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

[Andy Lutomirski]

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.

It would still be quite a change, though.