linux-kernel - Re: [PATCH] initrd: support erofs as initrd

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Message-ID: <20250321-komprimieren-dachgeschoss-75bc46d824d0@brauner>
Date: Fri, 21 Mar 2025 09:46:30 +0100
From: Christian Brauner <brauner@...nel.org>
To: Al Viro <viro@...iv.linux.org.uk>
Cc: julian.stecklina@...erus-technology.de, Christoph Hellwig <hch@....de>, 
	Linus Torvalds <torvalds@...ux-foundation.org>, Greg Kroah-Hartman <gregkh@...uxfoundation.org>, 
	"Rafael J. Wysocki" <rafael@...nel.org>, linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] initrd: support erofs as initrd

On Fri, Mar 21, 2025 at 02:08:26AM +0000, Al Viro wrote:
> On Thu, Mar 20, 2025 at 08:28:23PM +0100, Julian Stecklina via B4 Relay wrote:
> > From: Julian Stecklina <julian.stecklina@...erus-technology.de>
> > 
> > Add erofs detection to the initrd mount code. This allows systems to
> > boot from an erofs-based initrd in the same way as they can boot from
> > a squashfs initrd.
> > 
> > Just as squashfs initrds, erofs images as initrds are a good option
> > for systems that are memory-constrained.
> > 
> > Signed-off-by: Julian Stecklina <julian.stecklina@...erus-technology.de>
> 
> >  #include "do_mounts.h"
> >  #include "../fs/squashfs/squashfs_fs.h"
> > +#include "../fs/erofs/erofs_fs.h"
> 
> This is getting really unpleasant...
> 
> Folks, could we do something similar to initcalls - add a section
> (.init.text.rd_detect?) with array of pointers to __init functions
> that would be called by that thing in turn?  With filesystems that
> want to add that kind of stuff being about to do something like
> 
> static int __init detect_minix(struct file *file, void *buf, loff_t *pos, int start_block)
> {
> 	struct minix_super_block *minixsb = buf;
> 	initrd_fill_buffer(file, buf, pos, (start_block + 1) * BLOCK_SIZE);
> 	if (minixsb->s_magic == MINIX_SUPER_MAGIC ||
> 	    minixsb->s_magic == MINIX_SUPER_MAGIC2) {
> 		printk(KERN_NOTICE
> 			"RAMDISK: Minix filesystem found at block %d\n",
> 			start_block);
> 		return minixsb->s_nzones << minixsb->s_log_zone_size;
> 	}
> 	return -1;
> }
> 
> initrd_detect(detect_minix);
> 
> with the latter emitting a pointer to detect_minix into that new
> section?
> 
> initrd_fill_buffer() would be something along the lines of
> 
> 	if (*pos != wanted) {
> 		*pos = wanted;
> 		kernel_read(file, buf, 512, pos);
> 	}
> 
> I mean, we can keep adding those pieces there, but...

Very much agreed.