RE: [Xen-devel] [PATCH] Fix restore handling checks

[Dan Magenheimer <dan.magenheimer@xxxxxxxxxx>]

> From: Michal Novotny [mailto:minovotn@xxxxxxxxxx]
>
> Honestly I don't know about how tmem features work since I have them
> disabled because I'm unable to start any guest with them enabled
> because it fails on creation.

If you are booting Xen enabling tmem and NOT setting dom0_mem=
there was a bug just reported and fixed (or patch posted at
least).  If this is another problem with tmem (e.g. you ARE
setting both tmem and dom0_mem), please tell me more.

However, note that just enabling tmem in Xen doesn't do
anything.  Tmem must also be enabled in guests.

> Why should it give too many false-positives/false-negatives...

The issue is that in a tmem system (with self-ballooning enabled
which will be the default for tmem-enabled guests), all the
guests are dynamically changing in size between their maximum
memory and some minimum which may be smaller than the mem=
they were launched with.

And "free" memory as reported by Xen is also changing dynamically
due to guest tmem calls.

The only way to ensure a fixed amount of memory is available is:
1) freeze tmem
2) get free memory and "tmem freeable" memory and if there
   is enough
3) ALLOCATE the memory NOW
4) check to ensure the allocation worked
5) unfreeze tmem

The code in xends currently does this when launching new domains.

Tmem freeze only stops tmem from allocating more free memory
from Xen (by failing all tmem calls from tmem-enabled guests).
It doesn't stop guest ballooning activity.  So even the
"ALLOCATE the memory NOW" may fail... but that is acceptable
when launching a new domain.

Hope that helps,
Dan

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