On 06/26/2014 04:15 PM, Andy Lutomirski wrote: > So here's my mental image of how I might do this if I were doing it > entirely in userspace: I'd create a file or memfd for the bound tables > and another for the bound directory. These files would be *huge*: the > bound directory file would be 2GB and the bounds table file would be > 2^48 bytes or whatever it is. (Maybe even bigger?) > > Then I'd just map pieces of those files wherever they'd need to be, > and I'd make the mappings sparse. I suspect that you don't actually > want a vma for each piece of bound table that gets mapped -- the space > of vmas could end up incredibly sparse. So I'd at least map (in the > vma sense, not the pte sense) and entire bound table at a time. And > I'd probably just map the bound directory in one big piece. > > Then I'd populate it in the fault handler. > > This is almost what the code is doing, I think, modulo the files. > > This has one killer problem: these mappings need to be private (cowed > on fork). So memfd is no good. This essentially uses the page cache's radix tree as a parallel data structure in order to keep a vaddr->mpx_vma map. That's not a bad idea, but it is a parallel data structure that does not handle copy-on-write very well. I'm pretty sure we need the semantics that anonymous memory provides. > There's got to be an easyish way to > modify the mm code to allow anonymous maps with vm_ops. Maybe a new > mmap_region parameter or something? Maybe even a special anon_vma, > but I don't really understand how those work. Yeah, we very well might end up having to go down that path. > Also, egads: what happens when a bound table entry is associated with > a MAP_SHARED page? Bounds table entries are for pointers. Do we keep pointers inside of MAP_SHARED-mapped things? :) -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>