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[Malcolm Scott <launchpad@xxxxxxxxxxx>]

Apologies for the confusion; I had the right commit ID but pasted the
wrong description.  Now fixed, I think!

** Description changed:

  Commit 633d6f17cd91ad5bf2370265946f716e42d388c6 (aka
  38d30afb12140c0e3a446fe779dc9cd29548f313 in vivid) in Xen domU causes
  high resource requirements in the underlying target dom0 (migrating a
  64-bit domU involves a 1GB malloc in dom0, as well as a lot of
  unnecessary work).
  
  In my specific case, that 1GB malloc fails as my dom0s aren't big
  enough, causing all migrations to fail with the migrating VM suspended.
  
- This is fixed in https://marc.info/?l=git-commits-head&m=142800846201785&w=2 :
- "x86/xen: prepare p2m list for memory hotplug":
+ This is fixed in
+ http://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/?id=98dd166ea3a3c3b57919e20d9b0d1237fcd0349d
+ :
  
  """
- Commit 054954eb051f35e74b75a566a96fe756015352c8 ("xen: switch to linear
- virtual mapped sparse p2m list") introduced a regression regarding to
- memory hotplug for a pv-domain: as the virtual space for the p2m list
- is allocated for the to be expected memory size of the domain only,
- hotplugged memory above that size will not be usable by the domain.
+ x86/xen/p2m: hint at the last populated P2M entry"
  
- Correct this by using a configurable size for the p2m list in case of
- memory hotplug enabled (default supported memory size is 512 GB for
- 64 bit domains and 4 GB for 32 bit domains).
+ With commit 633d6f17cd91ad5bf2370265946f716e42d388c6 (x86/xen: prepare
+ p2m list for memory hotplug) the P2M may be sized to accomdate a much
+ larger amount of memory than the domain currently has.
  
- Signed-off-by: Juergen Gross <jgross@xxxxxxxx>
- Cc: <stable@xxxxxxxxxxxxxxx> # 3.19+
- Reviewed-by: Daniel Kiper <daniel.kiper@xxxxxxxxxx>
+ When saving a domain, the toolstack must scan all the P2M looking for
+ populated pages.  This results in a performance regression due to the
+ unnecessary scanning.
+ 
+ Instead of reporting (via shared_info) the maximum possible size of
+ the P2M, hint at the last PFN which might be populated.  This hint is
+ increased as new leaves are added to the P2M (in the expectation that
+ they will be used for populated entries).
+ 
  Signed-off-by: David Vrabel <david.vrabel@xxxxxxxxxx>
+ Cc: <stable@xxxxxxxxxxxxxxx> # 4.0+
  """

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https://bugs.launchpad.net/bugs/1542941

Title:
  Regression: problems migrating recent wily/vivid Xen VMs due to memory
  hotplug fix

Status in linux package in Ubuntu:
  Triaged

Bug description:
  Commit 633d6f17cd91ad5bf2370265946f716e42d388c6 (aka
  38d30afb12140c0e3a446fe779dc9cd29548f313 in vivid) in Xen domU causes
  high resource requirements in the underlying target dom0 (migrating a
  64-bit domU involves a 1GB malloc in dom0, as well as a lot of
  unnecessary work).

  In my specific case, that 1GB malloc fails as my dom0s aren't big
  enough, causing all migrations to fail with the migrating VM
  suspended.

  This is fixed in
  http://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/?id=98dd166ea3a3c3b57919e20d9b0d1237fcd0349d
  :

  """
  x86/xen/p2m: hint at the last populated P2M entry"

  With commit 633d6f17cd91ad5bf2370265946f716e42d388c6 (x86/xen: prepare
  p2m list for memory hotplug) the P2M may be sized to accomdate a much
  larger amount of memory than the domain currently has.

  When saving a domain, the toolstack must scan all the P2M looking for
  populated pages.  This results in a performance regression due to the
  unnecessary scanning.

  Instead of reporting (via shared_info) the maximum possible size of
  the P2M, hint at the last PFN which might be populated.  This hint is
  increased as new leaves are added to the P2M (in the expectation that
  they will be used for populated entries).

  Signed-off-by: David Vrabel <david.vrabel@xxxxxxxxxx>
  Cc: <stable@xxxxxxxxxxxxxxx> # 4.0+
  """

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