linux-kernel - Re: [PATCH v3 05/11] KVM: page track: introduce kvm_page_track_{add,remove}

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Date:	Fri, 19 Feb 2016 12:37:16 +0100
From:	Paolo Bonzini <pbonzini@...hat.com>
To:	Xiao Guangrong <guangrong.xiao@...ux.intel.com>
Cc:	gleb@...nel.org, mtosatti@...hat.com, kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org, kai.huang@...ux.intel.com,
	jike.song@...el.com
Subject: Re: [PATCH v3 05/11] KVM: page track: introduce
 kvm_page_track_{add,remove}_page

On 14/02/2016 12:31, Xiao Guangrong wrote:
> +	/* does tracking count wrap? */
> +	WARN_ON((count > 0) && (val + count < val));

This doesn't work, because "val + count" is an int.

> +	/* the last tracker has already gone? */
> +	WARN_ON((count < 0) && (val < !count));

Also, here any underflow should warn.

You can actually use the fact that val + count is an int like this:

    WARN_ON(val + count < 0 || val + count > USHRT_MAX)

and also please return if the warning fires.

> +void kvm_page_track_add_page(struct kvm *kvm, gfn_t gfn,
> +			     enum kvm_page_track_mode mode)
> +{
> +	struct kvm_memslots *slots;
> +	struct kvm_memory_slot *slot;
> +	int i;
> +
> +	for (i = 0; i < KVM_ADDRESS_SPACE_NUM; i++) {
> +		slots = __kvm_memslots(kvm, i);
> +
> +		slot = __gfn_to_memslot(slots, gfn);
> +		if (!slot)
> +			continue;
> +
> +		spin_lock(&kvm->mmu_lock);
> +		kvm_slot_page_track_add_page_nolock(kvm, slot, gfn, mode);
> +		spin_unlock(&kvm->mmu_lock);
> +	}
> +}

I don't think it is right to walk all address spaces.  The good news is
that you're not using kvm_page_track_{add,remove}_page at all as far as
I can see, so you can just remove them.

Also, when you will need it, I think it's better to move the
spin_lock/spin_unlock pair outside the for loop.  With this change,
perhaps it's better to leave it to the caller completely---but I cannot
say until I see the caller.

In the meanwhile, please leave out _nolock from the other functions' name.

Paolo