Re: [Xen-devel] Relevance of global lock (/var/lock/xl) in xl tool

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

Please try to avoid top posting, it breaks the flow of the
conversation.

+ Ian and Wei (tools maintainers)

On Mon, Jun 10, 2019 at 09:57:34PM +0200, Michał Leszczyński wrote:
> Hello,
> 
> I've tested the scenario with restoring too many domains having lock
> commented out. It turns out that in case when there is no memory left
> for XEN to allocate, all pending /xl restore/ commands will simply fail
> with an internal error.
> 
> As far as I understand, it's not going to do any damage unless I request
> too much memory?

Iff the lock is solely used to automatically adjust the memory balloon
for dom0 in order to allocate memory to use for other domains then as
long as you don't have autoballoon enabled it should be fine.

Failures during domain creation are always harmless, the only damage
is the domain not being actually created.

> If so, then there is some place for improvement, i.e. to make xl acquire
> the lock, allocate memory, release the lock and then start restoring the
> domain state. Maybe I could provide some pull request if I would manage
> to implement such behavior.

FWIW, there's a 'claim' hypercall which is already implemented and
used by libxl that should reserve memory for a domain beforehand.

Roger.

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