Re: [Xen-devel] [PATCH 1/2] x86/mem_sharing: reorder when pages are unlocked and released

[Tamas K Lengyel <tamas@xxxxxxxxxxxxx>]

On Fri, Apr 12, 2019 at 8:00 AM Andrew Cooper <andrew.cooper3@xxxxxxxxxx> wrote:
>
> On 12/04/2019 14:41, Tamas K Lengyel wrote:
> > On Fri, Apr 12, 2019 at 5:55 AM Jan Beulich <JBeulich@xxxxxxxx> wrote:
> >>>>> On 12.04.19 at 06:29, <tamas@xxxxxxxxxxxxx> wrote:
> >>> Patch 0502e0adae2 "x86: correct instances of PGC_allocated clearing" 
> >>> introduced
> >>> grabbing extra references for pages that drop references tied to 
> >>> PGC_allocated.
> >>> However, the way these extra references were grabbed were incorrect, 
> >>> resulting
> >>> in both share_pages and unshare_pages failing.
> >> I'm sorry for this.
> > It's my bad for not catching it earlier when I acked it. Reading the
> > patch it looked fine and made sense but evidently that's no substitute
> > for actually testing it.
>
> As an aside, do you have a one-paragraph introduction to how you use
> mem_sharing?

The use-case is malware analysis where we tend to deploy the same VM
image over and over. Mem_sharing saves resources as we don't have to
have say 4gb of memory available for each analysis session since only
a fraction of that memory actually gets touched during the analysis.
Right now it's still a bit clunky since we first have to restore the
VM image and then proceed with mem_sharing but there are other
improvements that are planned, such as VM forking
(https://xenproject.atlassian.net/projects/XEN/board?issue-key=XEN-89),
which would use mem_sharing but the deployment of new VMs would be
more streamlined.

>
> With any luck, we can try and avoid breaking it as often as we seem to.

Ideally :)

Thanks,
Tamas

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