linux-kernel - Re: [RFC PATCH 1/4] mm: use current as mmu notifier's owner

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Message-ID: <20250814124041.GD699432@nvidia.com>
Date: Thu, 14 Aug 2025 09:40:41 -0300
From: Jason Gunthorpe <jgg@...dia.com>
To: Mika Penttilä <mpenttil@...hat.com>
Cc: linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	David Hildenbrand <david@...hat.com>,
	Leon Romanovsky <leonro@...dia.com>,
	Alistair Popple <apopple@...dia.com>,
	Balbir Singh <balbirs@...dia.com>
Subject: Re: [RFC PATCH 1/4] mm: use current as mmu notifier's owner

On Thu, Aug 14, 2025 at 10:19:26AM +0300, Mika Penttilä wrote:
> When doing migration in combination with device fault handling,
> detect the case in the interval notifier.
> 
> Without that, we would livelock with our own invalidations
> while migrating and splitting pages during fault handling.
> 
> Note, pgmap_owner, used in some other code paths as owner for filtering,
> is not readily available for split path, so use current for this use case.
> Also, current and pgmap_owner, both being pointers to memory, can not be
> mis-interpreted to each other.
> 
> Cc: David Hildenbrand <david@...hat.com>
> Cc: Jason Gunthorpe <jgg@...dia.com>
> Cc: Leon Romanovsky <leonro@...dia.com>
> Cc: Alistair Popple <apopple@...dia.com>
> Cc: Balbir Singh <balbirs@...dia.com>
> 
> Signed-off-by: Mika Penttilä <mpenttil@...hat.com>
> ---
>  lib/test_hmm.c   | 5 +++++
>  mm/huge_memory.c | 6 +++---
>  mm/rmap.c        | 4 ++--
>  3 files changed, 10 insertions(+), 5 deletions(-)
> 
> diff --git a/lib/test_hmm.c b/lib/test_hmm.c
> index 761725bc713c..cd5c139213be 100644
> --- a/lib/test_hmm.c
> +++ b/lib/test_hmm.c
> @@ -269,6 +269,11 @@ static bool dmirror_interval_invalidate(struct mmu_interval_notifier *mni,
>  	    range->owner == dmirror->mdevice)
>  		return true;
>  
> +	if (range->event == MMU_NOTIFY_CLEAR &&
> +	    range->owner == current) {
> +		return true;
> +	}

I don't understand this, there is nothing in hmm that says only
current can call hmm_range_fault, and indeed most applications won't
even gurantee that.

So if this plan relies on something like the above in drivers I don't
see how it can work.

If this is just some hack for tests, try instead to find a solution
that more accurately matches what a real driver should do.

But this also seems overall troublesome to your goal, if you do a
migrate inside hmm_range_fault() it will generate an invalidation call
back and that will increment the seqlock and we will loop
hmm_range_fault() again which rewalks.

Jason