Re: [PATCH v3 1/2] KVM: arm64: ptdump: Check the page tables aren't freed when accessing

[Wei-Lin Chang]

On Thu, Sep 17, 2026 at 09:11:50AM +0100, Vincent Donnefort wrote:
> On Thu, Sep 17, 2026 at 12:03:36AM +0100, Wei-Lin Chang wrote:
> > An open debugfs file keeps the KVM structure alive, but does not prevent
> > mmu notifier release from freeing the stage-2 page tables when the VMM’s
> > address space is torn down. Therefore page tables belonging to the mmus
> > could have been freed when a thread opens or reads the ptdump files.
> > Take the mmu_lock and check mmu->pgt is still alive before accessing the
> > page tables.
> 
> As Sashiko said, the read_lock is probably enough, including the existing one in
> kvm_ptdump_guest_show()
> 
> With that change:
> 
> Reviewed-by: Vincent Donnefort <vdonnefort@xxxxxxxxxx>
> Tested-by: Vincent Donnefort <vdonnefort@xxxxxxxxxx>

Thanks for the review and testing!

I agree taking read_lock is enough for most of these, but changing
kvm_ptdump_guest_show() to read_lock could result in a dump showing
weird output e.g. 0-sized ranges. This happens when the dump reads a
block, and a parallel fault turns that block into a table, and the dump
descends into the table later.

This is debugfs afterall so I think it isn't a dealbreaker, but it adds
another purpose to this patch. Maybe we can change
kvm_ptdump_guest_show() into taking a read_lock when someone reports a
scalability problem when dumping the page tables.

I'll stick to changing the other ones into taking the read_lock now.

For future reference: KVM_PGTABLE_WALK_SHARED is required if we want to
change kvm_ptdump_guest_show() into taking a read_lock.

Thanks,
Wei-Lin Chang

>