[RFC KERNEL] initoverlayfs - a scalable initial filesystem

[Gao Xiang]

On 2023/12/12 15:35, Christoph Hellwig wrote:
> On Tue, Dec 12, 2023 at 08:50:56AM +0800, Gao Xiang wrote:
>> For non-virtualization cases, I guess you could try to use `memmap`
>> kernel option [2] to specify a memory region by bootloaders which
>> contains an EROFS rootfs and a customized init for booting as
>> erofs+overlayfs at least for `initoverlayfs`.  The main benefit is
>> that the memory region specified by the bootloader can be directly
>> used for mounting.  But I never tried if this option actually works.
>>
>> Furthermore, compared to traditional ramdisks, using direct address
>> can avoid page cache totally for uncompressed files like it can
>> just use unencoded data as mmaped memory.  For compressed files, it
>> still needs page cache to support mmaped access but we could adapt
>> more for persistent memory scenarios such as disable cache
>> decompression compared to previous block devices.
>>
>> I'm not sure if it's worth implementing this in kernelspace since
>> it's out of scope of an individual filesystem anyway.
> 
> IFF the use case turns out to be generally useful (it looks quite
> convoluted and odd to me), we could esily do an initdax concept where
> a chunk of memory passed by the bootloader is presented as a DAX device
> properly without memmap hacks.

I have no idea how it's faster than the current initramfs or initrd.
So if it's really useful, maybe some numbers can be posted first
with the current `memmap` hack and see it's worth going further with
some new infrastructure like initdax.

Thanks,
Gao Xiang