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981ee95cc1
access(2) remains commonly used, for example on exec:
access("/etc/ld.so.preload", R_OK)
or when running gcc: strace -c gcc empty.c
% time seconds usecs/call calls errors syscall
------ ----------- ----------- --------- --------- ----------------
0.00 0.000000 0 42 26 access
It falls down to do_faccessat without the AT_EACCESS flag, which in turn
results in allocation of new creds in order to modify fsuid/fsgid and
caps. This is a very expensive process single-threaded and most notably
multi-threaded, with numerous structures getting refed and unrefed on
imminent new cred destruction.
Turns out for typical consumers the resulting creds would be identical
and this can be checked upfront, avoiding the hard work.
An access benchmark plugged into will-it-scale running on Cascade Lake
shows:
test proc before after
access1 1 1310582 2908735 (+121%) # distinct files
access1 24 4716491 63822173 (+1353%) # distinct files
access2 24 2378041 5370335 (+125%) # same file
The above benchmarks are not integrated into will-it-scale, but can be
found in a pull request:
https://github.com/antonblanchard/will-it-scale/pull/36/files
Signed-off-by: Mateusz Guzik <mjguzik@gmail.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
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 Restructured Text 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.