[patch 104/163] mm/mmap: optimize a branch judgment in ksys_mmap_pgoff()

Andrew Morton
2020-08-07T06:23:03Z

From: Zhen Lei <thunder.leizhen@huawei.com>
Subject: mm/mmap: optimize a branch judgment in ksys_mmap_pgoff()

Look at the pseudo code below. It's very clear that, the judgement
"!is_file_hugepages(file)" at 3) is duplicated to the one at 1), we can
use "else if" to avoid it. And the assignment "retval = -EINVAL" at 2) is
only needed by the branch 3), because "retval" will be overwritten at 4).

No functional change, but it can reduce the code size. Maybe more clearer?
Before:
text data bss dec hex filename
28733 1590 1 30324 7674 mm/mmap.o

After:
text data bss dec hex filename
28701 1590 1 30292 7654 mm/mmap.o

====pseudo code====:
if (!(flags & MAP_ANONYMOUS)) {
...
1) if (is_file_hugepages(file))
len = ALIGN(len, huge_page_size(hstate_file(file)));
2) retval = -EINVAL;
3) if (unlikely(flags & MAP_HUGETLB && !is_file_hugepages(file)))
goto out_fput;
} else if (flags & MAP_HUGETLB) {
...
}
...

4) retval = vm_mmap_pgoff(file, addr, len, prot, flags, pgoff);
out_fput:
...
return retval;

Link: http://lkml.kernel.org/r/20200705080112.1405-1-thunder.leizhen@huawei.com
Signed-off-by: Zhen Lei <thunder.leizhen@huawei.com>
Reviewed-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

mm/mmap.c | 7 ++++---
1 file changed, 4 insertions(+), 3 deletions(-)

--- a/mm/mmap.c~mm-mmap-optimize-a-branch-judgment-in-ksys_mmap_pgoff
+++ a/mm/mmap.c
@@ -1562,11 +1562,12 @@ unsigned long ksys_mmap_pgoff(unsigned l
file = fget(fd);
if (!file)
return -EBADF;
- if (is_file_hugepages(file))
+ if (is_file_hugepages(file)) {
len = ALIGN(len, huge_page_size(hstate_file(file)));
- retval = -EINVAL;
- if (unlikely(flags & MAP_HUGETLB && !is_file_hugepages(file)))
+ } else if (unlikely(flags & MAP_HUGETLB)) {
+ retval = -EINVAL;
goto out_fput;
+ }
} else if (flags & MAP_HUGETLB) {
struct user_struct *user = NULL;
struct hstate *hs;
_