Hi,
This might be basic question, but just wanted to clarify this.
If grub stage 2 can mount the root filesystem in read only mode and load kernel and initramfs from it, why do we need initramfs to create something called temporary root filesystem in read only mode. Is it bcoz grub can't mount root filesystem to a mount point ?

I haven't looked at the code, but there is no requirement for a bootloader to "mount" anything - it simply needs to know how to find the kernel and (if present) the initrd and read them off disk. This used to be hard-coded - hence the problems for legacy grub when ext[34] extended the inode size. New filesystems (btrfs) also bring new inconsistencies.
It looks like grub2 gets around this with modules that can be (re-)written to accommodate changes.

The initrd primarily exists to ensure the necessary support for the kenel to mount the root are present - notably disk controller and such. All that has to be built into the initrd itself. Distros usually add other functionality - say PXE booting.
The initrd is entirely optional if the necessary support is buit in at kernel compile.

THanks for the reply.
I was talking about not so legacy Linux like rhel 5 and 6.
grub accesses kernel/initrd using (hd0,0)/boot/vmlinuz...and /boot/initrd.
this doesn't look like hardcoding, but rather reading from filesystem or else it won't even understand the directory structure.

Simple.

https://linuxjourney.com/lesson/boot-process-kernel

More detailed.

https://www.ibm.com/developerworks/l...trd/index.html

Well, those deal with how the kernel and initrd are built and how they operate once they are loaded. In order to get those files loaded, GRUB does have internal knowledge of filesystem structure and can parse that structure without, technically**, mounting it. It is much the same as the way fsck, debugfs, and the like parse the internal structure of an unmounted filesystem.

In the case of GRUB legacy, the stage 1.5 bootloader is built with knowledge of one filesystem, that being the filesystem that contains /boot.

GRUB2 starts out in a similar manner, but once the bootloader has gained access to the /boot/grub2 directory there are many loadable modules available to handle various filesystems as well as video drivers, networking, encryption, USB serial devices, etc. All filesystems are handled by parsing their structure and not by mounting them. Mounting would imply that there is some mount point directory on which to mount them, and there is not (yet) a root filesystem on which to do that.

** Note that the word "mount" here is being used in the specific sense of attaching a filesystem to a point on the overall directory tree and not in the more general sense of "make available for processing," such as mounting a device that contains a tar archive.

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Opps, grub legacy.

Thanks rknichols.
That clears my doubt..