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Message-ID: <aWieAOZx4Ghua6Bv@lstrano-desk.jf.intel.com>
Date: Wed, 14 Jan 2026 23:57:52 -0800
From: Matthew Brost <matthew.brost@...el.com>
To: Alistair Popple <apopple@...dia.com>
CC: Balbir Singh <balbirs@...dia.com>, Francois Dugast
<francois.dugast@...el.com>, <intel-xe@...ts.freedesktop.org>,
<dri-devel@...ts.freedesktop.org>, Zi Yan <ziy@...dia.com>, "adhavan
Srinivasan" <maddy@...ux.ibm.com>, Nicholas Piggin <npiggin@...il.com>,
Michael Ellerman <mpe@...erman.id.au>, "Christophe Leroy (CS GROUP)"
<chleroy@...nel.org>, Felix Kuehling <Felix.Kuehling@....com>, Alex Deucher
<alexander.deucher@....com>, Christian König
<christian.koenig@....com>, David Airlie <airlied@...il.com>, Simona Vetter
<simona@...ll.ch>, Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
Maxime Ripard <mripard@...nel.org>, Thomas Zimmermann <tzimmermann@...e.de>,
Lyude Paul <lyude@...hat.com>, Danilo Krummrich <dakr@...nel.org>, "David
Hildenbrand" <david@...nel.org>, Oscar Salvador <osalvador@...e.de>, "Andrew
Morton" <akpm@...ux-foundation.org>, Jason Gunthorpe <jgg@...pe.ca>, "Leon
Romanovsky" <leon@...nel.org>, Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
"Liam R . Howlett" <Liam.Howlett@...cle.com>, Vlastimil Babka
<vbabka@...e.cz>, Mike Rapoport <rppt@...nel.org>, Suren Baghdasaryan
<surenb@...gle.com>, Michal Hocko <mhocko@...e.com>,
<linuxppc-dev@...ts.ozlabs.org>, <kvm@...r.kernel.org>,
<linux-kernel@...r.kernel.org>, <amd-gfx@...ts.freedesktop.org>,
<nouveau@...ts.freedesktop.org>, <linux-mm@...ck.org>,
<linux-cxl@...r.kernel.org>
Subject: Re: [PATCH v5 1/5] mm/zone_device: Reinitialize large zone device
private folios
On Thu, Jan 15, 2026 at 06:13:15PM +1100, Alistair Popple wrote:
> On 2026-01-15 at 13:41 +1100, Matthew Brost <matthew.brost@...el.com> wrote...
> > 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@...el.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@...dia.com>
> > > > Cc: Alistair Popple <apopple@...dia.com>
> > > > Cc: adhavan Srinivasan <maddy@...ux.ibm.com>
> > > > Cc: Nicholas Piggin <npiggin@...il.com>
> > > > Cc: Michael Ellerman <mpe@...erman.id.au>
> > > > Cc: "Christophe Leroy (CS GROUP)" <chleroy@...nel.org>
> > > > Cc: Felix Kuehling <Felix.Kuehling@....com>
> > > > Cc: Alex Deucher <alexander.deucher@....com>
> > > > Cc: "Christian König" <christian.koenig@....com>
> > > > Cc: David Airlie <airlied@...il.com>
> > > > Cc: Simona Vetter <simona@...ll.ch>
> > > > Cc: Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>
> > > > Cc: Maxime Ripard <mripard@...nel.org>
> > > > Cc: Thomas Zimmermann <tzimmermann@...e.de>
> > > > Cc: Lyude Paul <lyude@...hat.com>
> > > > Cc: Danilo Krummrich <dakr@...nel.org>
> > > > Cc: David Hildenbrand <david@...nel.org>
> > > > Cc: Oscar Salvador <osalvador@...e.de>
> > > > Cc: Andrew Morton <akpm@...ux-foundation.org>
> > > > Cc: Jason Gunthorpe <jgg@...pe.ca>
> > > > Cc: Leon Romanovsky <leon@...nel.org>
> > > > Cc: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
> > > > Cc: Liam R. Howlett <Liam.Howlett@...cle.com>
> > > > Cc: Vlastimil Babka <vbabka@...e.cz>
> > > > Cc: Mike Rapoport <rppt@...nel.org>
> > > > Cc: Suren Baghdasaryan <surenb@...gle.com>
> > > > Cc: Michal Hocko <mhocko@...e.com>
> > > > Cc: Balbir Singh <balbirs@...dia.com>
> > > > Cc: linuxppc-dev@...ts.ozlabs.org
> > > > Cc: kvm@...r.kernel.org
> > > > Cc: linux-kernel@...r.kernel.org
> > > > Cc: amd-gfx@...ts.freedesktop.org
> > > > Cc: dri-devel@...ts.freedesktop.org
> > > > Cc: nouveau@...ts.freedesktop.org
> > > > Cc: linux-mm@...ck.org
> > > > Cc: linux-cxl@...r.kernel.org
> > > > Fixes: d245f9b4ab80 ("mm/zone_device: support large zone device private folios")
> > > > Signed-off-by: Matthew Brost <matthew.brost@...el.com>
> > > > Signed-off-by: Francois Dugast <francois.dugast@...el.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.
>
> I still don't follow - page == new_page == new_folio so isn't
> &new_page->_nr_pages the same as &new_folio->_nr_pages? I don't understand why
> we would care about the a second page here.
>
I just replied to another email—this is quite confusing, but let me try
here...
Memory layout of a folio:
page0
page1 <-- this is where _nr_pages is
...
So ((struct folio *)(new_page - 1))->_nr_pages is pointing to memory at
new_page but using casting to determine the _nr_pages location.
At this point, we have no idea if _nr_pages in new_page was set by a
prior larger folio, so we just blindly clear it, which is safe. This is
no different than what folio_reset_order() does; we just do it for every
single page’s memory within the orderi passed in.
Matt
> - Alistair
>
> >
> > 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
> > >
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