Dear mm folks, I have an issue, where it would be great to have a COW-backed virtual memory area within an userspace process. I know there's the possibility to have a file-backed MAP_SHARED vma, which is later duplicated with MAP_PRIVATE, but that's not exactly what I'm looking for. Say I have an anonymous page-aligned VMA a, with MAP_PRIVATE and PROT_RW. Userspace happily writes to/reads from it. At some point in time, I want to 'snapshot' that single VMA within the context of the process and without the need to fork(). Say there's something like a = mmap(0, len, PROT_RW, MAP_ANON | MAP_POPULATE, -1, 0); [... fill a ...] b = mmdup(a, len, PROT_READ); b shall be the new base pointer of a new VMA that is backed by COW mechanisms. After mmdup, those regular COW mechanisms do the rest: both VMAs (a and b) will fault on subsequent writes and duplicate the previously shared physical mapping, pretty much what cow_fault or shared_fault does. Afaict, this, or at least something like this is currently not supported by the kernel. Is that correct? If so, why? Generally spoken, is it a bad idea? I digged a bit into the mm code, and I think all the stuff that would be required is already there, so I wonder what I'm missing. This is some related work I found on that topic: https://sfb876.tu-dortmund.de/PublicPublicationFiles/kotthaus_2016a.pdf They implement mmapcopy(), which pretty much would fulfill my requirements. However, I still wonder why the kernel doesn't support something like that by default, so maybe some mm expert could shed light on this. Any suggestions welcome! Thanks Ralf
