On Sun, May 22, 2022 at 08:20:05AM -0700, Andy Lutomirski wrote: > On Sat, May 21, 2022, at 5:31 PM, Matthew Wilcox wrote: > > On Sat, May 21, 2022 at 03:19:24PM -0700, Andy Lutomirski wrote: > >> I can see a rather different use for this type of shared-pagetable > >> technology, though: monstrous MAP_SHARED mappings. For database and some VM > >> users, multiple processes will map the same file. If there was a way to > >> ensure appropriate alignment (or at least encourage it) and a way to handle > >> mappings that don't cover the whole file, then having multiple mappings > >> share the same page tables could be a decent efficiently gain. This doesn't > >> even need COW -- it's "just" pagetable sharing. > > > > The mshare proposal did not get a warm reception at LSFMM ;-( > > > > The conceptual model doesn't seem to work for the MM developers who were > > in the room. "Fear" was the most-used word. Not sure how we're going > > to get to a model of sharing page tables that doesn't scare people. > > FWIW, I didn’t like mshare. mshare was weird: it seemed to have > one mm own some page tables and other mms share them. I’m talking > about having a *file* own page tables and mms map them. This seems less > fear-inducing to me. Circular dependencies are impossible, mmap calls > don’t need to propagate, etc. OK, so that doesn't work for our use case. We need an object to own page tables that can be shared between different (co-operating) processes. Because we need the property that calling mprotect() changes the protection in all processes at the same time. Obviously we want that object to be referenced by a file descriptor, and it can also have a name. That object doesn't have to be an mm_struct. Maybe that would be enough of a change to remove the fear.
