[RFC PATCH 1/6] mm/mmu_notifier: Allow multiple struct mmu_interval_notifier passes

[Alistair Popple]

On Mon, Aug 18, 2025 at 01:46:55PM -0300, Jason Gunthorpe wrote:
> On Mon, Aug 18, 2025 at 09:44:01AM -0700, Matthew Brost wrote:
> > On Mon, Aug 18, 2025 at 01:36:17PM -0300, Jason Gunthorpe wrote:
> > > On Mon, Aug 18, 2025 at 09:25:20AM -0700, Matthew Brost wrote:
> > > > I think this choice makes sense: it allows embedding the wait state from
> > > > the initial notifier call into the pass structure. Patch [6] shows this
> > > > by attaching the issued TLB invalidation fences to the pass. Since a
> > > > single notifier may be invoked multiple times with different ranges but
> > > > the same seqno,
> > > 
> > > That should be explained, but also seems to be a bit of a different
> > > issue..
> > > 
> > > If the design is really to only have two passes and this linked list
> > > is about retaining state then there should not be so much freedom to
> > > have more passes.
> > 
> > I’ll let Thomas weigh in on whether we really need more than two passes;
> > my feeling is that two passes are likely sufficient. It’s also worth
> > noting that the linked list has an added benefit: the notifier tree only
> > needs to be walked once (a small time-complexity win).
> 
> You may end up keeping the linked list just with no way to add a third
> pass.

It seems to me though that linked list still adds unnecessary complexity. I
think this would all be much easier to follow if we just added two new callbacks
- invalidate_start() and invalidate_end() say.

Admitedly that would still require the linked list (or something similar) to
retain the ability to hold/pass a context between the start and end callbacks.
Which is bit annoying, it's a pity we need to allocate memory in a performance
sensitive path to effectively pass (at least in this case) a single pointer. I
can't think of any obvious solutions to that though.

> Jason
>