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

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Message-ID: <20250814130403.GF699432@nvidia.com>
Date: Thu, 14 Aug 2025 10:04:03 -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 03:53:00PM +0300, Mika Penttilä wrote:
> 
> On 8/14/25 15:40, Jason Gunthorpe wrote:
> > 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.
> 
> No it's the opposite, if we are ourselves the invalidator, don't care.

I think you've missed the point, you cannot use range->owner in any
way to tell "we are ourselves the invalidator". It is simply not the
meaning of range->owner.

> > 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.
> 
> That's the problem this solves.
> The semantics is "if we are the invalidator don't wait for invalidate end",
> aka don't mmu_interval_set_seq() that would make hang in the next mmu_interval_read_begin(),
> waiting the invalidate to end

I doubt we can skip mmu_interval_set_seq(), doing so can corrupt concurrent
hmm_range_fault in some other thread.

Nor, as I said, can we actually skip the invalidation toward HW
anyhow.

At the very least this commit message fails to explain the new locking
proposal, or justify why it can work.

Jason