Re: [PATCH v4 2/7] mm/zone_device: Add free_zone_device_folio_prepare() helper

[Balbir Singh <balbirs@nvidia.com>]

On 1/12/26 11:16, Matthew Brost wrote:
> On Mon, Jan 12, 2026 at 11:44:15AM +1100, Balbir Singh wrote:
>> On 1/12/26 06:55, Francois Dugast wrote:
>>> From: Matthew Brost <matthew.brost@intel.com>
>>>
>>> Add free_zone_device_folio_prepare(), a helper that restores large
>>> ZONE_DEVICE folios to a sane, initial state before freeing them.
>>>
>>> Compound ZONE_DEVICE folios overwrite per-page state (e.g. pgmap and
>>> compound metadata). Before returning such pages to the device pgmap
>>> allocator, each constituent page must be reset to a standalone
>>> ZONE_DEVICE folio with a valid pgmap and no compound state.
>>>
>>> Use this helper prior to folio_free() for device-private and
>>> device-coherent folios to ensure consistent device page state for
>>> subsequent allocations.
>>>
>>> Fixes: d245f9b4ab80 ("mm/zone_device: support large zone device private folios")
>>> Cc: Zi Yan <ziy@nvidia.com>
>>> Cc: David Hildenbrand <david@kernel.org>
>>> Cc: Oscar Salvador <osalvador@suse.de>
>>> Cc: Andrew Morton <akpm@linux-foundation.org>
>>> Cc: Balbir Singh <balbirs@nvidia.com>
>>> Cc: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
>>> Cc: Liam R. Howlett <Liam.Howlett@oracle.com>
>>> Cc: Vlastimil Babka <vbabka@suse.cz>
>>> Cc: Mike Rapoport <rppt@kernel.org>
>>> Cc: Suren Baghdasaryan <surenb@google.com>
>>> Cc: Michal Hocko <mhocko@suse.com>
>>> Cc: Alistair Popple <apopple@nvidia.com>
>>> Cc: linux-mm@kvack.org
>>> Cc: linux-cxl@vger.kernel.org
>>> Cc: linux-kernel@vger.kernel.org
>>> Suggested-by: Alistair Popple <apopple@nvidia.com>
>>> Signed-off-by: Matthew Brost <matthew.brost@intel.com>
>>> Signed-off-by: Francois Dugast <francois.dugast@intel.com>
>>> ---
>>>  include/linux/memremap.h |  1 +
>>>  mm/memremap.c            | 55 ++++++++++++++++++++++++++++++++++++++++
>>>  2 files changed, 56 insertions(+)
>>>
>>> diff --git a/include/linux/memremap.h b/include/linux/memremap.h
>>> index 97fcffeb1c1e..88e1d4707296 100644
>>> --- a/include/linux/memremap.h
>>> +++ b/include/linux/memremap.h
>>> @@ -230,6 +230,7 @@ static inline bool is_fsdax_page(const struct page *page)
>>>  
>>>  #ifdef CONFIG_ZONE_DEVICE
>>>  void zone_device_page_init(struct page *page, unsigned int order);
>>> +void free_zone_device_folio_prepare(struct folio *folio);
>>>  void *memremap_pages(struct dev_pagemap *pgmap, int nid);
>>>  void memunmap_pages(struct dev_pagemap *pgmap);
>>>  void *devm_memremap_pages(struct device *dev, struct dev_pagemap *pgmap);
>>> diff --git a/mm/memremap.c b/mm/memremap.c
>>> index 39dc4bd190d0..375a61e18858 100644
>>> --- a/mm/memremap.c
>>> +++ b/mm/memremap.c
>>> @@ -413,6 +413,60 @@ struct dev_pagemap *get_dev_pagemap(unsigned long pfn)
>>>  }
>>>  EXPORT_SYMBOL_GPL(get_dev_pagemap);
>>>  
>>> +/**
>>> + * free_zone_device_folio_prepare() - Prepare a ZONE_DEVICE folio for freeing.
>>> + * @folio: ZONE_DEVICE folio to prepare for release.
>>> + *
>>> + * ZONE_DEVICE pages/folios (e.g., device-private memory or fsdax-backed pages)
>>> + * can be compound. When freeing a compound ZONE_DEVICE folio, the tail pages
>>> + * must be restored to a sane ZONE_DEVICE state before they are released.
>>> + *
>>> + * This helper:
>>> + *   - Clears @folio->mapping and, for compound folios, clears each page's
>>> + *     compound-head state (ClearPageHead()/clear_compound_head()).
>>> + *   - Resets the compound order metadata (folio_reset_order()) and then
>>> + *     initializes each constituent page as a standalone ZONE_DEVICE folio:
>>> + *       * clears ->mapping
>>> + *       * restores ->pgmap (prep_compound_page() overwrites it)
>>> + *       * clears ->share (only relevant for fsdax; unused for device-private)
>>> + *
>>> + * If @folio is order-0, only the mapping is cleared and no further work is
>>> + * required.
>>> + */
>>> +void free_zone_device_folio_prepare(struct folio *folio)
>>> +{
>>> +	struct dev_pagemap *pgmap = page_pgmap(&folio->page);
>>> +	int order, i;
>>> +
>>> +	VM_WARN_ON_FOLIO(!folio_is_zone_device(folio), folio);
>>> +
>>> +	folio->mapping = NULL;
>>> +	order = folio_order(folio);
>>> +	if (!order)
>>> +		return;
>>> +
>>> +	folio_reset_order(folio);
>>> +
>>> +	for (i = 0; i < (1UL << order); i++) {
>>> +		struct page *page = folio_page(folio, i);
>>> +		struct folio *new_folio = (struct folio *)page;
>>> +
>>> +		ClearPageHead(page);
>>> +		clear_compound_head(page);
>>> +
>>> +		new_folio->mapping = NULL;
>>> +		/*
>>> +		 * Reset pgmap which was over-written by
>>> +		 * prep_compound_page().
>>> +		 */
>>> +		new_folio->pgmap = pgmap;
>>> +		new_folio->share = 0;	/* fsdax only, unused for device private */
>>> +		VM_WARN_ON_FOLIO(folio_ref_count(new_folio), new_folio);
>>> +		VM_WARN_ON_FOLIO(!folio_is_zone_device(new_folio), new_folio);
>>
>> Does calling the free_folio() callback on new_folio solve the issue you are facing, or is
>> that PMD_ORDER more frees than we'd like?
>>
> 
> No, calling free_folio() more often doesn’t solve anything—in fact, that
> would make my implementation explode. I explained this in detail here [1]
> to Zi.
> 
> To recap [1], my memory allocator has no visibility into individual
> pages or folios; it is DRM Buddy layered on top of TTM BO. This design
> allows VRAM to be allocated or evicted for both traditional GPU
> allocations (GEMs) and SVM allocations.
> 

I assume it is still backed by pages that are ref counted? I suspect you'd
need to convert one reference count to PMD_ORDER reference counts to make
this change work, or are the references not at page granularity? 

I followed the code through drm_zdd_pagemap_put() and zdd->refcount seemed
like a per folio refcount

> Now, to recap the actual issue: if device folios are not split upon free
> and are later reallocated with a different order in
> zone_device_page_init, the implementation breaks. This problem is not
> specific to Xe—Nouveau happens to always allocate at the same order, so
> it works by coincidence. Reallocating at a different order is valid
> behavior and must be supported.
> 

Agreed

> Matt
> 
> [1] https://patchwork.freedesktop.org/patch/697710/?series=159119&rev=3#comment_1282413
> 
>>> +	}
>>> +}
>>> +EXPORT_SYMBOL_GPL(free_zone_device_folio_prepare);
>>> +
>>>  void free_zone_device_folio(struct folio *folio)
>>>  {
>>>  	struct dev_pagemap *pgmap = folio->pgmap;
>>> @@ -454,6 +508,7 @@ void free_zone_device_folio(struct folio *folio)
>>>  	case MEMORY_DEVICE_COHERENT:
>>>  		if (WARN_ON_ONCE(!pgmap->ops || !pgmap->ops->folio_free))
>>>  			break;
>>> +		free_zone_device_folio_prepare(folio);
>>>  		pgmap->ops->folio_free(folio, order);
>>>  		percpu_ref_put_many(&folio->pgmap->ref, nr);
>>>  		break;
>>
>> Balbir