Re: [RFC PATCH v2 00/51] 1G page support for guest_memfd

[Michael Roth]

I'm not sure I'm hitting the same issue you were, but in order to fix
the race I was hitting I needed to grab the range look outside of the
kvm_gmem_get_folio() path so that it could provide mutual exclusion on
the allocation as well as the subsequent splitting of newly-allocation
hugepages.

Here's the patch I needed on top:

  https://github.com/mdroth/linux/commit/240e09e68fe61bb0dfad6a8e054a6aa9316a3660

I think this same issue exists for the THP implementation[1], where a
range lock built around filemap indicies instead of physical addresses
could maybe address both, but not sure it's worthwhile since THP has been
deemed non-upstreamable until general memory migration support is added
to gmem.

I'll dump the code below though for reference since I know some folks on
Cc have been asking about it, but it isn't yet in a state where it's
worth posting separately, but is at least relevant to this particular
discussion. For now, I've just piggy-backed off the filemap invalidate
write lock to serialize all allocations, but I've only hit the race
condition once for 2MB, it's a lot easier with 1GB using hugetlb.

[1]

The THP patches are currently on top of a snapshot of Ackerley’s hugetlb dev
tree. I’d originally planned to rebase on top of just the common
dependencies and posting upstream, but based on the latest guest_memfd/PUCK
calls, there is no chance of THP going upstream without first implementing
memory migration support for guest_memfd to deal with system-wide/cumulative
fragmentation. So I’m tabling that work, it’s just these 3 patches on top for
now:

  2ae099ef6977 KVM: guest_memfd: Serialize allocations when THP is enabled
  733f7a111699 [WIP] KVM: guest_memfd: Enable/fix hugepages for in-place conversion
  349aa261ac65 KVM: Add hugepage support for dedicated guest memory

The initial patch adds THP support for legacy/non-inplace, the remaining 2
enable it for inplace. There are various warnings/TODOs/debugs, I'm only
posting it for reference since I don't know when I'll get to a cleaned up
version since it's not clear it'll be useful in the near-term.

  Kernel:
    https://github.com/mdroth/linux/commits/snp-thp-rfc2-wip0

  QEMU:
    https://github.com/mdroth/qemu/commits/snp-hugetlb-v3wip0b

  To run QEMU with in-place conversion enabled you need the following option (SNP will default to legacy/non-inplace conversion otherwise):
    qemu ... -object sev-snp-guest,...,convert-in-place=true

  To enable hugepages when using either convert-in-place=false/true, a kvm module turns it on for now (flipping it on/off rapidly may help with simulating/testing low memory situations):

    echo 1 >/sys/module/kvm/gmem_2m_enabled

  This tree also supports SNP+hugetlbfs with the following in case you need it for comparison:

  For 2MB hugetlb:
    qemu ... \
      -object sev-snp-guest,...,convert-in-place=true,gmem-allocator=hugetlb,gmem-page-size=2097152

  For 1GB hugetlb:
    qemu ... \
      -object sev-snp-guest,...,convert-in-place=true,gmem-allocator=hugetlb,gmem-page-size=1073741824