[libvirt] [PATCH] qemuBuildMemoryBackendStr: Handle one more corner case

Martin Kletzander mkletzan at redhat.com
Fri Aug 11 05:34:10 UTC 2017

On Thu, Aug 10, 2017 at 05:43:13PM +0200, Michal Privoznik wrote:
>On 08/10/2017 03:44 PM, Martin Kletzander wrote:
>> On Tue, Aug 08, 2017 at 05:04:15PM +0200, Michal Privoznik wrote:
>>> https://bugzilla.redhat.com/show_bug.cgi?id=1458638
>>> This code is so complicated because we allow enabling the same
>>> bits at many places. Just like in this case: huge pages can be
>>> enabled by global <hugepages/> element under <memoryBacking> or
>>> on per <memory/> basis. To complicate things a bit more, users
>>> are allowed to not specify any specific page size in which case
>> s/to not specify/not to specify/
>> reads a bit nicely, or even
>> s/to not specify any/to specify no/ or "allowed to omit the page size"
>I'm going with the latter.
>> Choice is yours
>> Reviewed-by: Martin Kletzander <mkletzan at redhat.com>
>Pushed, thanks.
>> One more note that I told you in person, but I can't resist the urge to
>> share it more widely.  The code is gross because it can't be made much
>> better due to the fact that we keep the parsed data the same way they
>> are in the XML.  I think this could be made nicer if we were to fill all
>> the information during parsing.
>> When parsing /memory/hugepages we would fill in missing information
>> right away.  Then, when parsing /cpu/numa/cell we could copy that
>> information to the specific cells (if missing) and because the whole
>> definition is already available from parsing /memory/hugepages there's
>> less lookups.  Finally, when parsing /devices/memory/source we can
>> directly get missing info from the numa cell if nodemask is present or
>> global hugepages if it is not, and we don't have to lookup anything,
>> because it is already guaranteed to be filled in for the previous parts.
>> I hope it's understandable.  What I hope even more is that someone
>> refactors the code this way =)
>Right. One thing though that I haven't realized during our conversation
>is that we would need to keep track if the info is supplied by user or
>by this algorithm. The problem is the following: users can specify
><memoryBacking> <hugepages/> </memoryBacking>  in which case so called
>default huge page size is used. The default is whatever page size is
>printed in /proc/meminfo. For instance, on my local machine:
>$ cat /proc/meminfo | grep Hugepagesize:
>Hugepagesize:       2048 kB
>$ cat /proc/cpuinfo | grep -o pdpe1gb
>So the default huge page size is 2M but the processor supports also 1GB.
>However, on different machine the default may be something else. 2MB
>might not even be accessible there. Therefore if we would fill that info
>during parsing we might be screwed. Alternatively, if admin have 2MB
>enabled during define, but then prohibits 2MB in favour of 1GB, we are
>back in the problem. Hope it makes sense what I'm saying.

You don't have to copy the size, just whatever information you might
have.  We can talk it through later or I can explain myself with the
code, but I don't want to do the rework right now, to be honest.

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