Hi Lichen,
On Fri, 8 Aug 2025 at 09:55, Lichen Liu <lich...@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 <lich...@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.
> + 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
