On Tue, Jun 27, 2023 at 12:00:38PM -0400, Peter Xu wrote: > On Tue, Jun 27, 2023 at 12:52:34PM -0300, Jason Gunthorpe wrote: > > On Mon, Jun 26, 2023 at 03:04:21PM -0400, Peter Xu wrote: > > > On Mon, Jun 26, 2023 at 03:18:48PM -0300, Jason Gunthorpe wrote: > > > > On Mon, Jun 26, 2023 at 08:14:27PM +0200, David Hildenbrand wrote: > > > > > > > > > So we might have to implement the same page migration as gup does on > > > > > FOLL_LONGTERM here ... maybe there are more such cases/drivers that actually > > > > > require that handling when simply taking pages out of the memfd, believing > > > > > they can hold on to them forever. > > > > > > > > In general I would like to see an interface to FOLL_LONGTERM pin pages > > > > from a memfd. I would quite happily use that in iommufd as well. > > > > > > > > It solves some problems we have there with fork/exec/etc if the pages > > > > are not linked to a mm_struct. > > > > > > Afaiu any fd based approach should mean it'll never work with private > > > memories, while mm-based should be able to work on any kind. > > > > Is there a significant use case to open a memfd and then use > > MAP_PRIVATE? Why would anyone want to do that instead of just using > > normal mmap anonymous memory? > > I remember David Hildenbrand somewhere mentioned the use case where one > wants to snapshot a VM RAM into a file, then start multiple instances by > loading that VM RAM with MAP_PRIVATE, so it clones a bunch of snapshoted VM > running with a single RAM file shared as a template. Not a generic use > case, I guess. A file I can see, but a file is not a memfd, we are talking specifically about memfd, aren't we? > My question applies not only memfd but also in general - qemu by default > doesn't use memfd afaict, so it boils down to e.g. whether you'll target > the iommufd project to work in that case, where qemu uses anonymous > memory. I think this may change, as I understand it, the approach for confidential compute is to put the guest memory in a memfd... > Privately mapped file memory is only one of those kinds. I think memfd and related shmem-like objects are a reasonable target. We already know we should not FOLL_LONGTERM pin file backed pages. Jason
