On Fri, Aug 11, 2023 at 07:39:37PM +0200, David Hildenbrand wrote: > 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? Looks complete. I don't want to say so, but afaik postcopy should be "corner case" in most cases I'd say; people do still rely mostly on precopy. It's probably a matter of whether we'd like take the risk. -- Peter Xu
