Re: [PATCH v6 6/7] mm/madvise: employ mmget_still_valid for write lock

[Suren Baghdasaryan <surenb@google.com>]

On Tue, Feb 18, 2020 at 5:44 PM Minchan Kim <minchan@kernel.org> wrote:
>
> From: Oleksandr Natalenko <oleksandr@redhat.com>
>
> Do the very same trick as we already do since 04f5866e41fb. KSM hints
> will require locking mmap_sem for write since they modify vm_flags, so
> for remote KSM hinting this additional check is needed.
>
> Signed-off-by: Oleksandr Natalenko <oleksandr@redhat.com>
> Signed-off-by: Minchan Kim <minchan@kernel.org>
> ---
>  mm/madvise.c | 3 +++
>  1 file changed, 3 insertions(+)
>
> diff --git a/mm/madvise.c b/mm/madvise.c
> index f6d9b9e66243..c55a18fe71f9 100644
> --- a/mm/madvise.c
> +++ b/mm/madvise.c
> @@ -1118,6 +1118,8 @@ int do_madvise(struct task_struct *target_task, struct mm_struct *mm,
>         if (write) {
>                 if (down_write_killable(&mm->mmap_sem))
>                         return -EINTR;
> +               if (current->mm != mm && !mmget_still_valid(mm))

mmget_still_valid() seems pretty light-weight, so why not just use
that without checking that the mm belongs to the current process
first?

> +                       goto skip_mm;
>         } else {
>                 down_read(&mm->mmap_sem);
>         }
> @@ -1169,6 +1171,7 @@ int do_madvise(struct task_struct *target_task, struct mm_struct *mm,
>         }
>  out:
>         blk_finish_plug(&plug);
> +skip_mm:
>         if (write)
>                 up_write(&mm->mmap_sem);
>         else
> --
> 2.25.0.265.gbab2e86ba0-goog
>