Re: [RFC] Trylock the mmap_lock in vmf_anon_prepare()

[Jann Horn <jannh@google.com>]

On Mon, Apr 15, 2024 at 5:24 PM Matthew Wilcox <willy@infradead.org> wrote:
> It seems a shame to fallback if the mmap_lock is readily available.
> It doesn't blow up immediately in my testing ... thoughts?

Seems fine to me, since nothing below the trylocked mmap_lock does
vma_start_*().

> >From 70833af858ce41a80b5f0669648ee9f59b149a03 Mon Sep 17 00:00:00 2001
> From: "Matthew Wilcox (Oracle)" <willy@infradead.org>
> Date: Mon, 15 Apr 2024 11:13:41 -0400
> Subject: [PATCH] mm: Optimise vmf_anon_prepare() for VMAs without an anon_vma
>
> If the mmap_lock can be taken for read, there's no reason not to take
> it so that we can call __anon_vma_prepare().
>
> Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>

Reviewed-by: Jann Horn <jannh@google.com>

> ---
>  mm/memory.c | 8 ++++++--
>  1 file changed, 6 insertions(+), 2 deletions(-)
>
> diff --git a/mm/memory.c b/mm/memory.c
> index b9c23393fa9b..7075fb7f74da 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -3219,11 +3219,15 @@ vm_fault_t vmf_anon_prepare(struct vm_fault *vmf)
>         if (likely(vma->anon_vma))
>                 return 0;
>         if (vmf->flags & FAULT_FLAG_VMA_LOCK) {
> -               vma_end_read(vma);
> -               return VM_FAULT_RETRY;
> +               if (!mmap_read_trylock(vma->vm_mm)) {
> +                       vma_end_read(vma);
> +                       return VM_FAULT_RETRY;
> +               }
>         }
>         if (__anon_vma_prepare(vma))
>                 return VM_FAULT_OOM;
> +       if (vmf->flags & FAULT_FLAG_VMA_LOCK)
> +               mmap_read_unlock(vma->vm_mm);
>         return 0;
>  }
>
> --
> 2.43.0