Just something like open(/usr/include/sys/stat.h) causes five calls of generic_permission -> acl_permission_check -> in_group_p; if the compiler first tried /usr/local/include/..., that would be a few more. Altogether, on a bog-standard Ubuntu 20.04 install, a workload consisting of compiling lots of userspace programs (i.e., calling lots of short-lived programs that all need to get their shared libs mapped in, and the compilers poking around looking for system headers - lots of /usr/lib, /usr/bin, /usr/include/ accesses) puts in_group_p around 0.1% according to perf top. With an artificial load such as while true ; do find /usr/ -print0 | xargs -0 stat > /dev/null ; done that jumps to over 0.4%. System-installed files are almost always 0755 (directories and binaries) or 0644, so in most cases, we can avoid the binary search and the cost of pulling the cred->groups array and in_group_p() .text into the cpu cache. Signed-off-by: Rasmus Villemoes <linux@xxxxxxxxxxxxxxxxxx> --- fs/namei.c | 7 ++++++- 1 file changed, 6 insertions(+), 1 deletion(-) diff --git a/fs/namei.c b/fs/namei.c index d81f73ff1a8b..c6f0c6643db5 100644 --- a/fs/namei.c +++ b/fs/namei.c @@ -303,7 +303,12 @@ static int acl_permission_check(struct inode *inode, int mask) return error; } - if (in_group_p(inode->i_gid)) + /* + * If the "group" and "other" permissions are the same, + * there's no point calling in_group_p() to decide which + * set to use. + */ + if ((((mode >> 3) ^ mode) & 7) && in_group_p(inode->i_gid)) mode >>= 3; } -- 2.23.0
