Re: [PATCH v1 4/4] mm/memory: document restore_exclusive_pte()

[David Hildenbrand <david@redhat.com>]

On 30.01.25 14:31, Simona Vetter wrote:
> On Thu, Jan 30, 2025 at 10:37:06AM +0100, David Hildenbrand wrote:
>> On 30.01.25 01:27, Alistair Popple wrote:
>>> On Wed, Jan 29, 2025 at 12:58:02PM +0100, David Hildenbrand wrote:
>>>> Let's document how this function is to be used, and why the requirement
>>>> for the folio lock might maybe be dropped in the future.
>>>
>>> Sorry, only just catching up on your other thread. The folio lock was to ensure
>>> the GPU got a chance to make forward progress by mapping the page. Without it
>>> the CPU could immediately invalidate the entry before the GPU had a chance to
>>> retry the fault.
>>>> Obviously performance wise having such thrashing is terrible, so should
>>> really be avoided by userspace, but the lock at least allowed such programs
>>> to complete.
>>
>> Thanks for the clarification. So it's relevant that the MMU notifier in
>> remove_device_exclusive_entry() is sent after taking the folio lock.
>>
>> However, as soon as we drop the folio lock, remove_device_exclusive_entry()
>> will become active, lock the folio and trigger the MMU notifier.
>>
>> So the time it is actually mapped into the device is rather

I meant to say "rather short." :)

> 
> Looks like you cut off a bit here (or mail transport did that somewhere),
> but see my other reply I don't think this is a legit use-case. So we don't
> have to worry.

In that case, we would need the folio lock in the future.

> Well beyond documenting that if userspace concurrently thrashes
> the same page with both device atomics and cpu access it will stall real
> bad.

I'm curious, is locking between device-cpu or device-device something 
that can happen frequently? In that case, you would get that trashing 
naturally?

-- 
Cheers,

David / dhildenb