Re: [Qemu-devel] [Qemu-ppc] [PATCH] ppc: Yet another fix for the huge page support detection mechanism

[David Gibson <david@gibson.dropbear.id.au>]

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On Mon, Jul 18, 2016 at 10:59:44AM +0200, Greg Kurz wrote:
> On Mon, 18 Jul 2016 10:52:36 +1000
> David Gibson <david@gibson.dropbear.id.au> wrote:
> 
> > On Fri, Jul 15, 2016 at 10:10:25AM +0200, Thomas Huth wrote:
> > > Commit 86b50f2e1bef ("Disable huge page support if it is not available
> > > for main RAM") already made sure that huge page support is not announced
> > > to the guest if the normal RAM of non-NUMA configurations is not backed
> > > by a huge page filesystem. However, there is one more case that can go
> > > wrong: NUMA is enabled, but the RAM of the NUMA nodes are not configured
> > > with huge page support (and only the memory of a DIMM is configured with
> > > it). When QEMU is started with the following command line for example,
> > > the Linux guest currently crashes because it is trying to use huge pages
> > > on a memory region that does not support huge pages:
> > > 
> > >  qemu-system-ppc64 -enable-kvm ... -m 1G,slots=4,maxmem=32G -object \
> > >    memory-backend-file,policy=default,mem-path=/hugepages,size=1G,id=mem-mem1 \
> > >    -device pc-dimm,id=dimm-mem1,memdev=mem-mem1 -smp 2 \
> > >    -numa node,nodeid=0 -numa node,nodeid=1
> > > 
> > > To fix this issue, we've got to make sure to disable huge page support,
> > > too, when there is a NUMA node that is not using a memory backend with
> > > huge page support.
> > > 
> > > Fixes: 86b50f2e1befc33407bdfeb6f45f7b0d2439a740
> > > Signed-off-by: Thomas Huth <thuth@redhat.com>
> > > ---
> > >  target-ppc/kvm.c | 10 +++++++---
> > >  1 file changed, 7 insertions(+), 3 deletions(-)  
> > 
> > Applied to ppc-for-2.7, thanks.
> > 
> 
> It looks like my replies to this patch were ignored... no big deal
> though :)

I saw them.  IIUC, though, this patch still improves the situation,
even if it doesn't get every case right, so I was inclined to include
it earlier rather than later.

> 
> > > 
> > > diff --git a/target-ppc/kvm.c b/target-ppc/kvm.c
> > > index 884d564..7a8f555 100644
> > > --- a/target-ppc/kvm.c
> > > +++ b/target-ppc/kvm.c
> > > @@ -389,12 +389,16 @@ static long getrampagesize(void)
> > >  
> > >      object_child_foreach(memdev_root, find_max_supported_pagesize, &hpsize);
> > >  
> > > -    if (hpsize == LONG_MAX) {
> > > +    if (hpsize == LONG_MAX || hpsize == getpagesize()) {
> > >          return getpagesize();
> > >      }
> > >  
> > > -    if (nb_numa_nodes == 0 && hpsize > getpagesize()) {
> > > -        /* No NUMA nodes and normal RAM without -mem-path ==> no huge pages! */
> > > +    /* If NUMA is disabled or the NUMA nodes are not backed with a
> > > +     * memory-backend, then there is at least one node using "normal"
> > > +     * RAM. And since normal RAM has not been configured with "-mem-path"
> > > +     * (what we've checked earlier here already), we can not use huge pages!
> > > +     */
> > > +    if (nb_numa_nodes == 0 || numa_info[0].node_memdev == NULL) {
> > >          static bool warned;
> > >          if (!warned) {
> > >              error_report("Huge page support disabled (n/a for main memory).");  
> > 
> 



-- 
David Gibson			| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you.  NOT _the_ _other_
				| _way_ _around_!
http://www.ozlabs.org/~dgibson

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