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

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Message-ID: <e96dcfd4ce7c84a5b66ff9d5f082ea209266ce48.camel@linux.intel.com>
Date: Tue, 19 Aug 2025 13:33:40 +0200
From: Thomas Hellström <thomas.hellstrom@...ux.intel.com>
To: Alistair Popple <apopple@...dia.com>, Jason Gunthorpe <jgg@...pe.ca>
Cc: Matthew Brost <matthew.brost@...el.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 Tue, 2025-08-19 at 19:55 +1000, Alistair Popple wrote:
> 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.

One thing that the linked list avoids, though, is traversing the
interval tree two times. It has O(n*log(n)) whereas the linked list
overhead is just O(n_2pass).

> 
> 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.

One idea is for any two-pass notifier implementation to use a small
pool. That would also to some extent mitigate the risk of out-of-memory
with GFP_NOWAIT.

/Thomas


> 
> > Jason
> >