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

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Message-ID: <dd25041f-98e0-4bb5-bcd5-ba3507262c76@infradead.org>
Date: Thu, 14 Aug 2025 09:23:52 -0700
From: Randy Dunlap <rdunlap@...radead.org>
To: Lichen Liu <lichliu@...hat.com>, 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
Subject: Re: [PATCH RESEND] fs: Add 'rootfsflags' to set rootfs mount options

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