Re: [PATCH] mm: mmap: Merge vma after call_mmap() if possible

[Andrew Morton <akpm@linux-foundation.org>]

On Tue, 14 Jul 2020 11:07:44 +0800 linmiaohe <linmiaohe@huawei.com> wrote:

> From: Miaohe Lin <linmiaohe@huawei.com>
> 
> 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.
> 
> Signed-off-by: Hongxiang Lou <louhongxiang@huawei.com>
> Signed-off-by: Miaohe Lin <linmiaohe@huawei.com>
> ---
>  mm/mmap.c | 17 ++++++++++++++++-
>  1 file changed, 16 insertions(+), 1 deletion(-)
> 
> diff --git a/mm/mmap.c b/mm/mmap.c
> index 59a4682ebf3f..9568117471f8 100644
> --- a/mm/mmap.c
> +++ b/mm/mmap.c
> @@ -1689,7 +1689,7 @@ unsigned long mmap_region(struct file *file, unsigned long addr,
>  		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;
> @@ -1773,6 +1773,20 @@ unsigned long mmap_region(struct file *file, unsigned long addr,
>  		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;
> +				goto unmap_writable;

Shouldn't we update local variable `vm_flags' here, to pick up the
change?  And possibly `addr'?

> +			}
> +		}
> +
>  		/* Can addr have changed??
>  		 *
>  		 * Answer: Yes, several device drivers can do it in their
> @@ -1795,6 +1809,7 @@ unsigned long mmap_region(struct file *file, unsigned long addr,
>  	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)