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Message-ID: <20200206165116.GE695333@xz-x1>
Date: Thu, 6 Feb 2020 11:51:16 -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 Thu, Feb 06, 2020 at 08:28:18AM -0800, Sean Christopherson wrote:
> On Thu, Feb 06, 2020 at 11:14:15AM -0500, Peter Xu wrote:
> > 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...
>
> It's tempting to write the code like this (I know, because I did it):
>
> if (!mem->memory_size)
> return kvm_delete_memslot(kvm, mem, slot, as_id);
>
> new = *slot;
>
> Where @slot is a pointer to the memslot to be deleted. At first, second,
> and third glances, this seems perfectly sane.
>
> The issue is that slot was pulled from struct kvm_memslots.memslots, e.g.
>
> slot = &slots->memslots[index];
>
> Note that slots->memslots holds actual "struct kvm_memory_slot" objects,
> not pointers to slots. When update_memslots() sorts the slots, it swaps
> the actual slot objects, not pointers. I.e. after update_memslots(), even
> though @slot points at the same address, it's could be pointing at a
> different slot. As a result kvm_free_memslot() in kvm_delete_memslot()
> will free the dirty page info and arch-specific points for some random
> slot, not the intended slot, and will set npages=0 for that random slot.
Ah I see, thanks. Another alternative is we move the "old = *slot"
copy into kvm_delete_memslot(), which could be even clearer imo.
However I'm not sure whether it's a good idea to drop the test-by for
this. Considering that comment change should not affect it, would you
mind enrich the comment into something like this (or anything better)?
/*
* Make a full copy of the old memslot, the pointer will become stale
* when the memslots are re-sorted by update_memslots() in
* kvm_delete_memslot(), while to make the kvm_free_memslot() work as
* expected later on, we still need the cached memory slot.
*/
In all cases:
Reviewed-by: Peter Xu <peterx@...hat.com>
--
Peter Xu
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