fs: Add 'initramfs_options' to set initramfs mount options

JIRA: https://issues.redhat.com/browse/RHEL-121113
Upstream status: Linus

Conflicts: There's a fuzz 2 in fs/namespace.c with hunk #1 due to
   upstream commit 7f9bfaf ("fs: use xarray for old mount id") not
   being present in RHEL-9 and looks out of scope for this change.

commit 278033a
Author: Lichen Liu <lichliu@redhat.com>
Date:   Fri Aug 15 20:14:59 2025 +0800

    fs: Add 'initramfs_options' to set initramfs mount options

    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,
    initramfs_options, 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 initramfs_options=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 initramfs_options=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 initramfs_options 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 "initramfs_options"

    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>
    Link: https://lore.kernel.org/20250815121459.3391223-1-lichliu@redhat.com
    Tested-by: Rob Landley <rob@landley.net>
    Signed-off-by: Christian Brauner <brauner@kernel.org>

Signed-off-by: Ian Kent <ikent@redhat.com>

Author: Ian Kent

Documentation/admin-guide/kernel-parameters.txt

@@ -5765,6 +5765,9 @@
 
 	rootflags=	[KNL] Set root filesystem mount option string
 
+	initramfs_options= [KNL]
+                        Specify mount options for for the initramfs mount.
+
 	rootfstype=	[KNL] Set root filesystem type
 
 	rootwait	[KNL] Wait (indefinitely) for root device to show up.

fs/namespace.c

@@ -65,6 +65,15 @@ static int __init set_mphash_entries(char *str)
 }
 __setup("mphash_entries=", set_mphash_entries);
 
+static char * __initdata initramfs_options;
+static int __init initramfs_options_setup(char *str)
+{
+	initramfs_options = str;
+	return 1;
+}
+
+__setup("initramfs_options=", initramfs_options_setup);
+
 static u64 event;
 static DEFINE_IDA(mnt_id_ida);
 static DEFINE_IDA(mnt_group_ida);
@@ -4302,7 +4311,7 @@ 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", initramfs_options);
 	if (IS_ERR(mnt))
 		panic("Can't create rootfs");