Re: [RFC][Patch v8 0/7] KVM: Guest Free Page Hinting

[David Hildenbrand]

On 13.02.19 09:55, Wang, Wei W wrote:
> On Tuesday, February 12, 2019 5:24 PM, David Hildenbrand wrote:
>> Global means all VCPUs will be competing potentially for a single lock when
>> freeing/allocating a page, no? What if you have 64VCPUs allocating/freeing
>> memory like crazy?
> 
> I think the key point is that the 64 vcpus won't allocate/free on the same page simultaneously, so no need to have a global big lock, isnât it?
> I think atomic operations on the bitmap would be enough.

If you have to resize/alloc/coordinate who will report, you will need
locking. Especially, I doubt that there is an atomic xbitmap  (prove me
wrong :) ).

> 
>> (I assume some kind of locking is required even if the bitmap would be
>> atomic. Also, doesn't xbitmap mean that we eventually have to allocate
>> memory at places where we don't want to - e.g. from arch_free_page ?)
> 
> arch_free_pages is in free_pages_prepare, why can't we have memory allocation there?

I remember we were stumbling over some issues that were non-trivial. I
am not 100% sure yet anymore, but allocating memory while deep down in
the freeing part of MM core smells like "be careful".

> 
> It would also be doable to find a preferred place to preallocate some amount of memory for the bitmap.

That makes things very ugly. Especially, preallocation will most likely
require locking.

> 
>>
>> That's the big benefit of taking the pages of the buddy free list. Other VCPUs
>> won't stumble over them, waiting for them to get freed in the hypervisor.
> 
> As also mentioned above, I think other vcpus will not allocate/free on the same page that is in progress of being allocated/freed.

If a page is in the buddy but stuck in some other bitmap, there is
nothing stopping another VCPU from trying to allocate it. Nitesh has
been fighting with this problem already :)

> 
>> This sounds more like "the host requests to get free pages once in a while"
>> compared to "the host is always informed about free pages". At the time
>> where the host actually has to ask the guest (e.g. because the host is low on
>> memory), it might be to late to wait for guest action.
> 
> Option 1: Host asks for free pages:
> Not necessary to ask only when the host has been in memory pressure.
> This could be the orchestration layer's job to monitor the host memory usage.
> For example, people could set the condition "when 50% of the host memory
> has been used, start to ask a guest for some amount of free pages" 
> 
> Option 2: Guest actively offers free pages:
> Add a balloon callback to arch_free_page so that whenever a page gets freed its gfn
> will be filled into the balloon's report_vq and the host will take away the backing
> host page.
> 
> Both options can be implemented. But I think option 1 would be more
> efficient as the guest free pages are offered on demand.  

Yes, but as I mentioned this has other drawbacks. Relying on a a guest
to free up memory when you really need it is not going to work. It might
work for some scenarios but should not dictate the design. It is a good
start though if it makes things easier.

Enabling/disabling free page hintning by the hypervisor via some
mechanism is on the other hand a good idea. "I have plenty of free
space, don't worry".

> 
>> Nitesh uses MADV_FREE here (as far as I recall :) ), to only mark pages as
>> candidates for removal and if the host is low on memory, only scanning the
>> guest page tables is sufficient to free up memory.
>>
>> But both points might just be an implementation detail in the example you
>> describe.
> 
> Yes, it is an implementation detail. I think DONTNEED would be easier
> for the first step.
> 
>>
>>>
>>> In above 2), get_free_page_hints clears the bits which indicates that those
>> pages are not ready to be used by the guest yet. Why?
>>> This is because 3) will unmap the underlying physical pages from EPT.
>> Normally, when guest re-visits those pages, EPT violations and QEMU page
>> faults will get a new host page to set up the related EPT entry. If guest uses
>> that page before the page gets unmapped (i.e. right before step 3), no EPT
>> violation happens and the guest will use the same physical page that will be
>> unmapped and given to other host threads. So we need to make sure that
>> the guest free page is usable only after step 3 finishes.
>>>
>>> Back to arch_alloc_page(), it needs to check if the allocated pages
>>> have "1" set in the bitmap, if that's true, just clear the bits. Otherwise, it
>> means step 2) above has happened and step 4) hasn't been reached. In this
>> case, we can either have arch_alloc_page() busywaiting a bit till 4) is done
>> for that page Or better to have a balloon callback which prioritize 3) and 4)
>> to make this page usable by the guest.
>>
>> Regarding the latter, the VCPU allocating a page cannot do anything if the
>> page (along with other pages) is just being freed by the hypervisor.
>> It has to busy-wait, no chance to prioritize.
> 
> I meant this:
> With this approach, essentially the free pages have 2 states:
> ready free page: the page is on the free list and it has "1" in the bitmap
> non-ready free page: the page is on the free list and it has "0" in the bitmap
> Ready free pages are those who can be allocated to use.
> Non-ready free pages are those who are in progress of being reported to
> host and the related EPT mapping is about to be zapped. 
> 
> The non-ready pages are inserted into the report_vq and waiting for the
> host to zap the mappings one by one. After the mapping gets zapped
> (which means the backing host page has been taken away), host acks to
> the guest to mark the free page as ready free page (set the bit to 1 in the bitmap).

Yes, that's how I understood your approach. The interesting part is
where somebody finds a buddy page and wants to allocate it.

> 
> So the non-ready free page may happen to be used when they are waiting in
> the report_vq to be handled by the host to zap the mapping, balloon could
> have a fast path to notify the host:
> "page 0x1000 is about to be used, donât zap the mapping when you get
> 0x1000 from the report_vq"  /*option [1] */

This requires coordination and in any case there will be a scenario
where you have to wait for the hypervisor to eventually finish a madv
call. You can just try to make that scenario less likely.

What you propose is synchronous in the worst case. Getting pages of the
buddy makes it possible to have it done completely asynchronous. Nobody
allocating a page has to wait.

> 
> Or
> 
> "page 0x1000 is about to be used, please zap the mapping NOW, i.e. do 3) and 4) above,
> so that the free page will be marked as ready free page and the guest can use it".
> This option will generate an extra EPT violation and QEMU page fault to get a new host
> page to back the guest ready free page.

Again, coordination with the hypervisor while allocating a page. That is
to be avoided in any case.

> 
>>
>>>
>>> Using bitmaps to record free page hints don't need to take the free pages
>> off the buddy list and return them later, which needs to go through the long
>> allocation/free code path.
>>>
>>
>> Yes, but it means that any process is able to get stuck on such a page for as
>> long as it takes to report the free pages to the hypervisor and for it to call
>> madvise(pfn_start, DONTNEED) on any such page.
> 
> This only happens when the guest thread happens to get allocated on a page which is
> being reported to the host. Using option [1] above will avoid this.

I think getting pages out of the buddy system temporarily is the only
way we can avoid somebody else stumbling over a page currently getting
reported by the hypervisor. Otherwise, as I said, there are scenarios
where a allocating VCPU has to wait for the hypervisor to finish the
"freeing" task. While you can try to "speedup" that scenario -
"hypervisor please prioritize" you cannot avoid it. There will be busy
waiting.

I don't believe what you describe is going to work (especially the not
locking part when working with global resources).

What would be interesting is to see if something like a xbitmap could be
used instead of the per-vcpu list. Nitesh, do you remember what the
problem was with allocating memory from these hooks? Was it a locking issue?

Thanks!

> 
> Best,
> Wei
> 


-- 

Thanks,

David / dhildenb