linux/init
David Disseldorp 1274aea127 initramfs: add INITRAMFS_PRESERVE_MTIME Kconfig option
initramfs cpio mtime preservation, as implemented in commit 889d51a107
("initramfs: add option to preserve mtime from initramfs cpio images"),
uses a linked list to defer directory mtime processing until after all
other items in the cpio archive have been processed.  This is done to
ensure that parent directory mtimes aren't overwritten via subsequent
child creation.

The lkml link below indicates that the mtime retention use case was for
embedded devices with applications running exclusively out of initramfs,
where the 32-bit mtime value provided a rough file version identifier. 
Linux distributions which discard an extracted initramfs immediately after
the root filesystem has been mounted may want to avoid the unnecessary
overhead.

This change adds a new INITRAMFS_PRESERVE_MTIME Kconfig option, which can
be used to disable on-by-default mtime retention and in turn speed up
initramfs extraction, particularly for cpio archives with large directory
counts.

Benchmarks with a one million directory cpio archive extracted 20 times
demonstrated:
				mean extraction time (s)	std dev
INITRAMFS_PRESERVE_MTIME=y		3.808			 0.006
INITRAMFS_PRESERVE_MTIME unset		3.056			 0.004

The above extraction times were measured using ftrace (initcall_finish -
initcall_start) values for populate_rootfs() with initramfs_async
disabled.

[ddiss@suse.de: rebase atop dir_entry.name flexible array member and drop separate initramfs_mtime.h header]
Link: https://lkml.org/lkml/2008/9/3/424
Link: https://lkml.kernel.org/r/20220404093429.27570-4-ddiss@suse.de
Signed-off-by: David Disseldorp <ddiss@suse.de>
Reviewed-by: Martin Wilck <mwilck@suse.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Christian Brauner <christian.brauner@ubuntu.com>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
2022-05-09 18:29:19 -07:00
..
calibrate.c License cleanup: add SPDX GPL-2.0 license identifier to files with no license 2017-11-02 11:10:55 +01:00
do_mounts_initrd.c Merge branch 'hch.init_path' of git://git.kernel.org/pub/scm/linux/kernel/git/viro/vfs 2020-08-07 09:40:34 -07:00
do_mounts_rd.c init: add an init_unlink helper 2020-07-31 08:17:52 +02:00
do_mounts.c block: remove genhd.h 2022-02-02 07:49:59 -07:00
do_mounts.h init: add an init_mknod helper 2020-07-31 08:17:54 +02:00
init_task.c kcsan: Remove redundant zero-initialization of globals 2021-12-09 16:42:26 -08:00
initramfs.c initramfs: add INITRAMFS_PRESERVE_MTIME Kconfig option 2022-05-09 18:29:19 -07:00
Kconfig initramfs: add INITRAMFS_PRESERVE_MTIME Kconfig option 2022-05-09 18:29:19 -07:00
main.c Merge branch 'akpm' (patches from Andrew) 2022-03-24 14:14:07 -07:00
Makefile sched/preempt: Tell about PREEMPT_DYNAMIC on kernel headers 2022-03-11 15:36:35 +01:00
noinitramfs.c init: move usermodehelper_enable() to populate_rootfs() 2021-09-08 11:50:27 -07:00
version.c kbuild: add an elfnote for whether vmlinux is built with lto 2021-04-25 05:25:42 +09:00