Red Hat Bugzilla – Bug 499644
F-11 Xen 64-bit domU does not give memory back to the dom0
Last modified: 2010-06-28 08:25:34 EDT
Description of problem:
I'm following the test case here: https://fedoraproject.org/wiki/QA:Testcase_Virtualization_XenDomU_Cmdline_params. My F-11 Xen domU is configured to start with 2047MB of memory. However, while booting, I specified mem=1G on the kernel command-line, and the guest properly booted with only 1G of memory. However, it does *not* give the spare memory back to the hypervisor. That is, since we've set mem=, we can never balloon above that, so we should give that memory back to the HV so it can use it for other domains. This works as expected in the RHEL-5 domU kernel, and in upstream linux-2.6.18-xen.hg.
This one is much less important. It would be nice to get it to work, but it's not critical.
This bug appears to have been reported against 'rawhide' during the Fedora 11 development cycle.
Changing version to '11'.
More information and reason for this action is here:
Created attachment 358049 [details]
Patch for 2.6.29 kernel
Problem is caused by missing code for returning memory to HV. Patch adding this code to 2.6.29 kernel sent to Jeremy Fitzhardinge with CC na LKML and Xen-devel.
Thread is here:
Sounds like Jeremy wants to use the e820 map to find unused memory rather than just max_pfn
Yes, exactly. Miroslav, would you mind reworking your patch along those lines?
Created attachment 361207 [details]
Returning memory not contained in e820 map
General solution - returns all not mapped memory.
Grabbing this one to get the patch tested and into Fedora.
There should be optimized version of patch in Jeremy's repo. However, this version has wrong location of return_unused_memory - it is in xen_memory_setup not in xen_post_allocator_init.
branch rebase/core-freemem branch at git://git.kernel.org/pub/scm/linux/kernel/git/jeremy/xen.git
Indeed, the version of the patch in jeremy's repo does not actually seem to return the unused memory.
No, there's a problem with it. Aside from releasing a bit too early, the test to make sure the page being released actually belongs to the domain seems to always return false, so it never wants to release anything. I'm not sure why; it looks OK to me, but I haven't investigated it in detail yet.
Created attachment 395766 [details]
patch as committed to jeremy's repo
Attaching patch from Jeremy's repo for posterity.
I just had another look at this and noticed a rather obvious bug in the patch: "mfn = pfn_to_mfn(mfn);" rather than "mfn = pfn_to_mfn(*p*fn);". Fixing this makes it free memory properly as expected.
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