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

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Message-ID: <20250821-zirkel-leitkultur-2653cba2cd5b@brauner>
Date: Thu, 21 Aug 2025 10:24:11 +0200
From: Christian Brauner <brauner@...nel.org>
To: Lichen Liu <lichliu@...hat.com>
Cc: Christian Brauner <brauner@...nel.org>,
	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,
	viro@...iv.linux.org.uk,
	jack@...e.cz
Subject: Re: [PATCH v2] fs: Add 'rootfsflags' to set rootfs mount options

On Fri, 15 Aug 2025 20:14:59 +0800, 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 seems rather useful but I've renamed "rootfsflags" to
"initramfs_options" because "rootfsflags" is ambiguous and it's not
really just about flags.

Other than that I think it would make sense to just raise the limit to
90% for the root_fs_type mount. I'm not sure why this super privileged
code would only be allowed 50% by default.

---

Applied to the vfs-6.18.misc branch of the vfs/vfs.git tree.
Patches in the vfs-6.18.misc branch should appear in linux-next soon.

Please report any outstanding bugs that were missed during review in a
new review to the original patch series allowing us to drop it.

It's encouraged to provide Acked-bys and Reviewed-bys even though the
patch has now been applied. If possible patch trailers will be updated.

Note that commit hashes shown below are subject to change due to rebase,
trailer updates or similar. If in doubt, please check the listed branch.

tree:   https://git.kernel.org/pub/scm/linux/kernel/git/vfs/vfs.git
branch: vfs-6.18.misc

[1/1] fs: Add 'rootfsflags' to set rootfs mount options
      https://git.kernel.org/vfs/vfs/c/278033a225e1