Re: [PATCH 1/1] mm: fix vma_copy for !CONFIG_PER_VMA_LOCK

[Suren Baghdasaryan <surenb@google.com>]

On Tue, Dec 10, 2024 at 1:06 AM David Hildenbrand <david@redhat.com> wrote:
>
> On 09.12.24 23:10, Suren Baghdasaryan wrote:
> > vma_copy() function for !CONFIG_PER_VMA_LOCK configuration copies all
> > fields using memcpy() as opposed to CONFIG_PER_VMA_LOCK version which
> > copies only required fields. anon_vma_chain field should not be copied
> > and new vma should instead initialize it to an empty list. Fix this
> > by initializing anon_vma_chain inside vma_copy() function. The version
> > of vma_copy() for CONFIG_PER_VMA_LOCK is fine since it does not change
> > that field and anon_vma_chain of any new vma is already initialized and
> > empty.
>
> I'm wondering if there is sufficient reason to have two implementations
> to do the copying.
>
> How expensive would it be to simply use the CONFIG_PER_VMA_LOCK variant
> unconditionally? Is it even measurable in fork() micro-benchmarks?

Yeah, the benefit if any would be tiny. I'll try measuring the
difference and if it's not visible will remove the
!CONFIG_PER_VMA_LOCK version.
Thanks!

>
> --
> Cheers,
>
> David / dhildenb
>