Message-ID: <nvhjddzmdf34cbjnxmka6ir6smkokn7dlp2aa36ymvofmbo3qh@lugxvx5xlvud>
Date: Thu, 15 Jan 2026 22:05:00 +1100
From: Alistair Popple <apopple@...dia.com>
To: Matthew Brost <matthew.brost@...el.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>, Vlastimil Babka <vbabka@...e.cz>, 
	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 2026-01-15 at 18:43 +1100, Matthew Brost <matthew.brost@...el.com> wrote...
> On Thu, Jan 15, 2026 at 06:07:08PM +1100, Alistair Popple wrote:
> > On 2026-01-15 at 17:18 +1100, Matthew Brost <matthew.brost@...el.com> 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.
> > 
> > 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.

And obviously mine, because I (was) still not getting it and had typed up a
whole response and code walk through to show what was wrong, in the hope it
would help settle the misunderstanding. Which it did, because I discovered
where I was getting things wrong. But I've left the analysis below because it's
probably useful for others following along:

Walking through the code we have:

void zone_device_page_init(struct page *page, struct dev_pagemap *pgmap,
			   unsigned int order)
{

The first argument, page, is the first in a set of 1 << order contiguous
struct page. In the simplest case order == 0, meaning this function should only
initialise (ie. touch) a single struct page pointer which is passed as the first
argument to the function.

	struct page *new_page = page;

So now *new_page points to the single struct page we should touch.
	
	unsigned int i;

	VM_WARN_ON_ONCE(order > MAX_ORDER_NR_PAGES);

	for (i = 0; i < (1UL << order); ++i, ++new_page) {

order == 0, so this loop will only execute once.

		struct folio *new_folio = (struct folio *)new_page;

new_page still points to the single page we're initialising, and new_folio
points to the same page. Ie: &new_folio->page == new_page. There is a hazard
here because new_folio->__page_1, __page_2, etc. all point to pages we shouldn't
touch.

		new_page->flags.f &= ~0xffUL;	/* Clear possible order, page head */

Clears the flags, makes sense.

#ifdef NR_PAGES_IN_LARGE_FOLIO
		((struct folio *)(new_page - 1))->_nr_pages = 0;

If we break this down we have:

struct page *tmp_new_page = new_page - 1;

Which is the page before the one we're initialising and shouldn't be touched.
Then we cast to a folio:

struct folio *tmp_new_folio = (struct folio *) tmp_new_page;

And reset _nr_pages:

tmp_new_folio->_nr_pages = 0

And now I can see where I was confused - &tmp_new_folio->_nr_pages == &tmp_new_folio->__page_1->memcg_data == &new_page->memcg_data

So after both Balbir, probably yourself, and definitely myself scratching our
heads for way too long over this change I think we can conclude that the code as
is is way too confusing to merge without a lot more comments :-)

However why go through all this magic in the first place? Why not just treat
everything here as a page and just do

	new_page->memcg_data = 0

directly? That seems like the more straight forward approach. In fact given
all the confusion I wonder if it wouldn't be better to just do
memset(new_page, 0, sizeof(*new_page)) and reinitialise everything from
scratch.

> 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.

Yeah, I get it now. But I think just clearing memcg_data would be the easiest to
understand approach, especially if it had a comment explaining that it may have
previously been used for _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.

Oh, I get the above. I was thinking folio_reset_order() below would clear the
flags, but I see the folly there - that resets the flags for the next page.

> 
> > 		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.

Yep, I just noticed that. Thanks for pointing that out.

> All of the above is why is took me multiple hours to write 6 lines of
> code :).

And to review :) Good thing we don't get paid per SLOC of code right?

 - Alistair

> > > 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
> > > > > > 

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