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

[Lichen Liu <lichliu@redhat.com>]

On Thu, Aug 14, 2025 at 4:15 PM Askar Safin <safinaskar@zohomail.com> wrote:
>
> Lichen Liu <lichliu@redhat.com>:
> > 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@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;
> > +     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
>
> Thank you for this patch!
>
> I suggest periodically check linux-next to see whether the patch got there.
>
> If it was not applied in resonable time, then resend it.
> But this time, please, clearly specify tree, which should accept it.
> I think the most apropriate tree is VFS tree here.
> So, when resending please add linux-fsdevel@vger.kernel.org to CC and say in first paragraph
> in your mail that the patch is for VFS tree.
Thank You!

I checked the linux-next and it was not applied now. I will resend
this patch and CC linux-fsdevel@vger.kernel.org.

>
> --
> Askar Safin
>