them open. On a subdirectory export, subtree_check adds path verification to each request, and disabling it with no_subtree_check can reduce stale file handle problems when that path check is not part of the share's access boundary.

The setting belongs in the client option list in /etc/exports or an .exports drop-in under /etc/exports.d. Current nfs-utils defaults to no_subtree_check when neither option is written, but an explicit option makes the policy visible and prevents older or hardened export lines from carrying subtree_check forward unnoticed.

Change only the intended path and client selector, reload the active export table with exportfs -rv, and check the loaded options before client testing. If the exported path is a sensitive subdirectory of a larger filesystem and clients are not fully trusted, confirm that disabling subtree checking does not weaken the access boundary.

Steps to disable subtree checking for an NFS export:

Open a shell on the NFS server with an account that can use sudo.
Open the saved export definition that owns the path.
```
$ sudoedit /etc/exports.d/projects.exports
```
Use /etc/exports instead when the export is defined there. exportfs reads drop-in files under /etc/exports.d only when their names end in .exports.
Confirm that the matching client option list explicitly enables subtree checking.
```
/srv/nfs/projects 192.0.2.0/24(rw,sync,subtree_check,root_squash)
```
Keep the option list attached directly to the client selector, such as 192.0.2.0/24(rw,subtree_check), with no space before the opening parenthesis.
Replace subtree_check with no_subtree_check in that option list.
```
/srv/nfs/projects 192.0.2.0/24(rw,sync,no_subtree_check,root_squash)
```
Do not leave both subtree_check and no_subtree_check in the same client option list.
Reload the active export table from the saved files.
```
$ sudo exportfs -rv
exporting 192.0.2.0/24:/srv/nfs/projects
```
Fix any reported path, selector, or option warning before checking clients. Use exportfs -r for this change so deleted or narrowed exports are reconciled with the saved files.

Related: How to reload NFS exports
Verify that the loaded export includes no_subtree_check for the intended selector.
```
$ sudo exportfs -v
/srv/nfs/projects
		192.0.2.0/24(sync,wdelay,hide,no_subtree_check,sec=sys,rw,root_squash,no_all_squash)
```
The output should show no_subtree_check for the intended path and client selector. Defaults such as wdelay, hide, sec=sys, and no_all_squash may appear even when they are not written in the saved file.

Related: How to list NFS exports on a server
Related: How to mount an NFS export on Linux