Create and Resize a Safe tmpfs Mount

You will create a temporary filesystem with an 8 MiB limit, check its permissions and accounting, resize it without losing a test file, then unmount it cleanly. Allow about ten minutes. You need a Linux shell, mount, findmnt and df; mounting normally requires root privileges through sudo.

1. Check the local behaviour

The installed reference is Linux man-pages 6.7, dated 31 October 2023. This host is running Linux 6.8.0-139-generic with util-linux 2.42.4. The commands below use the interfaces provided by that combination.

A tmpfs keeps files in virtual memory. It can use swap when memory pressure requires it, consumes resources for the data actually stored, and has a size limit. It is not persistent storage: unmounting it loses its contents. The default limit is size=50% of physical RAM when neither size nor nr_blocks is supplied, so set an explicit limit for a scratch mount.

$ mount --version
mount from util-linux 2.42.4 (libmount 2.42.4: btrfs, namespaces, idmapping, fd-based-mount, statmount, statx, fanotify, assert, debug)
$ findmnt --version
findmnt from util-linux 2.42.4

The version text can differ on another distribution. That does not change the important safety rule: do not leave an unbounded or unexpectedly large tmpfs in place.

2. Prepare a private mount point

Choose a directory that is not already being used as a mount point. The example uses /tmp/dixon-tmpfs; replace it with a dedicated directory owned by the service or user that needs it.

$ sudo install -d -m 0755 /tmp/dixon-tmpfs
$ findmnt --noheadings --target /tmp/dixon-tmpfs

The second command should produce no output. If it shows an existing filesystem, stop and choose another directory. Do not mount over a directory containing files you need: those files become hidden until the new filesystem is unmounted.

The directory creation is the first privileged action. The later file operations can be unprivileged if the selected mode, owner and group permit them.

3. Mount a bounded tmpfs

Mount an 8 MiB instance with a root directory mode of 0700. The first tmpfs is the filesystem source name; it is a label here, not a block device.

$ sudo mount -t tmpfs -o size=8M,mode=0700 tmpfs /tmp/dixon-tmpfs
$ findmnt --noheadings --output FSTYPE,OPTIONS,TARGET /tmp/dixon-tmpfs
tmpfs rw,relatime,size=8192k,mode=700 /tmp/dixon-tmpfs

The M suffix means MiB for this option. The limit is rounded to whole pages, so the displayed value may be expressed in KiB. mode=0700 gives the mount root owner-only access; the initial owner and group are otherwise set from the mount defaults unless you pass uid= and gid=.

4. Write data and inspect the limit

Use ordinary commands to create a small file. This does not write to the underlying disk location while the tmpfs is mounted.

$ printf '%s\n' 'temporary test data' > /tmp/dixon-tmpfs/check.txt
$ cat /tmp/dixon-tmpfs/check.txt
temporary test data
$ df -h /tmp/dixon-tmpfs
Filesystem      Size  Used Avail Use% Mounted on
tmpfs           8.0M  4.0K  8.0M   1% /tmp/dixon-tmpfs

Your df output may use different rounding. Check the filesystem row, not a remembered byte count. The manpage also describes aggregate tmpfs use in Shmem in /proc/meminfo and the shared field from free; those figures include other shared-memory users, so df is the useful per-mount check.

5. Resize it without unmounting

tmpfs supports a remount that changes its size without discarding existing contents. Increase the limit to 16 MiB:

$ sudo mount -o remount,size=16M /tmp/dixon-tmpfs
$ findmnt --noheadings --output FSTYPE,OPTIONS,TARGET /tmp/dixon-tmpfs
tmpfs rw,relatime,size=16384k,mode=700 /tmp/dixon-tmpfs
$ cat /tmp/dixon-tmpfs/check.txt
temporary test data

A resize cannot make the limit smaller than the current usage. If a reduction fails, inspect usage with du -sh /tmp/dixon-tmpfs and remove only files you have confirmed are disposable. The remount itself needs privilege, and its settings must remain compatible with the original mount. For example, a mount created with noswap must keep that choice on remount; the local manpage records that option as available since Linux 6.4 and says swap is enabled by default otherwise.

6. Unmount and recover the directory

$ sudo umount /tmp/dixon-tmpfs
$ findmnt --noheadings --target /tmp/dixon-tmpfs
$ rmdir /tmp/dixon-tmpfs

After the unmount, findmnt should again print nothing. The directory is now gone. If you mounted over a directory that already contained files, they should reappear after unmounting.

If the mount is busy, find the process holding a file or working directory with sudo fuser -vm /tmp/dixon-tmpfs, leave that process safely, then retry sudo umount /tmp/dixon-tmpfs. Do not reach for a forced or lazy unmount in a routine cleanup: those choices can leave application state harder to reason about.

Common traps

• Expecting persistence: tmpfs is suitable for caches, scratch data and runtime files, not the only copy of a report or database.
• Assuming the default is small: without an explicit size, the limit is normally half of physical RAM.
• Confusing limit and current use: size=16M permits up to that limit; it does not allocate 16 MiB immediately.
• Using nr_blocks with a percentage: percentage syntax belongs to size, not nr_blocks or nr_inodes.
• Mounting over useful files: the old directory contents are hidden during the mount, not copied into tmpfs.