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

Jason Gunthorpe
2026-01-12T13:46:19Z

On Sun, Jan 11, 2026 at 07:51:01PM -0500, Zi Yan wrote:
> On 11 Jan 2026, at 19:19, Balbir Singh wrote:
>
> > 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.
> >>
>
> Hi Matthew,
>
> I think the wording is confusing, since the actual issue is that:
>
> 1. zone_device_page_init() calls prep_compound_page() to form a large folio,
> 2. but free_zone_device_folio() never reverse the course,
> 3. the undo of prep_compound_page() in free_zone_device_folio() needs to
> be done before driver callback ->folio_free(), since once ->folio_free()
> is called, the folio can be reallocated immediately,
> 4. after the undo of prep_compound_page(), folio_order() can no longer provide
> the original order information, thus, folio_free() needs that for proper
> device side ref manipulation.

There is something wrong with the driver if the "folio can be
reallocated immediately".

The flow generally expects there to be a driver allocator linked to
folio_free()

1) Allocator finds free memory
2) zone_device_page_init() allocates the memory and makes refcount=1
3) __folio_put() knows the recount 0.
4) free_zone_device_folio() calls folio_free(), but it doesn't
actually need to undo prep_compound_page() because *NOTHING* can
use the page pointer at this point.
5) Driver puts the memory back into the allocator and now #1 can
happen. It knows how much memory to put back because folio->order
is valid from #2
6) #1 happens again, then #2 happens again and the folio is in the
right state for use. The successor #2 fully undoes the work of the
predecessor #2.

If you have races where #1 can happen immediately after #3 then the
driver design is fundamentally broken and passing around order isn't
going to help anything.

If the allocator is using the struct page memory then step #5 should
also clean up the struct page with the allocator data before returning
it to the allocator.

I vaugely remember talking about this before in the context of the Xe
driver.. You can't just take an existing VRAM allocator and layer it
on top of the folios and have it broadly ignore the folio_free
callback.

Jsaon