Re: [PATCH v4 4/6] KVM: MMU: fast invalid all shadow pages

[Xiao Guangrong]

On 05/07/2013 03:50 AM, Marcelo Tosatti wrote:
> On Mon, May 06, 2013 at 11:39:11AM +0800, Xiao Guangrong wrote:
>> On 05/04/2013 08:52 AM, Marcelo Tosatti wrote:
>>> On Sat, May 04, 2013 at 12:51:06AM +0800, Xiao Guangrong wrote:
>>>> On 05/03/2013 11:53 PM, Marcelo Tosatti wrote:
>>>>> On Fri, May 03, 2013 at 01:52:07PM +0800, Xiao Guangrong wrote:
>>>>>> On 05/03/2013 09:05 AM, Marcelo Tosatti wrote:
>>>>>>
>>>>>>>> +
>>>>>>>> +/*
>>>>>>>> + * Fast invalid all shadow pages belong to @slot.
>>>>>>>> + *
>>>>>>>> + * @slot != NULL means the invalidation is caused the memslot specified
>>>>>>>> + * by @slot is being deleted, in this case, we should ensure that rmap
>>>>>>>> + * and lpage-info of the @slot can not be used after calling the function.
>>>>>>>> + *
>>>>>>>> + * @slot == NULL means the invalidation due to other reasons, we need
>>>>>>>> + * not care rmap and lpage-info since they are still valid after calling
>>>>>>>> + * the function.
>>>>>>>> + */
>>>>>>>> +void kvm_mmu_invalid_memslot_pages(struct kvm *kvm,
>>>>>>>> +				   struct kvm_memory_slot *slot)
>>>>>>>> +{
>>>>>>>> +	spin_lock(&kvm->mmu_lock);
>>>>>>>> +	kvm->arch.mmu_valid_gen++;
>>>>>>>> +
>>>>>>>> +	/*
>>>>>>>> +	 * All shadow paes are invalid, reset the large page info,
>>>>>>>> +	 * then we can safely desotry the memslot, it is also good
>>>>>>>> +	 * for large page used.
>>>>>>>> +	 */
>>>>>>>> +	kvm_clear_all_lpage_info(kvm);
>>>>>>>
>>>>>>> Xiao,
>>>>>>>
>>>>>>> I understood it was agreed that simple mmu_lock lockbreak while
>>>>>>> avoiding zapping of newly instantiated pages upon a
>>>>>>>
>>>>>>> 	if(spin_needbreak)
>>>>>>> 		cond_resched_lock()
>>>>>>>
>>>>>>> cycle was enough as a first step? And then later introduce root zapping
>>>>>>> along with measurements.
>>>>>>>
>>>>>>> https://lkml.org/lkml/2013/4/22/544
>>>>>>
>>>>>> Yes, it is.
>>>>>>
>>>>>> See the changelog in 0/0:
>>>>>>
>>>>>> " we use lock-break technique to zap all sptes linked on the
>>>>>> invalid rmap, it is not very effective but good for the first step."
>>>>>>
>>>>>> Thanks!
>>>>>
>>>>> Sure, but what is up with zeroing kvm_clear_all_lpage_info(kvm) and
>>>>> zapping the root? Only lock-break technique along with generation number 
>>>>> was what was agreed.
>>>>
>>>> Marcelo,
>>>>
>>>> Please Wait... I am completely confused. :(
>>>>
>>>> Let's clarify "zeroing kvm_clear_all_lpage_info(kvm) and zapping the root" first.
>>>> Are these changes you wanted?
>>>>
>>>> void kvm_mmu_invalid_memslot_pages(struct kvm *kvm,
>>>> 				   struct kvm_memory_slot *slot)
>>>> {
>>>> 	spin_lock(&kvm->mmu_lock);
>>>> 	kvm->arch.mmu_valid_gen++;
>>>>
>>>> 	/* Zero all root pages.*/
>>>> restart:
>>>> 	list_for_each_entry_safe(sp, node, &kvm->arch.active_mmu_pages, link) {
>>>> 		if (!sp->root_count)
>>>> 			continue;
>>>>
>>>> 		if (kvm_mmu_prepare_zap_page(kvm, sp, &invalid_list))
>>>> 			goto restart;
>>>> 	}
>>>>
>>>> 	/*
>>>> 	 * All shadow paes are invalid, reset the large page info,
>>>> 	 * then we can safely desotry the memslot, it is also good
>>>> 	 * for large page used.
>>>> 	 */
>>>> 	kvm_clear_all_lpage_info(kvm);
>>>>
>>>> 	kvm_mmu_commit_zap_page(kvm, &invalid_list);
>>>> 	spin_unlock(&kvm->mmu_lock);
>>>> }
>>>>
>>>> static void rmap_remove(struct kvm *kvm, u64 *spte)
>>>> {
>>>> 	struct kvm_mmu_page *sp;
>>>> 	gfn_t gfn;
>>>> 	unsigned long *rmapp;
>>>>
>>>> 	sp = page_header(__pa(spte));
>>>> +
>>>> +       /* Let invalid sp do not access its rmap. */
>>>> +	if (!sp_is_valid(sp))
>>>> +		return;
>>>> +
>>>> 	gfn = kvm_mmu_page_get_gfn(sp, spte - sp->spt);
>>>> 	rmapp = gfn_to_rmap(kvm, gfn, sp->role.level);
>>>> 	pte_list_remove(spte, rmapp);
>>>> }
>>>>
>>>> If yes, there is the reason why we can not do this that i mentioned before:
>>>>
>>>> after call kvm_mmu_invalid_memslot_pages(), the memslot->rmap will be destroyed.
>>>> Later, if host reclaim page, the mmu-notify handlers, ->invalidate_page and
>>>> ->invalidate_range_start, can not find any spte using the host page, then
>>>> Accessed/Dirty for host page is missing tracked.
>>>> (missing call kvm_set_pfn_accessed and kvm_set_pfn_dirty properly.)
>>>>
>>>> What's your idea?
>>>
>>>
>>> Step 1) Fix kvm_mmu_zap_all's behaviour: introduce lockbreak via
>>> spin_needbreak. Use generation numbers so that in case kvm_mmu_zap_all 
>>> releases mmu_lock and reacquires it again, only shadow pages 
>>> from the generation with which kvm_mmu_zap_all started are zapped (this
>>> guarantees forward progress and eventual termination).
>>>
>>> kvm_mmu_zap_generation()
>>> 	spin_lock(mmu_lock)
>>> 	int generation = kvm->arch.mmu_generation;
>>>
>>> 	for_each_shadow_page(sp) {
>>> 		if (sp->generation == kvm->arch.mmu_generation)
>>> 			zap_page(sp)
>>> 		if (spin_needbreak(mmu_lock)) {
>>> 			kvm->arch.mmu_generation++;
>>> 			cond_resched_lock(mmu_lock);
>>> 		}
>>> 	}
>>>
>>> kvm_mmu_zap_all()
>>> 	spin_lock(mmu_lock)
>>> 	for_each_shadow_page(sp) {
>>> 		if (spin_needbreak(mmu_lock)) {
>>> 			cond_resched_lock(mmu_lock);
>>> 		}
>>> 	}
>>>
>>> Use kvm_mmu_zap_generation for kvm_arch_flush_shadow_memslot.
>>> Use kvm_mmu_zap_all for kvm_mmu_notifier_release,kvm_destroy_vm.
>>>
>>> This addresses the main problem: excessively long hold times 
>>> of kvm_mmu_zap_all with very large guests.
>>>
>>> Do you see any problem with this logic? This was what i was thinking 
>>> we agreed.
>>
>> No. I understand it and it can work.
>>
>> Actually, it is similar with Gleb's idea that "zapping stale shadow pages
>> (and uses lock break technique)", after some discussion, we thought "only zap
>> shadow pages that are reachable from the slot's rmap" is better, that is this
>> patchset does.
>> (https://lkml.org/lkml/2013/4/23/73)
>>
>>>
>>> Step 2) Show that the optimization to zap only the roots is worthwhile
>>> via benchmarking, and implement it.
>>
>> This is what i am confused. I can not understand how "zap only the roots"
>> works. You mean these change?
>>
>> kvm_mmu_zap_generation()
>>  	spin_lock(mmu_lock)
>>  	int generation = kvm->arch.mmu_generation;
>>
>>  	for_each_shadow_page(sp) {
>> 		/* Change here. */
>> => 		if ((sp->generation == kvm->arch.mmu_generation) &&
>> =>		      sp->root_count)
>>  			zap_page(sp)
>>
>>  		if (spin_needbreak(mmu_lock)) {
>>  			kvm->arch.mmu_generation++;
>>  			cond_resched_lock(mmu_lock);
>>  		}
>>  	}
>>
>> If we do this, there will have shadow pages that are linked to invalid memslot's
>> rmap. How do we handle these pages and the mmu-notify issue?
>>
>> Thanks!
> 
> By "zap only roots" i mean zapping roots plus generation number on
> shadow pages. But this as a second step, after it has been demonstrated
> its worthwhile.

Marcelo,

Sorry for my stupidity, still do not understand. Could you please show me the
pseudocode and answer my questions above?





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