Re: [Xen-devel] [libvirt] [PATCH V3] libxl: fix dom0 balloon logic

[Jim Fehlig <jfehlig@xxxxxxxx>]

Martin Kletzander wrote:
> On Wed, Apr 01, 2015 at 11:08:54AM -0600, Jim Fehlig wrote:
>> Recent testing on large memory systems revealed a bug in the Xen xl
>> tool's freemem() function.  When autoballooning is enabled, freemem()
>> is used to ensure enough memory is available to start a domain,
>> ballooning dom0 if necessary.  When ballooning large amounts of memory
>> from dom0, freemem() would exceed its self-imposed wait time and
>> return an error.  Meanwhile, dom0 continued to balloon.  Starting the
>> domain later, after sufficient memory was ballooned from dom0, would
>> succeed.  The libvirt implementation in libxlDomainFreeMem() suffers
>> the same bug since it is modeled after freemem().
>>
>> In the end, the best place to fix the bug on the Xen side was to
>> slightly change the behavior of libxl_wait_for_memory_target().
>> Instead of failing after caller-provided wait_sec, the function now
>> blocks as long as dom0 memory ballooning is progressing.  It will return
>> failure only when more memory is needed to reach the target and wait_sec
>> have expired with no progress being made.  See xen.git commit fd3aa246.
>> There was a dicussion on how this would affect other libxl apps like
>> libvirt
>>
>> http://lists.xen.org/archives/html/xen-devel/2015-03/msg00739.html
>>
>> If libvirt containing this patch was build against a Xen containing
>> the old libxl_wait_for_memory_target() behavior, libxlDomainFreeMem()
>> will fail after 30 sec and domain creation will be terminated.
>> Without this patch and with old libxl_wait_for_memory_target() behavior,
>> libxlDomainFreeMem() does not succeed after 30 sec, but returns success
>> anyway.  Domain creation continues resulting in all sorts of fun stuff
>> like cpu soft lockups in the guest OS.  It was decided to properly fix
>> libxl_wait_for_memory_target(), and if anything improve the default
>> behavior of apps using the freemem reference impl in xl.
>>
>> xl was patched to accommodate the change in
>> libxl_wait_for_memory_target()
>> with xen.git commit 883b30a0.  This patch does the same in the libxl
>> driver.  While at it, I changed the logic to essentially match
>> freemem() in $xensrc/tools/libxl/xl_cmdimpl.c.  It was a bit cleaner
>> IMO and will make it easier to spot future, potentially interesting
>> divergences.
>>
>> Signed-off-by: Jim Fehlig <jfehlig@xxxxxxxx>
>> ---
>>
>> V3:
>> Remove unneeded local variable 'ret' in libxlDomainFreeMem.
>>
>
> ACK

Thanks!  I've pushed this now.

Regards,
Jim


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