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[Xen-devel] Re: [RFC PATCH 0/4] (Take 2): transcendent memory ("tmem") f

Dan Magenheimer wrote:
But this means that either the content of that page must have been
preserved somewhere or the discard fault handler has sufficient
information to go back and get the content from the source (e.g.
the filesystem).  Or am I misunderstanding?

As Rik said, it's the later.

With tmem, the equivalent of the "failure to access a discarded page"
is inline and synchronous, so if the tmem access "fails", the
normal code immediately executes.

Yup. This is the main difference AFAICT. It's really just API semantics within Linux.

You could clearly use the volatile state of CMM2 to implement tmem as an API in Linux. The get/put functions would set a flag such that if the discard handler was invoked as long as that operation happened, the operation could safely fail. That's why I claimed tmem is a subset of CMM2.

I suppose changing Linux to utilize the two tmem services
as described above is a semantic change.  But to me it
seems no more of a semantic change than requiring a new
special page fault handler because a page of memory might
disappear behind the OS's back.

But IMHO this is a corollary of the fundamental difference.  CMM2's
is more the "VMware" approach which is that OS's should never have
to be modified to run in a virtual environment.  (Oh, but maybe
modified just slightly to make the hypervisor a little less
clueless about the OS's resource utilization.)

While I always enjoy a good holy war, I'd like to avoid one here because I want to stay on the topic at hand.

If there was one change to tmem that would make it more palatable, for me it would be changing the way pools are "allocated". Instead of getting an opaque handle from the hypervisor, I would force the guest to allocate it's own memory and to tell the hypervisor that it's a tmem pool. You could then introduce semantics about whether the guest was allowed to directly manipulate the memory as long as it was in the pool. It would be required to access the memory via get/put functions that under Xen, would end up being a hypercall and a copy. Presumably you would do some tricks with ballooning to allocate empty memory in Xen and then use those addresses as tmem pools. On KVM, we could do something more clever.

The big advantage of keeping the tmem pool part of the normal set of guest memory is that you don't introduce new challenges with respect to memory accounting. Whether or not tmem is directly accessible from the guest, it is another memory resource. I'm certain that you'll want to do accounting of how much tmem is being consumed by each guest, and I strongly suspect that you'll want to do tmem accounting on a per-process basis. I also suspect that doing tmem limiting for things like cgroups would be desirable.

That all points to making tmem normal memory so that all that infrastructure can be reused. I'm not sure how well this maps to Xen guests, but it works out fine when the VMM is capable of presenting memory to the guest without actually allocating it (via overcommit).


Anthony Liguori

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