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Re: [Xen-devel] Proposed new "memory capacity claim" hypercall/feature

>>> On 07.11.12 at 23:17, Dan Magenheimer <dan.magenheimer@xxxxxxxxxx> wrote:
> It appears that the attempt to use 2MB and 1GB pages is done in
> the toolstack, and if the hypervisor rejects it, toolstack tries
> smaller pages.  Thus, if physical memory is highly fragmented
> (few or no order>=9 allocations available), this will result
> in one hypercall per 4k page so a 256GB domain would require
> 64 million hypercalls.  And, since AFAICT, there is no sane
> way to hold the heap_lock across even two hypercalls, speeding
> up the in-hypervisor allocation path, by itself, will not solve
> the TOCTOU race.

No, even in the absence of large pages, the tool stack will do 8M
allocations, just without requesting them to be contiguous.
Whether 8M is a suitable value is another aspect; that value may
predate hypercall preemption, and I don't immediately see why
the tool stack shouldn't be able to request larger chunks (up to
the whole amount at once).


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