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From:   Jann Horn <jannh@google.com>
Date:   Mon, 14 Aug 2023 16:54:01 +0200
Message-ID: <CAG48ez0XCXoXbSR6dyX7GUQYJKRFKQsHuye5q-PLU3-gR5of5A@mail.gmail.com>
Subject: Re: [PATCH v4 5/6] mm: always lock new vma before inserting into vma tree
To:     akpm@linux-foundation.org, Suren Baghdasaryan <surenb@google.com>
Cc:     torvalds@linux-foundation.org, willy@infradead.org,
        liam.howlett@oracle.com, david@redhat.com, peterx@redhat.com,
        ldufour@linux.ibm.com, vbabka@suse.cz, michel@lespinasse.org,
        jglisse@google.com, mhocko@suse.com, hannes@cmpxchg.org,
        dave@stgolabs.net, hughd@google.com, linux-kernel@vger.kernel.org,
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@akpm can you fix this up?

On Fri, Aug 4, 2023 at 5:27=E2=80=AFPM Suren Baghdasaryan <surenb@google.co=
m> wrote:
> While it's not strictly necessary to lock a newly created vma before
> adding it into the vma tree (as long as no further changes are performed
> to it), it seems like a good policy to lock it and prevent accidental
> changes after it becomes visible to the page faults. Lock the vma before
> adding it into the vma tree.
>
> Suggested-by: Jann Horn <jannh@google.com>
> Signed-off-by: Suren Baghdasaryan <surenb@google.com>
> Reviewed-by: Liam R. Howlett <Liam.Howlett@oracle.com>
> ---
>  mm/mmap.c | 7 +++++--
>  1 file changed, 5 insertions(+), 2 deletions(-)
>
> diff --git a/mm/mmap.c b/mm/mmap.c
> index 3937479d0e07..850a39dee075 100644
> --- a/mm/mmap.c
> +++ b/mm/mmap.c
> @@ -412,6 +412,8 @@ static int vma_link(struct mm_struct *mm, struct vm_a=
rea_struct *vma)
>         if (vma_iter_prealloc(&vmi))
>                 return -ENOMEM;
>
> +       vma_start_write(vma);
> +
>         if (vma->vm_file) {
>                 mapping =3D vma->vm_file->f_mapping;
>                 i_mmap_lock_write(mapping);

Something went wrong when this part of the patch was applied, because
of a conflict with "mm/mmap: move vma operations to mm_struct out of
the critical section of file mapping lock"; see how this patch ended
up in the mm tree:
https://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm.git/commit/?id=3D26=
cb4dafc13871ab68a4fb480ca1e19381cff392

> @@ -403,6 +403,8 @@ static int vma_link(struct mm_struct *mm, struct vm_a=
rea_struct *vma)
>
>   vma_iter_store(&vmi, vma);
>
> + vma_start_write(vma);
> +
>   if (vma->vm_file) {
>  mapping =3D vma->vm_file->f_mapping;
>  i_mmap_lock_write(mapping);

The "vma_start_write()" has to be ordered before the
"vma_iter_store(&vmi, vma)".