[PATCH 3/4] mremap: don't do uneccesary checks if new_len == old_len

From: Oleg Nesterov
Date: Fri Jun 19 2015 - 19:21:30 EST


The "new_len > old_len" branch in vma_to_resize() looks very confusing.
It only covers the VM_DONTEXPAND/pgoff checks but everything below is
equally unneeded if new_len == old_len.

Change this code to return if "new_len == old_len", new_len < old_len
is not possible, otherwise the code below is wrong anyway.

Signed-off-by: Oleg Nesterov <oleg@xxxxxxxxxx>
---
mm/mremap.c | 21 +++++++++++----------
1 files changed, 11 insertions(+), 10 deletions(-)

diff --git a/mm/mremap.c b/mm/mremap.c
index 492721c..2416721 100644
--- a/mm/mremap.c
+++ b/mm/mremap.c
@@ -342,6 +342,7 @@ static struct vm_area_struct *vma_to_resize(unsigned long addr,
{
struct mm_struct *mm = current->mm;
struct vm_area_struct *vma = find_vma(mm, addr);
+ unsigned long pgoff;

if (!vma || vma->vm_start > addr)
return ERR_PTR(-EFAULT);
@@ -353,17 +354,17 @@ static struct vm_area_struct *vma_to_resize(unsigned long addr,
if (old_len > vma->vm_end - addr)
return ERR_PTR(-EFAULT);

+ if (new_len == old_len)
+ return vma;
+
/* Need to be careful about a growing mapping */
- if (new_len > old_len) {
- unsigned long pgoff;
-
- if (vma->vm_flags & (VM_DONTEXPAND | VM_PFNMAP))
- return ERR_PTR(-EFAULT);
- pgoff = (addr - vma->vm_start) >> PAGE_SHIFT;
- pgoff += vma->vm_pgoff;
- if (pgoff + (new_len >> PAGE_SHIFT) < pgoff)
- return ERR_PTR(-EINVAL);
- }
+ pgoff = (addr - vma->vm_start) >> PAGE_SHIFT;
+ pgoff += vma->vm_pgoff;
+ if (pgoff + (new_len >> PAGE_SHIFT) < pgoff)
+ return ERR_PTR(-EINVAL);
+
+ if (vma->vm_flags & (VM_DONTEXPAND | VM_PFNMAP))
+ return ERR_PTR(-EFAULT);

if (vma->vm_flags & VM_LOCKED) {
unsigned long locked, lock_limit;
--
1.5.5.1

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