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| H A D | namei.c | 5fc475b7 Fri Jun 05 15:40:45 CDT 2020 Linus Torvalds <torvalds@linux-foundation.org> vfs: do not do group lookup when not necessary
Rasmus Villemoes points out that the 'in_group_p()' tests can be a
noticeable expense, and often completely unnecessary. A common
situation is that the 'group' bits are the same as the 'other' bits
wrt the permissions we want to test.
So rewrite 'acl_permission_check()' to not bother checking for group
ownership when the permission check doesn't care.
For example, if we're asking for read permissions, and both 'group' and
'other' allow reading, there's really no reason to check if we're part
of the group or not: either way, we'll allow it.
Rasmus says:
"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.
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"
Reported-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
5fc475b7 Fri Jun 05 15:40:45 CDT 2020 Linus Torvalds <torvalds@linux-foundation.org> vfs: do not do group lookup when not necessary
Rasmus Villemoes points out that the 'in_group_p()' tests can be a
noticeable expense, and often completely unnecessary. A common
situation is that the 'group' bits are the same as the 'other' bits
wrt the permissions we want to test.
So rewrite 'acl_permission_check()' to not bother checking for group
ownership when the permission check doesn't care.
For example, if we're asking for read permissions, and both 'group' and
'other' allow reading, there's really no reason to check if we're part
of the group or not: either way, we'll allow it.
Rasmus says:
"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.
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"
Reported-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
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