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

On 11.08.23 18:54, Peter Xu wrote: > 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. Right, and I'm thinking about if B is worth the effort. > > 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. libvirt certainly allows setting <access mode="shared"/> with <source type="file">. Could be that they also end up mapping "<hugepages>" to memory-backend-file instead of memory-backend-memfd (e.g., compatibility with older kernels?). > > In all cases, you and Igor should have the final grasp; no stand on a > strong opinon from my side. I do value your opinion, so I'm still trying to figure out if there are sane use cases that really need a new parameter. Let's recap: When opening the file R/O, resulting in fallocate() refusing to work: * virtio-balloon will fail to discard RAM but continue to "be alive" * virtio-mem will discard any private pages, but cannot free up disk blocks using fallocate. * postcopy would fail early Postcopy: * Works on shmem (MAP_SHARED / MAP_PRIVATE) * Works on hugetlb (MAP_SHARED / MAP_PRIVATE) * Does not work on file-backed memory (including MAP_PRIVATE) We can ignore virtio-mem for now. What remains is postcopy and virtio-balloon. memory-backend-file with MAP_PRIVATE on shmem/tmpfs results in a double memory consumption, so we can mostly cross that out as "sane use case". Rather make such users aware of that :D memory-backend-file with MAP_PRIVATE on hugetlb works. virtio-balloon is not really compatible with hugetlb, free-page-reporting might work (although quite non-nonsensical). So postcopy as the most important use case remains. memory-backend-file with MAP_PRIVATE on file-backed memory works. postcopy does not apply. virtio-balloon should work I guess. So the two use cases that are left are: * postcopy with hugetlb would fail * virtio-balloon with file-backed memory cannot free up disk blocks Am I missing a case?