linux-kernel - Re: [PATCH RESEND] fs: Add 'rootfsflags' to set rootfs mount options

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Message-ID: <CAPmSd0O=f24o0J6Q202qFv09YrvvAtVryLk68RbU9ncfMTS0Vw@mail.gmail.com>
Date: Fri, 15 Aug 2025 20:12:30 +0800
From: Lichen Liu <lichliu@...hat.com>
To: Randy Dunlap <rdunlap@...radead.org>
Cc: viro@...iv.linux.org.uk, brauner@...nel.org, jack@...e.cz, 
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org, 
	safinaskar@...omail.com, kexec@...ts.infradead.org, rob@...dley.net, 
	weilongchen@...wei.com, cyphar@...har.com, linux-api@...r.kernel.org, 
	zohar@...ux.ibm.com, stefanb@...ux.ibm.com, initramfs@...r.kernel.org
Subject: Re: [PATCH RESEND] fs: Add 'rootfsflags' to set rootfs mount options

Thanks Randy,

I will send a v2 with documentation.

On Fri, Aug 15, 2025 at 12:27 AM Randy Dunlap <rdunlap@...radead.org> wrote:
>
> Hi,
>
> On 8/14/25 3:34 AM, Lichen Liu wrote:
> > When CONFIG_TMPFS is enabled, the initial root filesystem is a tmpfs.
> > By default, a tmpfs mount is limited to using 50% of the available RAM
> > for its content. This can be problematic in memory-constrained
> > environments, particularly during a kdump capture.
> >
> > In a kdump scenario, the capture kernel boots with a limited amount of
> > memory specified by the 'crashkernel' parameter. If the initramfs is
> > large, it may fail to unpack into the tmpfs rootfs due to insufficient
> > space. This is because to get X MB of usable space in tmpfs, 2*X MB of
> > memory must be available for the mount. This leads to an OOM failure
> > during the early boot process, preventing a successful crash dump.
> >
> > This patch introduces a new kernel command-line parameter, rootfsflags,
> > which allows passing specific mount options directly to the rootfs when
> > it is first mounted. This gives users control over the rootfs behavior.
> >
> > For example, a user can now specify rootfsflags=size=75% to allow the
> > tmpfs to use up to 75% of the available memory. This can significantly
> > reduce the memory pressure for kdump.
> >
> > Consider a practical example:
> >
> > To unpack a 48MB initramfs, the tmpfs needs 48MB of usable space. With
> > the default 50% limit, this requires a memory pool of 96MB to be
> > available for the tmpfs mount. The total memory requirement is therefore
> > approximately: 16MB (vmlinuz) + 48MB (loaded initramfs) + 48MB (unpacked
> > kernel) + 96MB (for tmpfs) + 12MB (runtime overhead) ≈ 220MB.
> >
> > By using rootfsflags=size=75%, the memory pool required for the 48MB
> > tmpfs is reduced to 48MB / 0.75 = 64MB. This reduces the total memory
> > requirement by 32MB (96MB - 64MB), allowing the kdump to succeed with a
> > smaller crashkernel size, such as 192MB.
> >
> > An alternative approach of reusing the existing rootflags parameter was
> > considered. However, a new, dedicated rootfsflags parameter was chosen
> > to avoid altering the current behavior of rootflags (which applies to
> > the final root filesystem) and to prevent any potential regressions.
> >
> > This approach is inspired by prior discussions and patches on the topic.
> > Ref: https://www.lightofdawn.org/blog/?viewDetailed=00128
> > Ref: https://landley.net/notes-2015.html#01-01-2015
> > Ref: https://lkml.org/lkml/2021/6/29/783
> > Ref: https://www.kernel.org/doc/html/latest/filesystems/ramfs-rootfs-initramfs.html#what-is-rootfs
> >
> > Signed-off-by: Lichen Liu <lichliu@...hat.com>
> > Tested-by: Rob Landley <rob@...dley.net>
> > ---
> > Hi VFS maintainers,
> >
> > Resending this patch as it did not get picked up.
> > This patch is intended for the VFS tree.
> >
> >  fs/namespace.c | 11 ++++++++++-
> >  1 file changed, 10 insertions(+), 1 deletion(-)
> >
> > diff --git a/fs/namespace.c b/fs/namespace.c
> > index 8f1000f9f3df..e484c26d5e3f 100644
> > --- a/fs/namespace.c
> > +++ b/fs/namespace.c
> > @@ -65,6 +65,15 @@ static int __init set_mphash_entries(char *str)
> >  }
> >  __setup("mphash_entries=", set_mphash_entries);
> >
> > +static char * __initdata rootfs_flags;
> > +static int __init rootfs_flags_setup(char *str)
> > +{
> > +     rootfs_flags = str;
> > +     return 1;
> > +}
> > +
> > +__setup("rootfsflags=", rootfs_flags_setup);
>
> Please document this option (alphabetically) in
> Documentation/admin-guide/kernel-parameters.txt.
>
> Thanks.
>
> > +
> >  static u64 event;
> >  static DEFINE_XARRAY_FLAGS(mnt_id_xa, XA_FLAGS_ALLOC);
> >  static DEFINE_IDA(mnt_group_ida);
> > @@ -5677,7 +5686,7 @@ static void __init init_mount_tree(void)
> >       struct mnt_namespace *ns;
> >       struct path root;
> >
> > -     mnt = vfs_kern_mount(&rootfs_fs_type, 0, "rootfs", NULL);
> > +     mnt = vfs_kern_mount(&rootfs_fs_type, 0, "rootfs", rootfs_flags);
> >       if (IS_ERR(mnt))
> >               panic("Can't create rootfs");
> >
>
> --
> ~Randy
>
>