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

["Michael S. Tsirkin" <mst@redhat.com>]

On Wed, Feb 13, 2019 at 06:59:24PM +0100, David Hildenbrand wrote:
> >>>
> >>>> 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, don’t 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.
> > 
> > Right - there has to be waiting. But it does not have to be busy -
> > if you can defer page use until interrupt, that's one option.
> > Further if you are ready to exit to hypervisor it does not have to be
> > busy waiting.  In particular right now virtio does not have a capability
> > to stop queue processing by device.  We could add that if necessary.  In
> > that case, you would stop queue and detach buffers.  It is already
> > possible by reseting the balloon.  Naturally there is no magic - you
> > exit to hypervisor and block there. It's not all that great
> > in that VCPU does not run at all. But it is not busy waiting.
> 
> Of course, you can always yield to the hypervisor and not call it busy
> waiting. From the guest point of view, it is busy waiting. The VCPU is
> to making progress. If I am not wrong, one can easily construct examples
> where all VCPUs in the guest are waiting for the hypervisor to
> madv(dontneed) pages. I don't like that approach
> 
> Especially if temporarily getting pages out of the buddy resolves these
> issues and seems to work.

Well hypervisor can send a singla and interrupt the dontneed work.
But yes I prefer not blocking the VCPU too.

I also prefer MADV_FREE generally.

> 
> -- 
> 
> Thanks,
> 
> David / dhildenb