linux-kernel - Re: [RFC PATCH 1/6] mm/mmu_notifier: Allow multiple struct mmu_interval

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Message-ID: <20250818164655.GJ599331@ziepe.ca>
Date: Mon, 18 Aug 2025 13:46:55 -0300
From: Jason Gunthorpe <jgg@...pe.ca>
To: Matthew Brost <matthew.brost@...el.com>
Cc: Thomas Hellström <thomas.hellstrom@...ux.intel.com>,
	intel-xe@...ts.freedesktop.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Simona Vetter <simona.vetter@...ll.ch>,
	Dave Airlie <airlied@...il.com>, dri-devel@...ts.freedesktop.org,
	linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	Christian König <christian.koenig@....com>
Subject: Re: [RFC PATCH 1/6] mm/mmu_notifier: Allow multiple struct
 mmu_interval_notifier passes

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.

Jason