5.4-stable review patch. If anyone has any objections, please let me know.
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From: Lichen Liu lichliu@redhat.com
[ Upstream commit 278033a225e13ec21900f0a92b8351658f5377f2 ]
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.ht...
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: Sasha Levin sashal@kernel.org --- 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 9975dcab99c35..5c2594d7c9ac9 100644 --- a/Documentation/admin-guide/kernel-parameters.txt +++ b/Documentation/admin-guide/kernel-parameters.txt @@ -4409,6 +4409,9 @@
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. diff --git a/fs/namespace.c b/fs/namespace.c index c87f847c959d9..3c6f0586ae218 100644 --- a/fs/namespace.c +++ b/fs/namespace.c @@ -62,6 +62,15 @@ 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); @@ -3829,7 +3838,7 @@ 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");