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7c179096e7Stats from nd->depth usage during the venerable kernel build collected like so:
bpftrace -e 'kprobe:terminate_walk,kprobe:walk_component,kprobe:legitimize_links
{ @[probe] = lhist(((struct nameidata *)arg0)->depth, 0, 8, 1); }'
@[kprobe:legitimize_links]:
[0, 1) 6554906 |@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@|
[1, 2) 3534 | |
@[kprobe:terminate_walk]:
[0, 1) 12153664 |@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@|
@[kprobe:walk_component]:
[0, 1) 53075749 |@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@|
[1, 2) 971421 | |
[2, 3) 84946 | |
Additionally a custom probe was added for depth within link_path_walk():
bpftrace -e 'kprobe:link_path_walk_probe { @[probe] = lhist(arg0, 0, 8, 1); }'
@[kprobe:link_path_walk_probe]:
[0, 1) 7528231 |@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@|
[1, 2) 407905 |@@ |
Given these results:
1. terminate_walk() is called towards the end of the lookup and in this
test it never had any links to clean up.
2. legitimize_links() is also called towards the end of lookup and most
of the time there s 0 depth. Patch consumers to avoid calling into it
in that case.
3. walk_component() is typically called with WALK_MORE and zero depth,
checked in that order. Check depth first and predict it is 0.
4. link_path_walk() also does not deal with a symlink most of the time
when !*name
Signed-off-by: Mateusz Guzik <mjguzik@gmail.com>
Link: https://patch.msgid.link/20251119142954.2909394-1-mjguzik@gmail.com
Signed-off-by: Christian Brauner <brauner@kernel.org>
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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.