Re: [PATCH v2 2/9] mm: Allow per-VMA locks on file-backed VMAs

[Suren Baghdasaryan <surenb@google.com>]

On Tue, Jul 11, 2023 at 1:21 PM Matthew Wilcox (Oracle)
<willy@infradead.org> wrote:
>
> The fault path will immediately fail in handle_mm_fault(), so this
> is the minimal step which allows the per-VMA lock to be taken on
> file-backed VMAs.  There may be a small performance reduction as a
> little unnecessary work will be done on each page fault.  See later
> patches for the improvement.
>
> Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>

Reviewed-by: Suren Baghdasaryan <surenb@google.com>

> ---
>  mm/memory.c | 11 ++++++-----
>  1 file changed, 6 insertions(+), 5 deletions(-)
>
> diff --git a/mm/memory.c b/mm/memory.c
> index 2c7967632866..f2dcc695f54e 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -5247,6 +5247,11 @@ vm_fault_t handle_mm_fault(struct vm_area_struct *vma, unsigned long address,
>                 goto out;
>         }
>
> +       if ((flags & FAULT_FLAG_VMA_LOCK) && !vma_is_anonymous(vma)) {
> +               vma_end_read(vma);
> +               return VM_FAULT_RETRY;
> +       }
> +
>         /*
>          * Enable the memcg OOM handling for faults triggered in user
>          * space.  Kernel faults are handled more gracefully.
> @@ -5418,12 +5423,8 @@ struct vm_area_struct *lock_vma_under_rcu(struct mm_struct *mm,
>         if (!vma)
>                 goto inval;
>
> -       /* Only anonymous vmas are supported for now */
> -       if (!vma_is_anonymous(vma))
> -               goto inval;
> -
>         /* find_mergeable_anon_vma uses adjacent vmas which are not locked */
> -       if (!vma->anon_vma)
> +       if (vma_is_anonymous(vma) && !vma->anon_vma)
>                 goto inval;
>
>         if (!vma_start_read(vma))
> --
> 2.39.2
>