Re: [PATCH] mm: lock newly mapped VMA with corrected ordering

[Suren Baghdasaryan <surenb@google.com>]

On Sat, Jul 8, 2023 at 4:04 PM Hugh Dickins <hughd@google.com> wrote:
>
> Lockdep is certainly right to complain about
> (&vma->vm_lock->lock){++++}-{3:3}, at: vma_start_write+0x2d/0x3f
>                but task is already holding lock:
> (&mapping->i_mmap_rwsem){+.+.}-{3:3}, at: mmap_region+0x4dc/0x6db
> Invert those to the usual ordering.

Doh! Thanks Hugh!
I totally forgot to run this with lockdep enabled :(

>
> Fixes: 33313a747e81 ("mm: lock newly mapped VMA which can be modified after it becomes visible")
> Cc: stable@vger.kernel.org
> Signed-off-by: Hugh Dickins <hughd@google.com>
> ---
>  mm/mmap.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/mm/mmap.c b/mm/mmap.c
> index 84c71431a527..3eda23c9ebe7 100644
> --- a/mm/mmap.c
> +++ b/mm/mmap.c
> @@ -2809,11 +2809,11 @@ unsigned long mmap_region(struct file *file, unsigned long addr,
>         if (vma_iter_prealloc(&vmi))
>                 goto close_and_free_vma;
>
> +       /* Lock the VMA since it is modified after insertion into VMA tree */
> +       vma_start_write(vma);
>         if (vma->vm_file)
>                 i_mmap_lock_write(vma->vm_file->f_mapping);
>
> -       /* Lock the VMA since it is modified after insertion into VMA tree */
> -       vma_start_write(vma);
>         vma_iter_store(&vmi, vma);
>         mm->map_count++;
>         if (vma->vm_file) {
> --
> 2.35.3