[PATCH] fs: Add 'rootfsflags' to set rootfs mount options

[Dave Young]

On Fri, 8 Aug 2025 at 10:30, Dave Young <dyoung at redhat.com> wrote:
>
> Hi Lichen,
>
> On Fri, 8 Aug 2025 at 09:55, Lichen Liu <lichliu at redhat.com> 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 at redhat.com>
> > ---
> >  fs/namespace.c | 11 ++++++++++-
> >  1 file changed, 10 insertions(+), 1 deletion(-)
> >
> > diff --git a/fs/namespace.c b/fs/namespace.c
> > index ddfd4457d338..a450db31613e 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;
>
> I do see there are a few similar usages in init/do_mounts.c, probably
> it is old stuff and it just works.  But I think making rootfs_flags as
> an array and copying str into it is the right way.

Another question, may need fs people to clarify.  If the mount is
tmpfs and it is also rootfs,  could it use 100% of the memory by
default, and then no need for an extra param?    I feel that there is
no point to reserve memory if it is a fully memory based file system.

>
> > +       return 1;
> > +}
> > +
> > +__setup("rootfsflags=", rootfs_flags_setup);
> > +
> >  static u64 event;
> >  static DEFINE_XARRAY_FLAGS(mnt_id_xa, XA_FLAGS_ALLOC);
> >  static DEFINE_IDA(mnt_group_ida);
> > @@ -6086,7 +6095,7 @@ 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");
> >
> > --
> > 2.50.1
> >
> >
> Thanks
> Dave