linux-kernel - Re: [PATCH v2] fs: Add 'rootfsflags' to set rootfs mount options

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <CAPmSd0OHB3orya=3V1icz4ZzOoAwHuWTqON_Dk+EaEcACtVtbg@mail.gmail.com>
Date: Wed, 20 Aug 2025 23:17:20 +0800
From: Lichen Liu <lichliu@...hat.com>
To: viro@...iv.linux.org.uk, brauner@...nel.org, jack@...e.cz
Cc: 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, 
	corbet@....net, linux-doc@...r.kernel.org
Subject: Re: [PATCH v2] fs: Add 'rootfsflags' to set rootfs mount options

Hi all, do you have any comments for this v2 patch?

Thanks,
Lichen

On Fri, Aug 15, 2025 at 8:15 PM Lichen Liu <lichliu@...hat.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.
>
> Also add documentation for the new kernel parameter "rootfsflags"
>
> 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>
> ---
> Changes in v2:
>   - Add documentation for the new kernel parameter.
>
>  Documentation/admin-guide/kernel-parameters.txt |  3 +++
>  fs/namespace.c                                  | 11 ++++++++++-
>  2 files changed, 13 insertions(+), 1 deletion(-)
>
> diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
> index fb8752b42ec8..0c00f651d431 100644
> --- a/Documentation/admin-guide/kernel-parameters.txt
> +++ b/Documentation/admin-guide/kernel-parameters.txt
> @@ -6220,6 +6220,9 @@
>
>         rootflags=      [KNL] Set root filesystem mount option string
>
> +       rootfsflags=    [KNL] Set initial root filesystem mount option string
> +                       (e.g. tmpfs for initramfs)
> +
>         rootfstype=     [KNL] Set root filesystem type
>
>         rootwait        [KNL] Wait (indefinitely) for root device to show up.
> 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);
> +
>  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");
>
> --
> 2.47.0
>