Message-ID: <c034a80b-1fb5-4e20-b9d8-6f079cdc1d09@suse.cz>
Date: Fri, 16 Jan 2026 17:13:50 +0100
From: Vlastimil Babka <vbabka@...e.cz>
To: Matthew Brost <matthew.brost@...el.com>,
 Alistair Popple <apopple@...dia.com>
Cc: 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>, Mike Rapoport
 <rppt@...nel.org>, Suren Baghdasaryan <surenb@...gle.com>,
 Michal Hocko <mhocko@...e.com>, Balbir Singh <balbirs@...dia.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 1/15/26 07:18, Matthew Brost 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 <francois.dugast@...el.com> 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.
>> > 
>> > Thanks for fixing, a couple of minor comments below.
>> > 
>> > > 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 */
>> > 
>> > 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
>> 
> 
> 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;
>  #endif

the flags field includes also zone and node id, so clearing that will break
the pages's node+zone association, that's probably why...

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