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

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Message-ID: <2da9464b-3b3d-46bd-a68f-bfef1226bbf6@redhat.com>
Date: Thu, 14 Aug 2025 15:53:00 +0300
From: Mika Penttilä <mpenttil@...hat.com>
To: Jason Gunthorpe <jgg@...dia.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 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.

> 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

>
> Jason
>
--Mika