linux-kernel - [PATCH 1/1] mm/mmap: optimize a branch judgment in ksys_mmap

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Message-ID: <20200705080112.1405-1-thunder.leizhen@huawei.com>
Date:   Sun, 5 Jul 2020 16:01:12 +0800
From:   Zhen Lei <thunder.leizhen@...wei.com>
To:     Andrew Morton <akpm@...ux-foundation.org>,
        linux-mm <linux-mm@...ck.org>,
        linux-kernel <linux-kernel@...r.kernel.org>
CC:     Zhen Lei <thunder.leizhen@...wei.com>
Subject: [PATCH 1/1] 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;

Signed-off-by: Zhen Lei <thunder.leizhen@...wei.com>
---
 mm/mmap.c | 7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/mm/mmap.c b/mm/mmap.c
index 59a4682ebf3faed..a7bb2039738528f 100644
--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -1562,11 +1562,12 @@ unsigned long ksys_mmap_pgoff(unsigned long addr, unsigned long len,
 		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;
-- 
1.8.3