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"failed to free memory for the domain" from "xl create"

  • To: Ian Jackson <iwj@xxxxxxxxxxxxxx>, Wei Liu <wl@xxxxxxx>, Anthony Perard <anthony.perard@xxxxxxxxxx>
  • From: Jan Beulich <jbeulich@xxxxxxxx>
  • Date: Thu, 19 Aug 2021 16:09:36 +0200
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  • Delivery-date: Thu, 19 Aug 2021 14:09:57 +0000
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besides there not having been any indication what has gone wrong
(if anything in the first place), I wonder in how far the operation
of freemem() / libxl_wait_for_memory_target() is fully suitable for
all possible scenarios. I did create the guest in question on a
Dom0 running in strict mode on large IOMMU page mappings (as far as
they didn't get shattered by that point). If we consider the case
of Dom0 ballooning out a page that was mapped by a 1G mapping, then
the shattering of the 1G page as well as one of the resulting 2M
pages would result in _one less_ free pages in the end: The
ballooned out page was freed at the price of the allocation of two
pages to become page table ones. This looks to undermine the
"forward progress" determination, which I consider fragile anyway
as long as other parallel operations potentially consuming memory
aren't locked out.

Does one of you have thoughts towards possible improvements here?

Thanks, Jan



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