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



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 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
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.

Michal

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