linux-kernel - Re: [PATCH v5 12/19] KVM: Move memslot deletion to helper function

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Message-ID: <20200206161415.GA695333@xz-x1>
Date:   Thu, 6 Feb 2020 11:14:15 -0500
From:   Peter Xu <peterx@...hat.com>
To:     Sean Christopherson <sean.j.christopherson@...el.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Paul Mackerras <paulus@...abs.org>,
        Christian Borntraeger <borntraeger@...ibm.com>,
        Janosch Frank <frankja@...ux.ibm.com>,
        David Hildenbrand <david@...hat.com>,
        Cornelia Huck <cohuck@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, Marc Zyngier <maz@...nel.org>,
        James Morse <james.morse@....com>,
        Julien Thierry <julien.thierry.kdev@...il.com>,
        Suzuki K Poulose <suzuki.poulose@....com>,
        linux-mips@...r.kernel.org, kvm@...r.kernel.org,
        kvm-ppc@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        kvmarm@...ts.cs.columbia.edu, linux-kernel@...r.kernel.org,
        Christoffer Dall <christoffer.dall@....com>,
        Philippe Mathieu-Daudé <f4bug@...at.org>
Subject: Re: [PATCH v5 12/19] KVM: Move memslot deletion to helper function

On Tue, Jan 21, 2020 at 02:31:50PM -0800, Sean Christopherson wrote:
> Move memslot deletion into its own routine so that the success path for
> other memslot updates does not need to use kvm_free_memslot(), i.e. can
> explicitly destroy the dirty bitmap when necessary.  This paves the way
> for dropping @dont from kvm_free_memslot(), i.e. all callers now pass
> NULL for @dont.
> 
> Add a comment above the code to make a copy of the existing memslot
> prior to deletion, it is not at all obvious that the pointer will become
> stale during sorting and/or installation of new memslots.

Could you help explain a bit on this explicit comment?  I can follow
up with the patch itself which looks all correct to me, but I failed
to catch what this extra comment wants to emphasize...

Thanks,

> 
> Note, kvm_arch_commit_memory_region() allows an architecture to free
> resources when moving a memslot or changing its flags, e.g. x86 frees
> its arch specific memslot metadata during commit_memory_region().

-- 
Peter Xu