[patch 112/163] mm: mmap: merge vma after call_mmap() if possible

Andrew Morton
2020-08-07T06:23:37Z

From: Miaohe Lin <linmiaohe@huawei.com>
Subject: mm: mmap: merge vma after call_mmap() if possible

The vm_flags may be changed after call_mmap() because drivers may set some
flags for their own purpose. As a result, we failed to merge the adjacent
vma due to the different vm_flags as userspace can't pass in the same one.
Try to merge vma after call_mmap() to fix this issue.

Link: http://lkml.kernel.org/r/1594954065-23733-1-git-send-email-linmiaohe@huawei.com
Signed-off-by: Hongxiang Lou <louhongxiang@huawei.com>
Signed-off-by: Miaohe Lin <linmiaohe@huawei.com>
Reviewed-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

mm/mmap.c | 22 +++++++++++++++++++++-
1 file changed, 21 insertions(+), 1 deletion(-)

--- a/mm/mmap.c~mm-mmap-merge-vma-after-call_mmap-if-possible
+++ a/mm/mmap.c
@@ -1690,7 +1690,7 @@ unsigned long mmap_region(struct file *f
struct list_head *uf)
{
struct mm_struct *mm = current->mm;
- struct vm_area_struct *vma, *prev;
+ struct vm_area_struct *vma, *prev, *merge;
int error;
struct rb_node **rb_link, *rb_parent;
unsigned long charged = 0;
@@ -1774,6 +1774,25 @@ unsigned long mmap_region(struct file *f
if (error)
goto unmap_and_free_vma;

+ /* If vm_flags changed after call_mmap(), we should try merge vma again
+ * as we may succeed this time.
+ */
+ if (unlikely(vm_flags != vma->vm_flags && prev)) {
+ merge = vma_merge(mm, prev, vma->vm_start, vma->vm_end, vma->vm_flags,
+ NULL, vma->vm_file, vma->vm_pgoff, NULL, NULL_VM_UFFD_CTX);
+ if (merge) {
+ fput(file);
+ vm_area_free(vma);
+ vma = merge;
+ /* Update vm_flags and possible addr to pick up the change. We don't
+ * warn here if addr changed as the vma is not linked by vma_link().
+ */
+ addr = vma->vm_start;
+ vm_flags = vma->vm_flags;
+ goto unmap_writable;
+ }
+ }
+
/* Can addr have changed??
*
* Answer: Yes, several device drivers can do it in their
@@ -1796,6 +1815,7 @@ unsigned long mmap_region(struct file *f
vma_link(mm, vma, prev, rb_link, rb_parent);
/* Once vma denies write, undo our temporary denial count */
if (file) {
+unmap_writable:
if (vm_flags & VM_SHARED)
mapping_unmap_writable(file->f_mapping);
if (vm_flags & VM_DENYWRITE)
_