linux-kernel - Re: [PATCH v6 2/2] init/do_mounts.c: create second mount for initramfs

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Date:   Mon, 7 Jun 2021 05:15:24 -0700
From:   menglong8.dong@...il.com
To:     Christian Brauner <christian.brauner@...ntu.com>
Cc:     Menglong Dong <menglong8.dong@...il.com>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Kees Cook <keescook@...omium.org>,
        Sami Tolvanen <samitolvanen@...gle.com>, johan@...nel.org,
        ojeda@...nel.org, jeyu@...nel.org, masahiroy@...nel.org,
        joe@...ches.com, Jan Kara <jack@...e.cz>, hare@...e.de,
        Jens Axboe <axboe@...nel.dk>, tj@...nel.org,
        gregkh@...uxfoundation.org, song@...nel.org,
        NeilBrown <neilb@...e.de>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        Barret Rhoden <brho@...gle.com>, f.fainelli@...il.com,
        palmerdabbelt@...gle.com, wangkefeng.wang@...wei.com,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Steven Rostedt <rostedt@...dmis.org>, vbabka@...e.cz,
        Alexander Potapenko <glider@...gle.com>, pmladek@...e.com,
        johannes.berg@...el.com,
        "Eric W. Biederman" <ebiederm@...ssion.com>, jojing64@...il.com,
        terrelln@...com, geert@...ux-m68k.org,
        linux-fsdevel@...r.kernel.org, LKML <linux-kernel@...r.kernel.org>,
        Luis Chamberlain <mcgrof@...nel.org>, arnd@...db.de,
        Chris Down <chris@...isdown.name>, mingo@...nel.org,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        Josh Triplett <josh@...htriplett.org>
Subject: Re: [PATCH v6 2/2] init/do_mounts.c: create second mount for
 initramfs

On Mon, Jun 07, 2021 at 12:31:47PM +0200, Christian Brauner wrote:
> On Sat, Jun 05, 2021 at 10:47:07PM +0800, Menglong Dong wrote:
[...]
> > 
> > I think it's necessary, as I explained in the third patch. When the rootfs
> > is a block device, ramfs is used in init_mount_tree() unconditionally,
> > which can be seen from the enable of is_tmpfs.
> > 
> > That makes sense, because rootfs will not become the root if a block
> > device is specified by 'root' in boot cmd, so it makes no sense to use
> > tmpfs, because ramfs is more simple.
> > 
> > Here, I make rootfs as ramfs for the same reason: the first mount is not
> > used as the root, so make it ramfs which is more simple.
> 
> Ok. If you don't mind I'd like to pull and test this before moving
> further. (Btw, I talked about this at Plumbers before btw.)
> What did you use for testing this? Any way you can share it?

Ok, no problem definitely. I tested this function in 3 way mainly:

1. I debug the kernel with qemu and gdb, and trace the the whole
   process, to ensure that there is no abnormal situation.
2. I tested pivot_root() in initramfs and ensured that it can be
   used normally. What's more, I also tested docker and ensured
   container can run normally without 'DOCKER_RAMDISK=yes' set in
   initramfs.
3. I tried to enable and disable CONFIG_INITRAMFS_MOUNT, and
   ensured that the system can boot successfully from initramfs, initrd
   and sda.

What's more, our team is going to test it comprehensively, such as
ltp, etc.

Thanks!
Menglong Dong