[SOLVED] How does kernel know which filesystems to mount where?

movrax · 04-10-2022, 10:19 AM

Hello,

I was wondering: where is information about mountpoints stored? i.e. how does the kernel know, for example, that "/dev/sda0p1" is mounted on "/" or that "/dev/sda0p2" is mounted on "/efi".

Thanks

pan64 · 04-10-2022, 10:23 AM

for example: /etc/fstab

hazel · 04-10-2022, 11:03 AM

I'm not sure it's that simple. On one level, one of the startup scripts calls mount -a (mount all) and /etc/fstab tells the mount command which partitions are included and where they are to go. That's simple. But to access the data on the disks, the kernel has to know which partition to go to when a mountpoint appears in a path. How does it know that? afaik the kernel doesn't read data out of files like fstab.

movrax · 04-10-2022, 11:04 AM

Quote:

Originally Posted by pan64

for example: /etc/fstab

/etc/fstab is already a filesystem path, isn't it? Maybe my reasoning is not correct here, but I assume has to know where "/etc" is mounted from somewhere else.

teckk · 04-10-2022, 11:23 AM

https://www.kernel.org/doc/html/late...ems/index.html
https://www.win.tue.nl/~aeb/linux/lk/lk-8.html

Code:

cat /proc/filesystems

https://kernel.org/doc/html/v5.18-rc...ems/ubifs.html

etc.

Edit:
https://systemd.io/
https://www.linux.com/training-tutor...using-systemd/
https://github.com/systemd/systemd
https://wiki.archlinux.org/title/Systemd

michaelk · 04-10-2022, 11:36 AM

As I understand the boot process basically /etc is part of the root filesystem and the kernel knows where to find the root filesystem because it is passed as a kernel option. During the init process the kernel initializes all of the storage systems and mounts the root filesystem, as the init process continues the /etc/fstab is read and the rest of the filesystems are mounted.

jailbait · 04-10-2022, 02:58 PM

Quote:

Originally Posted by movrax

/etc/fstab is already a filesystem path, isn't it? Maybe my reasoning is not correct here, but I assume has to know where "/etc" is mounted from somewhere else.

Some directories must be in the / partition. /etc is one of those directories.

pan64 · 04-10-2022, 11:43 PM

Quote:

Originally Posted by movrax

/etc/fstab is already a filesystem path, isn't it? Maybe my reasoning is not correct here, but I assume has to know where "/etc" is mounted from somewhere else.

It was already explained, /etc should be available during that init phase, therefore it need to be located on the root filesystem.
Additionally when you boot you need to specify the location of the root filesystem and the kernel itself in the boot loader (grub, lilo, whatever), so when the kernel starts the identifier of the root filesystem is passed.
You can read about it (for example) here: https://en.wikipedia.org/wiki/Linux_startup_process (and explained much better).