linux-kernel - [RFC] mm/mremap: Remove redundant checks inside vma

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Date:   Mon, 10 Jul 2017 16:40:59 +0530
From:   Anshuman Khandual <khandual@...ux.vnet.ibm.com>
To:     linux-mm@...ck.org, linux-kernel@...r.kernel.org
Cc:     akpm@...ux-foundation.org, mike.kravetz@...cle.com
Subject: [RFC] mm/mremap: Remove redundant checks inside vma_expandable()

As 'delta' is an unsigned long, 'end' (vma->vm_end + delta) cannot
be less than 'vma->vm_end'. Checking for availability of virtual
address range at the end of the VMA for the incremental size is
also reduntant at this point. Hence drop them both.

Signed-off-by: Anshuman Khandual <khandual@...ux.vnet.ibm.com>
---

The following test program achieves fatser execution time with
this change.

#include <stdio.h>
#include <string.h>
#include <unistd.h>
#include <errno.h>
#include <sys/mman.h>
#include <sys/time.h>

#define ALLOC_SIZE 0x10000UL
#define MAX_COUNT 1024 * 1024

int main(int argc, char *argv[])
{
        unsigned long count;
        char *ptr;

        ptr = mmap(NULL, ALLOC_SIZE, PROT_READ | PROT_WRITE, MAP_PRIVATE| MAP_ANONYMOUS, -1, 0);
        if (ptr == MAP_FAILED) {
                perror("map() failed");
                return -1;
        }
        memset(ptr, 0, ALLOC_SIZE);

        for (count = 1; count <= MAX_COUNT; count++) {
                ptr =  (char *) mremap(ptr, ALLOC_SIZE * count, ALLOC_SIZE * (count + 1), 1);
                if (ptr == MAP_FAILED) {
                        perror("mremap() failed");
                        printf("At %lu size", ALLOC_SIZE * (count + 1));
                        return -1;
                }
                /*
                memset(ptr, 0, ALLOC_SIZE * (count + 1));
                */
        }


        for (count = MAX_COUNT; count > 1; count--) {
                ptr =  (char *) mremap(ptr, ALLOC_SIZE * count, ALLOC_SIZE * (count - 1), 1);
                if (ptr == MAP_FAILED) {
                        perror("mremap() failed");
                        printf("At %lu size", ALLOC_SIZE * (count - 1));
                        return -1;
                }
                /*
                memset(ptr, 0, ALLOC_SIZE * (count - 1));
                */
        }
        return 0;
}


 mm/mremap.c | 9 +++------
 1 file changed, 3 insertions(+), 6 deletions(-)

diff --git a/mm/mremap.c b/mm/mremap.c
index cd8a1b1..b937c28 100644
--- a/mm/mremap.c
+++ b/mm/mremap.c
@@ -487,12 +487,9 @@ static unsigned long mremap_to(unsigned long addr, unsigned long old_len,
 static int vma_expandable(struct vm_area_struct *vma, unsigned long delta)
 {
 	unsigned long end = vma->vm_end + delta;
-	if (end < vma->vm_end) /* overflow */
-		return 0;
-	if (vma->vm_next && vma->vm_next->vm_start < end) /* intersection */
-		return 0;
-	if (get_unmapped_area(NULL, vma->vm_start, end - vma->vm_start,
-			      0, MAP_FIXED) & ~PAGE_MASK)
+
+	/* Intersection with next VMA */
+	if (vma->vm_next && vma->vm_next->vm_start < end)
 		return 0;
 	return 1;
 }
-- 
1.8.5.2