Re: [PATCH v2 3/6] mm: Handle shared faults under the VMA lock

[Suren Baghdasaryan <surenb@google.com>]

On Fri, Oct 6, 2023 at 12:53 PM Matthew Wilcox (Oracle)
<willy@infradead.org> wrote:
>
> There are many implementations of ->fault and some of them depend on
> mmap_lock being held.  All vm_ops that implement ->map_pages() end up
> calling filemap_fault(), which I have audited to be sure it does not rely
> on mmap_lock.  So (for now) key off ->map_pages existing as a flag to
> indicate that it's safe to call ->fault while only holding the vma lock.
>
> Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>

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

> ---
>  mm/memory.c | 22 ++++++++++++++++++----
>  1 file changed, 18 insertions(+), 4 deletions(-)
>
> diff --git a/mm/memory.c b/mm/memory.c
> index cff78c496728..a9b0c135209a 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -3042,6 +3042,21 @@ static inline void wp_page_reuse(struct vm_fault *vmf)
>         count_vm_event(PGREUSE);
>  }
>
> +/*
> + * We could add a bitflag somewhere, but for now, we know that all
> + * vm_ops that have a ->map_pages have been audited and don't need
> + * the mmap_lock to be held.
> + */
> +static inline vm_fault_t vmf_can_call_fault(const struct vm_fault *vmf)
> +{
> +       struct vm_area_struct *vma = vmf->vma;
> +
> +       if (vma->vm_ops->map_pages || !(vmf->flags & FAULT_FLAG_VMA_LOCK))
> +               return 0;
> +       vma_end_read(vma);
> +       return VM_FAULT_RETRY;
> +}
> +
>  static vm_fault_t vmf_anon_prepare(struct vm_fault *vmf)
>  {
>         struct vm_area_struct *vma = vmf->vma;
> @@ -4669,10 +4684,9 @@ static vm_fault_t do_shared_fault(struct vm_fault *vmf)
>         vm_fault_t ret, tmp;
>         struct folio *folio;
>
> -       if (vmf->flags & FAULT_FLAG_VMA_LOCK) {
> -               vma_end_read(vma);
> -               return VM_FAULT_RETRY;
> -       }
> +       ret = vmf_can_call_fault(vmf);
> +       if (ret)
> +               return ret;
>
>         ret = __do_fault(vmf);
>         if (unlikely(ret & (VM_FAULT_ERROR | VM_FAULT_NOPAGE | VM_FAULT_RETRY)))
> --
> 2.40.1
>