Re: [libvirt] [PATCH V2] libxl: fix dom0 balloon logic

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(a)suse.com&gt; --- V2: Actually use libxl_wait_for_memory_target(), instead of libxl_wait_for_free_memory() src/libxl/libxl_domain.c | 55 +++++++++++++++++++++++------------------------- 1 file changed, 26 insertions(+), 29 deletions(-) diff --git a/src/libxl/libxl_domain.c b/src/libxl/libxl_domain.c index 407a9bd..a1739aa 100644 --- a/src/libxl/libxl_domain.c +++ b/src/libxl/libxl_domain.c @@ -1121,38 +1121,39 @@ libxlDomainFreeMem(libxlDomainObjPrivatePtr priv, libxl_domain_config *d_config) { uint32_t needed_mem; uint32_t free_mem; - size_t i; - int ret = -1; + int ret;

int tries = 3; int wait_secs = 10; - if ((ret = libxl_domain_need_memory(priv->ctx, &d_config->b_info, - &needed_mem)) >= 0) { - for (i = 0; i < tries; ++i) { - if ((ret = libxl_get_free_memory(priv->ctx, &free_mem)) < 0) - break; + ret = libxl_domain_need_memory(priv->ctx, &d_config->b_info, &needed_mem); + if (ret < 0) + goto error;