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From:   Suren Baghdasaryan <surenb@google.com>
Date:   Wed, 28 Dec 2022 09:10:20 -0800
Message-ID: <CAJuCfpFUh4qGqePueUd5snz27nxLUPehQeAmbkshheno==KtcA@mail.gmail.com>
Subject: Re: [QUESTION] about the maple tree and current status of mmap_lock scalability
To:     Hyeonggon Yoo <42.hyeyoo@gmail.com>
Cc:     linux-mm@kvack.org, liam.howlett@oracle.com, willy@infradead.org,
        ldufour@linux.ibm.com, michel@lespinasse.org, vbabka@suse.cz,
        linux-kernel@vger.kernel.org
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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