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 <li...@rasmusvillemoes.dk>
---
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
