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

[Alistair Popple <apopple@nvidia.com>]

On 2026-01-12 at 12:16 +1100, Matthew Brost <matthew.brost@intel.com> 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)

I don't really like the naming here - we're not preparing a folio to be
freed, from the core-mm perspective the folio is already free. This is just
reinitialising the folio metadata ready for the driver to reuse it, which may
actually involve just recreating a compound folio.

So maybe zone_device_folio_reinitialise()? Or would it be possible to
roll this into a zone_device_folio_init() type function (similar to
zone_device_page_init()) that just deals with everything at allocation time?

> > > +{
> > > +	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.
> 
> 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.

I agree it's probably by coincidence but it is a perfectly valid design to
always just (re)allocate at the same order and not worry about having to
reinitialise things to different orders.

 - Alistair

> 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