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

[Greg Kurz]

On Mon, 18 Jul 2016 12:44:09 +0200
Greg Kurz <address@hidden> wrote:

> On Mon, 18 Jul 2016 11:33:16 +0200
> Thomas Huth <address@hidden> wrote:
> 
> > On 18.07.2016 11:26, Greg Kurz wrote:  
> > > On Mon, 18 Jul 2016 11:04:39 +0200
> > > Thomas Huth <address@hidden> wrote:
> > >     
> > >> On 18.07.2016 10:59, Greg Kurz wrote:    
> > >>> On Mon, 18 Jul 2016 10:52:36 +1000
> > >>> David Gibson <address@hidden> 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 <address@hidden>
> > >>>>> ---
> > >>>>>  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'll try to come up with an additional patch that fixes the remaining
> > >> problem that you've found... Meanwhile, did you find out why you get
> > >> that assertion that I was not able to recreate? Could you maybe post the
> > >> exact command line to trigger that assertion?
> > >>    
> > > 
> > > I hit the assertion when I specify pc-dimm devices on the command line:
> > > 
> > > qemu-system-ppc64 -enable-kvm ... -m 2G,slots=4,maxmem=32G \
> > >  -object 
> > > memory-backend-file,policy=default,mem-path=/mnt/kvm_hugepage,size=1G,id=mem-mem1
> > >  \
> > >  -device pc-dimm,id=dimm-mem1,memdev=mem-mem1 \
> > >  -object 
> > > memory-backend-file,policy=default,mem-path=/mnt,size=1G,id=mem-mem2 \
> > >  -device pc-dimm,id=dimm-mem2,memdev=mem-mem2 \
> > >  -smp 2 -numa node,nodeid=0,memdev=mem-mem1 \
> > >  -numa node,nodeid=1,memdev=mem-mem2    
> > 
> > FWIW, with that command line, I still don't get an assertion but a
> > normal error message:
> > 
> > qemu-system-ppc64: -device pc-dimm,id=dimm-mem1,memdev=mem-mem1: can't
> > use already busy memdev: mem-mem1
> > 
> >  Thomas
> >   
> 
> I hit the assertion with this exact command line:
> 
> qemu-system-ppc64 -machine pseries,accel=kvm \
> -m 1G,slots=4,maxmem=32G \
> -object 
> memory-backend-file,policy=default,mem-path=/dev/hugepages,size=1G,id=mem-mem1
>  \
> -device pc-dimm,id=dimm-mem1,memdev=mem-mem1 \
> -numa node,nodeid=0,memdev=mem-mem1 \
> -S
> 
> QEMU was built against David's ppc-for-2.7 branch (commit 159d2e39a).
> 
> But I get the very same error as you with QEMU 2.6... regression ?
> 

Bisect leads to:

commit 2aece63c8a9d2c3a8ff41d2febc4cdeff2633331
Author: Xiao Guangrong <address@hidden>
Date:   Wed Jul 13 12:18:06 2016 +0800

    hostmem: detect host backend memory is being used properly

... any idea ?

> And BTW, I'm not sure to understand why it is wrong to specify both pc-dimm 
> and
> numa pointing to the same memory backend.
> 
> Cheers.
> 
> --
> Greg

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