Re: [PATCH v3 3/7] mm: Split device-private and coherent folios before freeing

[Matthew Brost]

On Fri, Jan 09, 2026 at 12:28:22PM -0500, Zi Yan wrote:
> On 9 Jan 2026, at 6:09, Mika Penttilä wrote:
> 
> > Hi,
> >
> > On 1/9/26 10:54, Francois Dugast wrote:
> >
> >> From: Matthew Brost <matthew.brost@xxxxxxxxx>
> >>
> >> Split device-private and coherent folios into individual pages before
> >> freeing so that any order folio can be formed upon the next use of the
> >> pages.
> >>
> >> Cc: Balbir Singh <balbirs@xxxxxxxxxx>
> >> Cc: Alistair Popple <apopple@xxxxxxxxxx>
> >> Cc: Zi Yan <ziy@xxxxxxxxxx>
> >> Cc: David Hildenbrand <david@xxxxxxxxxx>
> >> Cc: Oscar Salvador <osalvador@xxxxxxx>
> >> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> >> Cc: linux-mm@xxxxxxxxx
> >> Cc: linux-cxl@xxxxxxxxxxxxxxx
> >> Cc: linux-kernel@xxxxxxxxxxxxxxx
> >> Signed-off-by: Matthew Brost <matthew.brost@xxxxxxxxx>
> >> Signed-off-by: Francois Dugast <francois.dugast@xxxxxxxxx>
> >> ---
> >>  mm/memremap.c | 2 ++
> >>  1 file changed, 2 insertions(+)
> >>
> >> diff --git a/mm/memremap.c b/mm/memremap.c
> >> index 63c6ab4fdf08..7289cdd6862f 100644
> >> --- a/mm/memremap.c
> >> +++ b/mm/memremap.c
> >> @@ -453,6 +453,8 @@ void free_zone_device_folio(struct folio *folio)
> >>  	case MEMORY_DEVICE_COHERENT:
> >>  		if (WARN_ON_ONCE(!pgmap->ops || !pgmap->ops->folio_free))
> >>  			break;
> >> +
> >> +		folio_split_unref(folio);
> >>  		pgmap->ops->folio_free(folio);
> >>  		percpu_ref_put_many(&folio->pgmap->ref, nr);
> >>  		break;
> >
> > This breaks folio_free implementations like nouveau_dmem_folio_free
> > which checks the folio order and act upon that.
> > Maybe add an order parameter to folio_free or let the driver handle the split?

'let the driver handle the split?' - I had consisder this as an option.

> 
> Passing an order parameter might be better to avoid exposing core MM internals
> by asking drivers to undo compound pages.
> 

It looks like Nouveau tracks free folios and free pages—something Xe’s
device memory allocator (DRM Buddy) cannot do. I guess this answers my
earlier question of how Nouveau avoids hitting the same bug as Xe / GPU
SVM with respect to reusing folios. It appears Nouveau prefers not to
split the folio, so I’m leaning toward moving this call into the
driver’s folio_free function.

Matt

> Best Regards,
> Yan, Zi