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

Matthew Brost
2026-01-15T05:57:53Z

On Thu, Jan 15, 2026 at 04:27:26PM +1100, Alistair Popple wrote:
> On 2026-01-15 at 06:19 +1100, Francois Dugast <francois.dugast@intel.com> 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.
>
> Thanks for fixing, a couple of minor comments below.
>
> > 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 */
>
> This seems odd to me, mainly due to the "magic" number. Why not just clear
> the flags entirely? Or at least explicitly just clear the flags you care about
> which would remove the need for the comment and should let you use the proper
> PageFlag functions.
>

I'm copying this from folio_reset_order [1]. My paranoia about touching
anything related to struct page is high, so I did the same thing
folio_reset_order does here.

[1] https://elixir.bootlin.com/linux/v6.18.5/source/include/linux/mm.h#L1075

> > +#ifdef NR_PAGES_IN_LARGE_FOLIO
> > + ((struct folio *)(new_page - 1))->_nr_pages = 0;
> > +#endif
> > + new_folio->mapping = NULL;
> > + new_folio->pgmap = pgmap; /* Also clear compound head */
> > + new_folio->share = 0; /* fsdax only, unused for device private */
>
> It would be nice if the FS DAX code actually used this as well. Is there a
> reason that change was dropped from the series?
>

I don't have a test platform for FS DAX. In prior revisions, I was just
moving existing FS DAX code to a helper, which I felt confident about.

This revision is slightly different, and I don't feel comfortable
modifying FS DAX code without a test platform. I agree we should update
FS DAX, but that should be done in a follow-up with coordinated testing.

Matt

> > + 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().
> > --
> > 2.43.0
> >