Re: [PATCH 0/2] xen/mm: limit in-place scrubbing

["Roger Pau Monné" <roger.pau@citrix.com>]

On Fri, Jan 09, 2026 at 10:29:20AM +0000, Andrew Cooper wrote:
> On 09/01/2026 10:15 am, Jan Beulich wrote:
> > On 08.01.2026 18:55, Roger Pau Monne wrote:
> >> In XenServer we have seen the watchdog occasionally triggering during
> >> domain creation if 1GB pages are scrubbed in-place during physmap
> >> population.
> > That's pretty extreme - writing to 1Gb of memory can't really take over 5s,
> > can it?
> 
> Sure it can.
> 
> > Is there lock contention involved?
> 
> Almost certainly, and it's probably the more relevant aspect in this case.

Possibly.  I can tell Edwin to give me his reproduction.  There's also
the map_domain_page() page aspect of this operation.  On big enough
systems this will cause a fair amount of stress to the map cache,
since each page is mapped, scrubbed and unmapped.  I don't think
however the systems on which we have seen this to be using the map
cache (it was on debug=n builds with less than 5TB of memory).

> > Or is this when very many CPUs
> > try to do the same in parallel?
> 
> The scenario is reboot of a VM when Xapi is doing NUMA placement using
> per-node claims.

Not exclusively.  We have reports of this also happening without any
claims or NUMA placements being used.

AFAICT it's possibly triggered when doing reboots of multiple VMs in
parallel, and all reports of it I've seen it's on multi-node NUMA
systems.  I wonder if scrubbing a 1G remote page in 4K chunks is
killing the intra-node bandwidth.

Thanks, Roger.