Re: [PATCH v5 1/5] mm/zone_device: Reinitialize large zone device private folios

[Matthew Brost <matthew.brost@intel.com>]

On Thu, Jan 15, 2026 at 01:36:11PM +1100, Balbir Singh wrote:
> On 1/15/26 06:19, Francois Dugast wrote:
> > From: Matthew Brost <matthew.brost@intel.com>
> > 
> > Reinitialize metadata for large zone device private folios in
> > zone_device_page_init prior to creating a higher-order zone device
> > private folio. This step is necessary when the folio’s order changes
> > dynamically between zone_device_page_init calls to avoid building a
> > corrupt folio. As part of the metadata reinitialization, the dev_pagemap
> > must be passed in from the caller because the pgmap stored in the folio
> > page may have been overwritten with a compound head.
> > 
> > Cc: Zi Yan <ziy@nvidia.com>
> > Cc: Alistair Popple <apopple@nvidia.com>
> > Cc: adhavan Srinivasan <maddy@linux.ibm.com>
> > Cc: Nicholas Piggin <npiggin@gmail.com>
> > Cc: Michael Ellerman <mpe@ellerman.id.au>
> > Cc: "Christophe Leroy (CS GROUP)" <chleroy@kernel.org>
> > Cc: Felix Kuehling <Felix.Kuehling@amd.com>
> > Cc: Alex Deucher <alexander.deucher@amd.com>
> > Cc: "Christian König" <christian.koenig@amd.com>
> > Cc: David Airlie <airlied@gmail.com>
> > Cc: Simona Vetter <simona@ffwll.ch>
> > Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
> > Cc: Maxime Ripard <mripard@kernel.org>
> > Cc: Thomas Zimmermann <tzimmermann@suse.de>
> > Cc: Lyude Paul <lyude@redhat.com>
> > Cc: Danilo Krummrich <dakr@kernel.org>
> > Cc: David Hildenbrand <david@kernel.org>
> > Cc: Oscar Salvador <osalvador@suse.de>
> > Cc: Andrew Morton <akpm@linux-foundation.org>
> > Cc: Jason Gunthorpe <jgg@ziepe.ca>
> > Cc: Leon Romanovsky <leon@kernel.org>
> > 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: Balbir Singh <balbirs@nvidia.com>
> > Cc: linuxppc-dev@lists.ozlabs.org
> > Cc: kvm@vger.kernel.org
> > Cc: linux-kernel@vger.kernel.org
> > Cc: amd-gfx@lists.freedesktop.org
> > Cc: dri-devel@lists.freedesktop.org
> > Cc: nouveau@lists.freedesktop.org
> > Cc: linux-mm@kvack.org
> > Cc: linux-cxl@vger.kernel.org
> > Fixes: d245f9b4ab80 ("mm/zone_device: support large zone device private folios")
> > Signed-off-by: Matthew Brost <matthew.brost@intel.com>
> > Signed-off-by: Francois Dugast <francois.dugast@intel.com>
> > ---
> >  arch/powerpc/kvm/book3s_hv_uvmem.c       |  2 +-
> >  drivers/gpu/drm/amd/amdkfd/kfd_migrate.c |  2 +-
> >  drivers/gpu/drm/drm_pagemap.c            |  2 +-
> >  drivers/gpu/drm/nouveau/nouveau_dmem.c   |  2 +-
> >  include/linux/memremap.h                 |  9 ++++++---
> >  lib/test_hmm.c                           |  4 +++-
> >  mm/memremap.c                            | 20 +++++++++++++++++++-
> >  7 files changed, 32 insertions(+), 9 deletions(-)
> > 
> > diff --git a/arch/powerpc/kvm/book3s_hv_uvmem.c b/arch/powerpc/kvm/book3s_hv_uvmem.c
> > index e5000bef90f2..7cf9310de0ec 100644
> > --- a/arch/powerpc/kvm/book3s_hv_uvmem.c
> > +++ b/arch/powerpc/kvm/book3s_hv_uvmem.c
> > @@ -723,7 +723,7 @@ static struct page *kvmppc_uvmem_get_page(unsigned long gpa, struct kvm *kvm)
> >  
> >  	dpage = pfn_to_page(uvmem_pfn);
> >  	dpage->zone_device_data = pvt;
> > -	zone_device_page_init(dpage, 0);
> > +	zone_device_page_init(dpage, &kvmppc_uvmem_pgmap, 0);
> >  	return dpage;
> >  out_clear:
> >  	spin_lock(&kvmppc_uvmem_bitmap_lock);
> > diff --git a/drivers/gpu/drm/amd/amdkfd/kfd_migrate.c b/drivers/gpu/drm/amd/amdkfd/kfd_migrate.c
> > index af53e796ea1b..6ada7b4af7c6 100644
> > --- a/drivers/gpu/drm/amd/amdkfd/kfd_migrate.c
> > +++ b/drivers/gpu/drm/amd/amdkfd/kfd_migrate.c
> > @@ -217,7 +217,7 @@ svm_migrate_get_vram_page(struct svm_range *prange, unsigned long pfn)
> >  	page = pfn_to_page(pfn);
> >  	svm_range_bo_ref(prange->svm_bo);
> >  	page->zone_device_data = prange->svm_bo;
> > -	zone_device_page_init(page, 0);
> > +	zone_device_page_init(page, page_pgmap(page), 0);
> >  }
> >  
> >  static void
> > diff --git a/drivers/gpu/drm/drm_pagemap.c b/drivers/gpu/drm/drm_pagemap.c
> > index 03ee39a761a4..c497726b0147 100644
> > --- a/drivers/gpu/drm/drm_pagemap.c
> > +++ b/drivers/gpu/drm/drm_pagemap.c
> > @@ -201,7 +201,7 @@ static void drm_pagemap_get_devmem_page(struct page *page,
> >  					struct drm_pagemap_zdd *zdd)
> >  {
> >  	page->zone_device_data = drm_pagemap_zdd_get(zdd);
> > -	zone_device_page_init(page, 0);
> > +	zone_device_page_init(page, zdd->dpagemap->pagemap, 0);
> >  }
> >  
> >  /**
> > diff --git a/drivers/gpu/drm/nouveau/nouveau_dmem.c b/drivers/gpu/drm/nouveau/nouveau_dmem.c
> > index 58071652679d..3d8031296eed 100644
> > --- a/drivers/gpu/drm/nouveau/nouveau_dmem.c
> > +++ b/drivers/gpu/drm/nouveau/nouveau_dmem.c
> > @@ -425,7 +425,7 @@ nouveau_dmem_page_alloc_locked(struct nouveau_drm *drm, bool is_large)
> >  			order = ilog2(DMEM_CHUNK_NPAGES);
> >  	}
> >  
> > -	zone_device_folio_init(folio, order);
> > +	zone_device_folio_init(folio, page_pgmap(folio_page(folio, 0)), order);
> >  	return page;
> >  }
> >  
> > diff --git a/include/linux/memremap.h b/include/linux/memremap.h
> > index 713ec0435b48..e3c2ccf872a8 100644
> > --- a/include/linux/memremap.h
> > +++ b/include/linux/memremap.h
> > @@ -224,7 +224,8 @@ 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 zone_device_page_init(struct page *page, struct dev_pagemap *pgmap,
> > +			   unsigned int order);
> >  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);
> > @@ -234,9 +235,11 @@ bool pgmap_pfn_valid(struct dev_pagemap *pgmap, unsigned long pfn);
> >  
> >  unsigned long memremap_compat_align(void);
> >  
> > -static inline void zone_device_folio_init(struct folio *folio, unsigned int order)
> > +static inline void zone_device_folio_init(struct folio *folio,
> > +					  struct dev_pagemap *pgmap,
> > +					  unsigned int order)
> >  {
> > -	zone_device_page_init(&folio->page, order);
> > +	zone_device_page_init(&folio->page, pgmap, order);
> >  	if (order)
> >  		folio_set_large_rmappable(folio);
> >  }
> > diff --git a/lib/test_hmm.c b/lib/test_hmm.c
> > index 8af169d3873a..455a6862ae50 100644
> > --- a/lib/test_hmm.c
> > +++ b/lib/test_hmm.c
> > @@ -662,7 +662,9 @@ static struct page *dmirror_devmem_alloc_page(struct dmirror *dmirror,
> >  			goto error;
> >  	}
> >  
> > -	zone_device_folio_init(page_folio(dpage), order);
> > +	zone_device_folio_init(page_folio(dpage),
> > +			       page_pgmap(folio_page(page_folio(dpage), 0)),
> > +			       order);
> >  	dpage->zone_device_data = rpage;
> >  	return dpage;
> >  
> > diff --git a/mm/memremap.c b/mm/memremap.c
> > index 63c6ab4fdf08..6f46ab14662b 100644
> > --- a/mm/memremap.c
> > +++ b/mm/memremap.c
> > @@ -477,10 +477,28 @@ void free_zone_device_folio(struct folio *folio)
> >  	}
> >  }
> >  
> > -void zone_device_page_init(struct page *page, unsigned int order)
> > +void zone_device_page_init(struct page *page, struct dev_pagemap *pgmap,
> > +			   unsigned int order)
> >  {
> > +	struct page *new_page = page;
> > +	unsigned int i;
> > +
> >  	VM_WARN_ON_ONCE(order > MAX_ORDER_NR_PAGES);
> >  
> > +	for (i = 0; i < (1UL << order); ++i, ++new_page) {
> > +		struct folio *new_folio = (struct folio *)new_page;
> > +
> > +		new_page->flags.f &= ~0xffUL;	/* Clear possible order, page head */
> > +#ifdef NR_PAGES_IN_LARGE_FOLIO
> > +		((struct folio *)(new_page - 1))->_nr_pages = 0;
> > +#endif
> 
> Not sure I follow the new_page - 1? What happens when order is 0?
> 

This is just to get _nr_pages in the new_page as folio->_nr_pages is in
the folio's second page. So it just modifying itself. I agree this is a
bit goofy but couldn't think of a better way to do this. In the page
structure this is the memcg_data field on most builds.

Matt

> > +		new_folio->mapping = NULL;
> > +		new_folio->pgmap = pgmap;	/* Also clear compound head */
> > +		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);
> > +	}
> > +
> >  	/*
> >  	 * Drivers shouldn't be allocating pages after calling
> >  	 * memunmap_pages().
> 
> I wish we did not have to pass in the pgmap, but I can see why
> we can't rely on the existing pgmap
> 
> Balbir
>