Re: [PATCH v4 1/7] mm/zone_device: Add order argument to folio_free callback

[Balbir Singh <balbirs@nvidia.com>]

On 1/12/26 08:35, Matthew Wilcox wrote:
> On Sun, Jan 11, 2026 at 09:55:40PM +0100, Francois Dugast wrote:
>> The core MM splits the folio before calling folio_free, restoring the
>> zone pages associated with the folio to an initialized state (e.g.,
>> non-compound, pgmap valid, etc...). The order argument represents the
>> folio’s order prior to the split which can be used driver side to know
>> how many pages are being freed.
> 
> This really feels like the wrong way to fix this problem.
> 

This stems from a special requirement, freeing is done in two phases

1. Free the folio -> inform the driver (which implies freeing the backing device memory)
2. Return the folio back, split it back to single order folios

The current code does not do 2. 1 followed by 2 does not work for
Francois since the backing memory can get reused before we reach step 2.
The proposed patch does 2 followed 1, but doing 2 means we've lost the 
folio order and thus the old order is passed in. Although, I wonder if the
backing folio's zone_device_data can be used to encode any order information
about the device side allocation.

@Francois, I hope I did not miss anything in the explanation above.

> I think someone from the graphics side really needs to take the lead on
> understanding what the MM is doing (both currently and in the future).
> I'm happy to work with you, but it feels like there's a lot of churn right
> now because there's a lot of people working on this without understanding
> the MM side of things (and conversely, I don't think (m)any people on the
> MM side really understand what graphics cards are trying to accomplish).
> 

I suspect you are referring to folio specialization and/or downsizing?

> Who is that going to be?  I'm happy to get on the phone with someone.

Happy to work with you, but I am not the authority on graphics, I can speak
to zone device folios. I suspect we'd need to speak to more than one person.

Balbir