Re: [RFC PATCH 0/6] Introduce Copy-On-Write to Page Table

[Matthew Wilcox <willy@infradead.org>]

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.