Re: [PATCH v1 1/3] softmmu/physmem: fallback to opening guest RAM file as readonly in a MAP_PRIVATE mapping

[Peter Xu]

On Fri, Aug 11, 2023 at 06:25:14PM +0200, David Hildenbrand wrote:
> On 11.08.23 18:22, Peter Xu wrote:
> > On Fri, Aug 11, 2023 at 06:17:05PM +0200, David Hildenbrand wrote:
> > > We wouldn't touch "-mem-path".
> > 
> > But still the same issue when someone uses -object memory-backend-file for
> > hugetlb, mapping privately, expecting ram discard to work?
> > 
> > Basically I see that example as, "hugetlb" in general made the private
> > mapping over RW file usable, so forbidden that anywhere may take a risk.
> 
> These users can be directed to using hugetlb
> 
> a) using MAP_SHARED
> b) using memory-backend-memfd, if MAP_PRIVATE is desired
> 
> Am I missing any important use case? Are we being a bit to careful about
> virtio-balloon and postcopy simply not being available for these corner
> cases?

The current immediate issue is not really mem=rw + fd=rw + private case
(which was a known issue), but how to make mem=rw + fd=ro + private work
for ThinnerBloger, iiuc.

I'd just think it safer to expose that cap to solve problem A (vm
templating) without affecting problem B (fallcate-over-private not working
right), when B is uncertain.

I'm also copy Daniel & libvirt list in case there's quick comment from
there. Say, maybe libvirt never use private mapping on hugetlb files over
memory-backend-file at all, then it's probably fine.

In all cases, you and Igor should have the final grasp; no stand on a
strong opinon from my side.

-- 
Peter Xu