Re: [QUESTION] about the maple tree and current status of mmap_lock scalability

[Suren Baghdasaryan <surenb@google.com>]

Hi Hyeonggon,

On Wed, Dec 28, 2022 at 4:49 AM Hyeonggon Yoo <42.hyeyoo@gmail.com> wrote:
>
> Hello mm folks,
>
> I have a few questions about the current status of mmap_lock scalability.
>
> =============================================================
> What is currently causing the kernel to use mmap_lock to protect the maple tree?
> =============================================================
>
> I understand that the long-term goal is to remove the need for mmap_lock in readers
> while traversing the maple tree, using techniques such as RCU or SPF.
> What is the biggest obstacle preventing this from being achieved at this time?

Maple tree has an RCU mode which does not need mmap_lock for
traversal. Liam and I were testing it recently and Liam fixed a number
of issues to enable it. It seems stable now and the fixes are
incorporated into the "per-vma locks" patchset which I prepared in
this branch: https://github.com/surenbaghdasaryan/linux/tree/per_vma_lock.
I haven't posted this patchset upstream yet but it's pretty much ready
to go. I'm planning to post it in early January.
Thanks,
Suren.

>
> ==================================================
> How does the maple tree provide RCU-safe manipulation of VMAs?
> ==================================================
>
> Is it similar to the approach suggested in the RCUVM paper (replacing the original
> root node with a new root node that shares most of its nodes and deferring
> the freeing of stale nodes using RCU)?
>
> I'm having difficulty understanding the design of the maple tree in this regard.
>
> [RCUVM paper] https://pdos.csail.mit.edu/papers/rcuvm:asplos12.pdf
>
> Thank you for your time.
>
> ---
> Hyeonggon