On Thu, Jan 15, 2026 at 06:07:08PM +1100, Alistair Popple wrote:
> On 2026-01-15 at 17:18 +1100, Matthew Brost <[email protected]> wrote...
> > On Wed, Jan 14, 2026 at 09:57:31PM -0800, Matthew Brost wrote:
> > > On Thu, Jan 15, 2026 at 04:27:26PM +1100, Alistair Popple wrote:
> > > > On 2026-01-15 at 06:19 +1100, Francois Dugast
> > > > <[email protected]> wrote...
> > > > > From: Matthew Brost <[email protected]>
> > > > >
> > > > > 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 <[email protected]>
> > > > > Cc: Alistair Popple <[email protected]>
> > > > > Cc: adhavan Srinivasan <[email protected]>
> > > > > Cc: Nicholas Piggin <[email protected]>
> > > > > Cc: Michael Ellerman <[email protected]>
> > > > > Cc: "Christophe Leroy (CS GROUP)" <[email protected]>
> > > > > Cc: Felix Kuehling <[email protected]>
> > > > > Cc: Alex Deucher <[email protected]>
> > > > > Cc: "Christian König" <[email protected]>
> > > > > Cc: David Airlie <[email protected]>
> > > > > Cc: Simona Vetter <[email protected]>
> > > > > Cc: Maarten Lankhorst <[email protected]>
> > > > > Cc: Maxime Ripard <[email protected]>
> > > > > Cc: Thomas Zimmermann <[email protected]>
> > > > > Cc: Lyude Paul <[email protected]>
> > > > > Cc: Danilo Krummrich <[email protected]>
> > > > > Cc: David Hildenbrand <[email protected]>
> > > > > Cc: Oscar Salvador <[email protected]>
> > > > > Cc: Andrew Morton <[email protected]>
> > > > > Cc: Jason Gunthorpe <[email protected]>
> > > > > Cc: Leon Romanovsky <[email protected]>
> > > > > Cc: Lorenzo Stoakes <[email protected]>
> > > > > Cc: Liam R. Howlett <[email protected]>
> > > > > Cc: Vlastimil Babka <[email protected]>
> > > > > Cc: Mike Rapoport <[email protected]>
> > > > > Cc: Suren Baghdasaryan <[email protected]>
> > > > > Cc: Michal Hocko <[email protected]>
> > > > > Cc: Balbir Singh <[email protected]>
> > > > > Cc: [email protected]
> > > > > Cc: [email protected]
> > > > > Cc: [email protected]
> > > > > Cc: [email protected]
> > > > > Cc: [email protected]
> > > > > Cc: [email protected]
> > > > > Cc: [email protected]
> > > > > Cc: [email protected]
> > > > > Fixes: d245f9b4ab80 ("mm/zone_device: support large zone device
> > > > > private folios")
> > > > > Signed-off-by: Matthew Brost <[email protected]>
> > > > > Signed-off-by: Francois Dugast <[email protected]>
> > > > > ---
> > > > > 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.
>
> So why not just use folio_reset_order() below?
>
> > >
> > > [1]
> > > https://elixir.bootlin.com/linux/v6.18.5/source/include/linux/mm.h#L1075
> > >
> >
> > This immediately hangs my first SVM test...
> >
> > diff --git a/mm/memremap.c b/mm/memremap.c
> > index 6f46ab14662b..ef8c56876cf5 100644
> > --- a/mm/memremap.c
> > +++ b/mm/memremap.c
> > @@ -488,7 +488,7 @@ void zone_device_page_init(struct page *page, struct
> > dev_pagemap *pgmap,
> > 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 */
> > + new_page->flags.f = 0;
> > #ifdef NR_PAGES_IN_LARGE_FOLIO
> > ((struct folio *)(new_page - 1))->_nr_pages = 0;
>
> This seems wrong to me - I saw your reply to Balbir but for an order-0 page
> isn't this going to access a completely different, possibly already allocated,
> page?
>
No — it accesses itself (new_page). It just uses some odd memory tricks
for this, which I agree isn’t the best thing I’ve ever written, but it
was the least-worst idea I had. I didn’t design the folio/page field
aliasing; I understand why it exists, but it still makes my head hurt.
folio->_nr_pages is page + 1 for reference (new_page after this math).
Again, if I touched this memory directly in new_page, it’s most likely
memcg_data, but that is hidden behind a Kconfig.
This just blindly implementing part of folio_reset_order which clears
_nr_pages.
> > #endif
> >
> > I can walk through exactly which flags need to be cleared, but my
> > feeling is that likely any flag that the order field overloads and can
> > possibly encode should be cleared—so bits 0–7 based on the existing
> > code.
> >
> > How about in a follow-up we normalize setting / clearing the order flag
> > field with a #define and an inline helper?
>
> Ie: Would something like the following work:
>
> ClearPageHead(new_page);
Any of these bit could possibly be set the order field in a folio, which
modifies page + 1 flags field.
PG_locked, /* Page is locked. Don't touch. */
PG_writeback, /* Page is under writeback */
PG_referenced,
PG_uptodate,
PG_dirty,
PG_lru,
PG_head, /* Must be in bit 6 */
PG_waiters, /* Page has waiters, check its waitqueue. Must
be bit #7 and in the same byte as "PG_locked" */
So a common order-9 (2MB) folio would have PG_locked | PG_uptodate set.
Now we get stuck on the next page lock because PG_locked is set.
Offhand, I don’t know if different orders—which set different bits—cause
any nasty issues either. So I figured the safest thing was clear any
bits which folio order can set within subsequent page's memory flags
like folio_reset_order does.
> clear_compound_head(new_page);
> folio_reset_order(new_folio);
>
> Which would also deal with setting _nr_pages.
>
folio_reset_order(new_folio) would set _nr_pages in the memory that is
new_page + 1. So let's say that page has a ref count + memcg_data, now
that memory is corrupted and will crash the kernel.
All of the above is why is took me multiple hours to write 6 lines of
code :).
> > Matt
> >
> > > > > +#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.
>
> Fair enough, I figured something like this might be your answer :-) You
> could update it and ask people with access to such a system to test it though
> (unfortunately my setup has bit-rotted beyond repair).
>
> But I'm ok leaving to for a future change.
>
I did a quick grep in fs/dax.c and don’t see zone_device_page_init
called there. It probably could be used if it’s creating compound pages
and drop the open-coded reinit when shared == 0, but yeah, that’s not
something I can blindly code without testing.
I can try to put something together for people to test soonish.
Matt
> > >
> > > 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
> > > > >
