Re: [PATCH 1/6] mm: Allow per-VMA locks on file-backed VMAs

[Suren Baghdasaryan <surenb@google.com>]

On Tue, Apr 4, 2023 at 6:59 AM 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>
> ---
>  mm/memory.c | 9 ++++-----
>  1 file changed, 4 insertions(+), 5 deletions(-)
>
> diff --git a/mm/memory.c b/mm/memory.c
> index fdaec7772fff..f726f85f0081 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -5223,6 +5223,9 @@ vm_fault_t handle_mm_fault(struct vm_area_struct *vma, unsigned long address,
>                                             flags & FAULT_FLAG_REMOTE))
>                 return VM_FAULT_SIGSEGV;
>
> +       if ((flags & FAULT_FLAG_VMA_LOCK) && !vma_is_anonymous(vma))
> +               return VM_FAULT_RETRY;
> +

There are count_vm_event(PGFAULT) and count_memcg_event_mm(vma->vm_mm,
PGFAULT) earlier in this function. Returning here and retrying I think
will double-count this page fault. Returning before this accounting
should fix this issue.

>         /*
>          * Enable the memcg OOM handling for faults triggered in user
>          * space.  Kernel faults are handled more gracefully.
> @@ -5275,12 +5278,8 @@ 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
>