a virtual file system that behaves like /dev/null

Mostly based on the ramfs example from the linux kernel and the lwnfs file system.

A virtual filesystem that behaves like /dev/null, but on the VFS layer of the kernel. Existing alternatives are mostly based on FUSE which do not perform good because of user/kernel space mapping operations.

It can handle regular file operations like mkdir/rmdir/ln but writing to files does not store any data.

In my tests i could get up to 3.85 Gib/s so it should be fine for performance tests that need directory operations.

# make
make -C /lib/modules/4.18.5/build M=/home/abi/lwnfs modules
make[1]: Entering directory '/usr/src/linux-headers-4.18.5'
  Building modules, stage 2.
  MODPOST 1 modules
make[1]: Leaving directory '/usr/src/linux-headers-4.18.5'

# insmod nullfs.ko 
# mkdir /sinkhole
# mount -t nullfs none /sinkhole/
# mkdir /sinkhole/testdir
# touch /sinkhole/testdir/myfile
# echo foobar > /sinkhole/testdir/myfile
# ls -lah /sinkhole/testdir/myfile
-rw-r--r-- 1 root root 0 Nov  8 20:17 /sinkhole/testdir/myfile
# cat /sinkhole/testdir/myfile
# cat /dev/zero | pv > /sinkhole/testdir/myfile
11.1GiB 0:00:04 [3.85GiB/s] [     <=>      ] 
# cat /sinkhole/testdir/myfile
# 

File size is preserved to work around applications that do size checks:

# stat -c%s proxmox.iso 
641517568
# cp proxmox.iso /sinkhole/
# stat -c%s /sinkhole/proxmox.iso 
641517568

There is the possiblity to exclude certain files from beeing sent into the void. For example if the file matching "fstab" should be kept in memory, one can mount nullfs with the "write=" option.

# mount -t nullfs none /sinkhole/ -o write=fstab
# cp /etc/fstab /sinkhole/
# wc -l /sinkhole/fstab 
14 /sinkhole/fstab
# cp /etc/passwd /sinkhole/
# wc -l /sinkhole/passwd 
0 /sinkhole/passwd

Another option is using the sysfs interface to change the exclude string after the module has been loaded:

 echo foo  > /sys/fs/nullfs/exclude 

Keep in mind that file data is kept in memory and no boundary checks are done, so this might fill up your RAM in case you exclude big files from beeing nulled.

• replace simple_statfs call with real one, show free space of a directory that can be passed during kernel module load
• return the size of the files with contents like /dev/zero
• support multiple parameters for write= option
• allow regex for write= option via trace.h