Re: [PATCH 4/6] mm: Handle COW faults under the VMA lock

From: Suren Baghdasaryan <surenb@google.com>
To: "Matthew Wilcox (Oracle)" <willy@infradead.org>
Cc: Andrew Morton <akpm@linux-foundation.org>, linux-mm@kvack.org
Subject: Re: [PATCH 4/6] mm: Handle COW faults under the VMA lock
Date: Wed, 27 Sep 2023 17:53:35 -0700	[thread overview]
Message-ID: <CAJuCfpHg_P8Y=5N2QCotXKh+f650kBh3nv=41YZyJhKXQfNPfg@mail.gmail.com> (raw)
In-Reply-To: <20230927052505.2855872-5-willy@infradead.org>

On Tue, Sep 26, 2023 at 10:25 PM Matthew Wilcox (Oracle)
<willy@infradead.org> wrote:
>
> If the page is not currently present in the page tables, we need to call
> the page fault handler to find out which page we're supposed to COW,
> so we need to both check that there is already an anon_vma and that the
> fault handler doesn't need the mmap_lock.
>
> Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
> ---
>  mm/memory.c | 12 +++++-------
>  1 file changed, 5 insertions(+), 7 deletions(-)
>
> diff --git a/mm/memory.c b/mm/memory.c
> index 0f3da4889230..02231a9394ed 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -4639,13 +4639,11 @@ static vm_fault_t do_cow_fault(struct vm_fault *vmf)
>         struct vm_area_struct *vma = vmf->vma;
>         vm_fault_t ret;
>
> -       if (vmf->flags & FAULT_FLAG_VMA_LOCK) {
> -               vma_end_read(vma);
> -               return VM_FAULT_RETRY;
> -       }
> -
> -       if (unlikely(anon_vma_prepare(vma)))
> -               return VM_FAULT_OOM;
> +       ret = vmf_maybe_unlock_vma(vmf);
> +       if (!ret)
> +               ret = vmf_anon_prepare(vmf);

Same comment as in [PATCH 2/6]. If we reach here with
FAULT_FLAG_VMA_LOCK then vma->anon_vma will not be NULL unless we
change lock_vma_under_rcu().

> +       if (ret)
> +               return ret;
>
>         vmf->cow_page = alloc_page_vma(GFP_HIGHUSER_MOVABLE, vma, vmf->address);
>         if (!vmf->cow_page)
> --
> 2.40.1
>