linux-kernel - Re: [PATCH 1/7] KVM: arm64: Move hyp_pool locking out of refcount helpers

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Message-ID: <YLY2mjxGCSyunnhV@google.com>
Date:   Tue, 1 Jun 2021 13:31:06 +0000
From:   Quentin Perret <qperret@...gle.com>
To:     Marc Zyngier <maz@...nel.org>
Cc:     will@...nel.org, james.morse@....com, alexandru.elisei@....com,
        catalin.marinas@....com, suzuki.poulose@....com,
        linux-arm-kernel@...ts.infradead.org, kvmarm@...ts.cs.columbia.edu,
        kernel-team@...roid.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/7] KVM: arm64: Move hyp_pool locking out of refcount
 helpers

On Tuesday 01 Jun 2021 at 13:02:00 (+0100), Marc Zyngier wrote:
> On Thu, 27 May 2021 13:51:28 +0100,
> Quentin Perret <qperret@...gle.com> wrote:
> > 
> > The hyp_page refcount helpers currently rely on the hyp_pool lock for
> > serialization. However, this means the refcounts can't be changed from
> > the buddy allocator core as it already holds the lock, which means pages
> > have to go through odd transient states.
> > 
> > For example, when a page is freed, its refcount is set to 0, and the
> > lock is transiently released before the page can be attached to a free
> > list in the buddy tree. This is currently harmless as the allocator
> > checks the list node of each page to see if it is available for
> > allocation or not, but it means the page refcount can't be trusted to
> > represent the state of the page even if the pool lock is held.
> > 
> > In order to fix this, remove the pool locking from the refcount helpers,
> > and move all the logic to the buddy allocator. This will simplify the
> > removal of the list node from struct hyp_page in a later patch.
> 
> Is there any chance some documentation could be added so that we have
> a record of what the locking boundaries are? Something along the line
> of what we have in arch/arm64/kvm/vgic/vgic.c, for example.

Sounds like a good idea, I'll go write something.

Cheers,
Quentin