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

[Martin Kletzander <mkletzan@xxxxxxxxxx>]

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

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