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

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Message-ID: <87sg2123pj.wl-maz@kernel.org>
Date:   Tue, 01 Jun 2021 13:02:00 +0100
From:   Marc Zyngier <maz@...nel.org>
To:     Quentin Perret <qperret@...gle.com>
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 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.

Thanks,

	M.

-- 
Without deviation from the norm, progress is not possible.