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

[Thomas Huth <thuth@redhat.com>]

On 15.07.2016 11:28, Greg Kurz wrote:
> On Fri, 15 Jul 2016 10:10:25 +0200
> Thomas Huth <thuth@redhat.com> 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
> 
> According to http://patchwork.ozlabs.org/patch/584741/ , it is best worded
> 
> "Broken in commit 86b50f2e1bef"

Using the "Fixes:" syntax is a well-known practise with the Linux kernel
(see
http://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/tree/Documentation/SubmittingPatches?id=HEAD#n187),
so I don't see the point why we should introduce another syntax for QEMU
here. And if we do, it should be documented on
http://qemu-project.org/Contribute/SubmitAPatch at least.

 Thomas