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

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Date:   Sat, 8 Jul 2023 16:10:07 -0700
From:   Suren Baghdasaryan <surenb@...gle.com>
To:     Hugh Dickins <hughd@...gle.com>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        akpm@...ux-foundation.org, regressions@...mhuis.info,
        bagasdotme@...il.com, jacobly.alt@...il.com, willy@...radead.org,
        liam.howlett@...cle.com, david@...hat.com, peterx@...hat.com,
        ldufour@...ux.ibm.com, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, linuxppc-dev@...ts.ozlabs.org,
        linux-arm-kernel@...ts.infradead.org, gregkh@...uxfoundation.org,
        regressions@...ts.linux.dev, stable@...r.kernel.org
Subject: Re: [PATCH] mm: lock newly mapped VMA with corrected ordering

On Sat, Jul 8, 2023 at 4:04 PM Hugh Dickins <hughd@...gle.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@...r.kernel.org
> Signed-off-by: Hugh Dickins <hughd@...gle.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