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Lichen Liu 3b72a03bf5 fs: Add 'initramfs_options' to set initramfs mount options 
[ Upstream commit 278033a225 ]

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: Sasha Levin <sashal@kernel.org>
History 2025-10-19 16:37:43 +02:00
arch arm64: dts: qcom: qcs615: add missing dt property in QUP SEs 2025-10-19 16:37:42 +02:00
block blk-crypto: fix missing blktrace bio split events 2025-10-19 16:37:23 +02:00
certs sign-file,extract-cert: use pkcs11 provider for OPENSSL MAJOR >= 3 2024-09-20 19:52:48 +03:00
crypto crypto: essiv - Check ssize for decryption and in-place encryption 2025-10-19 16:37:14 +02:00
Documentation fs: Add 'initramfs_options' to set initramfs mount options 2025-10-19 16:37:43 +02:00
drivers irqchip/sifive-plic: Avoid interrupt ID 0 handling during suspend/resume 2025-10-19 16:37:43 +02:00
fs fs: Add 'initramfs_options' to set initramfs mount options 2025-10-19 16:37:43 +02:00
include mm/ksm: fix incorrect KSM counter handling in mm_struct during fork 2025-10-19 16:37:42 +02:00
init init: handle bootloader identifier in kernel parameters 2025-10-19 16:37:27 +02:00
io_uring io_uring/zcrx: increment fallback loop src offset 2025-10-19 16:37:14 +02:00
ipc vfs-6.17-rc1.mmap_prepare 2025-07-28 13:43:25 -07:00
kernel pid: Add a judgment for ns null in pid_nr_ns 2025-10-19 16:37:43 +02:00
lib lib/genalloc: fix device leak in of_gen_pool_get() 2025-10-19 16:37:28 +02:00
LICENSES LICENSES: Replace the obsolete address of the FSF in the GFDL-1.2 2025-07-24 11:15:39 +02:00
mm mm/damon/lru_sort: use param_ctx for damon_attrs staging 2025-10-19 16:37:38 +02:00
net mptcp: reset blackhole on success with non-loopback ifaces 2025-10-19 16:37:36 +02:00
rust cpufreq: Make drivers using CPUFREQ_ETERNAL specify transition latency 2025-10-19 16:37:00 +02:00
samples 15 hotfixes. 11 are cc:stable and the remainder address post-6.16 issues 2025-09-17 21:34:26 -07:00
scripts kbuild: Add '.rel.*' strip pattern for vmlinux 2025-10-19 16:37:15 +02:00
security KEYS: trusted_tpm1: Compare HMAC values in constant time 2025-10-19 16:37:28 +02:00
sound ASoC: SOF: Intel: Read the LLP via the associated Link DMA channel 2025-10-19 16:37:09 +02:00
tools selftests: mptcp: join: validate C-flag + def limit 2025-10-19 16:37:36 +02:00
usr usr/include: openrisc: don't HDRTEST bpf_perf_event.h 2025-05-12 15:03:17 +09:00
virt Merge tag 'kvm-x86-no_assignment-6.17' of https://github.com/kvm-x86/linux into HEAD 2025-07-29 08:36:42 -04:00
.clang-format Linux 6.15-rc5 2025-05-06 16:39:25 +10:00
.clippy.toml rust: clean Rust 1.88.0's warning about clippy::disallowed_macros configuration 2025-05-07 00:11:47 +02:00
.cocciconfig scripts: add Linux .cocciconfig for coccinelle 2016-07-22 12:13:39 +02:00
.editorconfig .editorconfig: remove trim_trailing_whitespace option 2024-06-13 16:47:52 +02:00
.get_maintainer.ignore MAINTAINERS: remove Alyssa Rosenzweig 2025-09-18 21:17:31 +02:00
.gitattributes .gitattributes: set diff driver for Rust source code files 2023-05-31 17:48:25 +02:00
.gitignore gitignore: allow .pylintrc to be tracked 2025-07-02 17:10:04 -06:00
.mailmap 7 hotfixes. 4 are cc:stable and the remainder address post-6.16 issues or 2025-09-28 09:32:00 -07:00
.pylintrc docs: add a .pylintrc file with sys path for docs scripts 2025-04-09 12:10:33 -06:00
.rustfmt.toml rust: add .rustfmt.toml 2022-09-28 09:02:20 +02:00
COPYING COPYING: state that all contributions really are covered by this file 2020-02-10 13:32:20 -08:00
CREDITS MAINTAINERS: retire Boris from TLS maintainers 2025-08-26 17:36:01 -07:00
Kbuild drm: ensure drm headers are self-contained and pass kernel-doc 2025-02-12 10:44:43 +02:00
Kconfig io_uring: Rename KConfig to Kconfig 2025-02-19 14:53:27 -07:00
MAINTAINERS i2c-for-6.17-rc8 2025-09-28 13:19:23 -07:00
Makefile Linux 6.17.3 2025-10-15 12:04:23 +02:00
README README: Fix spelling 2024-03-18 03:36:32 -06:00

README 

Linux kernel
============

There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.

In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``.  The formatted documentation can also be read online at:

    https://www.kernel.org/doc/html/latest/

There are various text files in the Documentation/ subdirectory,
several of them using the reStructuredText markup notation.

Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.