Filesystems/tmpfs

Overview

Ramfs is a RAM-based filesystem. There is no backing store.

The block size of the filesystem is PAGE_CACHE_SIZE which is same as PAGE_SIZE. Thus each file occupies at least one page of memory.

mounting options

It supports following options. For details, look at shmem_parse_options()

• size=N[KkMmGg] or size=N%: Specifies maximum size of the filesystem. By default, tmpfs filesystem uses up to half of physical memory. max_blocks parameter is calculated out of specified size or percentage. Since size of each block is PAGE_CACHE_SIZE, so max_blocks = size of requested memory / PAGE_CACHE_SIZE.
• nr_blocks=N: Specifies maximum number of blocks in the tmpfs filesystem. Like size option, this sets max_blocks parameter.
• nr_inodes=N: Specifies maximum number of inodes in the filesystem. max_inodes parameter is set to the specified value.
• mode=Octal: Specifies mode parameter. The filesystem is mounted using specified mode.
• uid=N: Sets uid parameter. The mounted filesystem will be owned by specified user.
• gid=N: Sets gid parameter. The mounted filesystem will be owned by specified group.
• mpol=policy: Sets mpol parameter to specified memory policy.

mounting filesystem

Following command mounts a tmpfs filesystem on /tmp.

mount -t tmpfs tmpfs /tmp

To limit the size of tmpfs to 64 MByte,

mount -t tmpfs tmpfs /tmp -o size=64M

To resize the mounted tmpfs to 128 MByte,

mount -t tmpfs tmpfs /tmp -o size=128M,remount

To mount the filesystem on start-up, edit /etc/fstab

none /tmp tmpfs size=64M 0 0

Implementation

ramfs is implemented by files under mm/shmem.c.

Registration of ramfs filesystem

When ramfs module is loaded, it registers ramfs filesystem. inode.c defines an initialization function for the module.

int __init init_tmpfs(void)
{
        ......
        error = register_filesystem(&tmpfs_fs_type);
        ......
}

tmpfs_fs_type is defined as follows.

static struct file_system_type tmpfs_fs_type = {
        .owner          = THIS_MODULE,
        .name           = "tmpfs",
        .mount          = shmem_mount,
        .kill_sb        = kill_litter_super,
};

Mounting tmpfs

int shmem_fill_super(struct super_block *sb, void *data, int silent)
{
        ......

Reference

• Kernel documents: [[1]]