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

[Zi Yan]

On 9 Jan 2026, at 15:03, Matthew Brost wrote:

> On Fri, Jan 09, 2026 at 02:23:49PM -0500, Zi Yan wrote:
>> On 9 Jan 2026, at 14:08, Matthew Brost wrote:
>>
>>> On Fri, Jan 09, 2026 at 01:53:33PM -0500, Zi Yan wrote:
>>>> On 9 Jan 2026, at 13:26, Matthew Brost wrote:
>>>>
>>>>> 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.
>>>>
>>>> No, that creates asymmetric page handling and is error prone.
>>>>
>>>
>>> I agree it is asymmetric and symmetric is likely better.
>>>
>>>> In addition, looking at nouveau’s implementation in
>>>> nouveau_dmem_page_alloc_locked(), it gets a folio from drm->dmem->free_folios,
>>>> which is never split, and passes it to zone_device_folio_init(). This
>>>> is wrong, since if the folio is large, it will go through prep_compound_page()
>>>> again. The bug has not manifested because there is only order-9 large folios.
>>>> Once mTHP support is added, how is nouveau going to allocate a order-4 folio
>>>> from a free order-9 folio? Maintain a per-order free folio list and
>>>> reimplement a buddy allocator? Nevertheless, nouveau’s implementation
>>>
>>> The way Nouveau handles memory allocations here looks wrong to me—it
>>> should probably use DRM Buddy and convert a block buddy to pages rather
>>> than tracking a free folio list and free page list. But this is not my
>>> driver.
>>>
>>>> is wrong by calling prep_compound_page() on a folio (already compound page).
>>>>
>>>
>>> I don’t disagree that this implementation is questionable.
>>>
>>> So what’s the suggestion here—add folio order to folio_free just to
>>> accommodate Nouveau’s rather odd memory allocation algorithm? That
>>> doesn’t seem right to me either.
>>
>> Splitting the folio in free_zone_device_folio() and passing folio order
>> to folio_free() make sense to me, since after the split, the folio passed
>
> If this is concensous / direction - I can do this but a tree wide
> change.
>
> I do have another question for everyone here - do we think this spliting
> implementation should be considered a Fixes so this can go into 6.19?

IMHO, this should be a fix, since it is wrong to call prep_compound_page()
on a large folio. IIUC this seems to only affect nouveau now, I will let
them to decide.

>
>> to folio_free() contains no order information, but just the used-to-be
>> head page and the remaining 511 pages are free. How does Intel Xe driver
>> handle it without knowing folio order?
>>
>
> It’s a bit convoluted, but folio/page->zone_device_data points to a
> reference-counted object in GPU SVM. When the object’s reference count
> drops to zero, we callback into the driver layer to release the memory.
> In Xe, this is a TTM BO that resolves to a DRM Buddy allocation, which
> is then released. If it’s not clear, our original allocation size
> determines the granularity at which we free the backing store.
>
>> Do we really need the order info in ->folio_free() if the folio is split
>> in free_zone_device_folio()? free_zone_device_folio() should just call
>> ->folio_free() 2^order times to free individual page.
>>
>
> No. If it’s a higher-order folio—let’s say a 2MB folio—we have one
> reference to our GPU SVM object, so we can free the backing in a single
> ->folio_free call.
>
> Now, if that folio gets split at some point into 4KB pages, then we’d
> have 512 references to this object set up in the ->folio_split calls.
> We’d then expect 512 ->folio_free() calls. Same case here: if, for
> whatever reason, we can’t create a 2MB device page during a 2MB
> migration and need to create 512 4KB pages so we'd have 512 references
> to our GPU SVM object.

Thank you for the explanation. Adding folio order to ->folio_free() makes
sense to me now.

Best Regards,
Yan, Zi