Message-ID: <lwbeop6muq4tbrdauwfi42nw5jss7yvgbmls546sxvygivpcwg@persiopzpqed>
Date: Tue, 13 Jan 2026 13:06:02 +1100
From: Alistair Popple <apopple@...dia.com>
To: Matthew Brost <matthew.brost@...el.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>, David Hildenbrand <david@...nel.org>, 
	Oscar Salvador <osalvador@...e.de>, Andrew Morton <akpm@...ux-foundation.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>, linux-mm@...ck.org, 
	linux-cxl@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 2/7] mm/zone_device: Add
 free_zone_device_folio_prepare() helper

On 2026-01-13 at 12:40 +1100, Matthew Brost <matthew.brost@...el.com> wrote...
> On Tue, Jan 13, 2026 at 12:35:31PM +1100, Alistair Popple wrote:
> > On 2026-01-13 at 12:07 +1100, Matthew Brost <matthew.brost@...el.com> wrote...
> > > On Tue, Jan 13, 2026 at 11:43:51AM +1100, Alistair Popple wrote:
> > > > On 2026-01-13 at 11:23 +1100, Matthew Brost <matthew.brost@...el.com> wrote...
> > > > > On Tue, Jan 13, 2026 at 10:58:27AM +1100, Alistair Popple wrote:
> > > > > > On 2026-01-12 at 12:16 +1100, Matthew Brost <matthew.brost@...el.com> wrote...
> > > > > > > On Mon, Jan 12, 2026 at 11:44:15AM +1100, Balbir Singh wrote:
> > > > > > > > On 1/12/26 06:55, Francois Dugast wrote:
> > > > > > > > > From: Matthew Brost <matthew.brost@...el.com>
> > > > > > > > > 
> > > > > > > > > Add free_zone_device_folio_prepare(), a helper that restores large
> > > > > > > > > ZONE_DEVICE folios to a sane, initial state before freeing them.
> > > > > > > > > 
> > > > > > > > > Compound ZONE_DEVICE folios overwrite per-page state (e.g. pgmap and
> > > > > > > > > compound metadata). Before returning such pages to the device pgmap
> > > > > > > > > allocator, each constituent page must be reset to a standalone
> > > > > > > > > ZONE_DEVICE folio with a valid pgmap and no compound state.
> > > > > > > > > 
> > > > > > > > > Use this helper prior to folio_free() for device-private and
> > > > > > > > > device-coherent folios to ensure consistent device page state for
> > > > > > > > > subsequent allocations.
> > > > > > > > > 
> > > > > > > > > Fixes: d245f9b4ab80 ("mm/zone_device: support large zone device private folios")
> > > > > > > > > Cc: Zi Yan <ziy@...dia.com>
> > > > > > > > > Cc: David Hildenbrand <david@...nel.org>
> > > > > > > > > Cc: Oscar Salvador <osalvador@...e.de>
> > > > > > > > > Cc: Andrew Morton <akpm@...ux-foundation.org>
> > > > > > > > > Cc: Balbir Singh <balbirs@...dia.com>
> > > > > > > > > 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: Alistair Popple <apopple@...dia.com>
> > > > > > > > > Cc: linux-mm@...ck.org
> > > > > > > > > Cc: linux-cxl@...r.kernel.org
> > > > > > > > > Cc: linux-kernel@...r.kernel.org
> > > > > > > > > Suggested-by: Alistair Popple <apopple@...dia.com>
> > > > > > > > > Signed-off-by: Matthew Brost <matthew.brost@...el.com>
> > > > > > > > > Signed-off-by: Francois Dugast <francois.dugast@...el.com>
> > > > > > > > > ---
> > > > > > > > >  include/linux/memremap.h |  1 +
> > > > > > > > >  mm/memremap.c            | 55 ++++++++++++++++++++++++++++++++++++++++
> > > > > > > > >  2 files changed, 56 insertions(+)
> > > > > > > > > 
> > > > > > > > > diff --git a/include/linux/memremap.h b/include/linux/memremap.h
> > > > > > > > > index 97fcffeb1c1e..88e1d4707296 100644
> > > > > > > > > --- a/include/linux/memremap.h
> > > > > > > > > +++ b/include/linux/memremap.h
> > > > > > > > > @@ -230,6 +230,7 @@ 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 free_zone_device_folio_prepare(struct folio *folio);
> > > > > > > > >  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);
> > > > > > > > > diff --git a/mm/memremap.c b/mm/memremap.c
> > > > > > > > > index 39dc4bd190d0..375a61e18858 100644
> > > > > > > > > --- a/mm/memremap.c
> > > > > > > > > +++ b/mm/memremap.c
> > > > > > > > > @@ -413,6 +413,60 @@ struct dev_pagemap *get_dev_pagemap(unsigned long pfn)
> > > > > > > > >  }
> > > > > > > > >  EXPORT_SYMBOL_GPL(get_dev_pagemap);
> > > > > > > > >  
> > > > > > > > > +/**
> > > > > > > > > + * free_zone_device_folio_prepare() - Prepare a ZONE_DEVICE folio for freeing.
> > > > > > > > > + * @folio: ZONE_DEVICE folio to prepare for release.
> > > > > > > > > + *
> > > > > > > > > + * ZONE_DEVICE pages/folios (e.g., device-private memory or fsdax-backed pages)
> > > > > > > > > + * can be compound. When freeing a compound ZONE_DEVICE folio, the tail pages
> > > > > > > > > + * must be restored to a sane ZONE_DEVICE state before they are released.
> > > > > > > > > + *
> > > > > > > > > + * This helper:
> > > > > > > > > + *   - Clears @folio->mapping and, for compound folios, clears each page's
> > > > > > > > > + *     compound-head state (ClearPageHead()/clear_compound_head()).
> > > > > > > > > + *   - Resets the compound order metadata (folio_reset_order()) and then
> > > > > > > > > + *     initializes each constituent page as a standalone ZONE_DEVICE folio:
> > > > > > > > > + *       * clears ->mapping
> > > > > > > > > + *       * restores ->pgmap (prep_compound_page() overwrites it)
> > > > > > > > > + *       * clears ->share (only relevant for fsdax; unused for device-private)
> > > > > > > > > + *
> > > > > > > > > + * If @folio is order-0, only the mapping is cleared and no further work is
> > > > > > > > > + * required.
> > > > > > > > > + */
> > > > > > > > > +void free_zone_device_folio_prepare(struct folio *folio)
> > > > > > 
> > > > > > I don't really like the naming here - we're not preparing a folio to be
> > > > > > freed, from the core-mm perspective the folio is already free. This is just
> > > > > > reinitialising the folio metadata ready for the driver to reuse it, which may
> > > > > > actually involve just recreating a compound folio.
> > > > > > 
> > > > > > So maybe zone_device_folio_reinitialise()? Or would it be possible to
> > > > > 
> > > > > zone_device_folio_reinitialise - that works for me... but seem like
> > > > > everyone has a opinion. 
> > > > 
> > > > Well of course :) There are only two hard problems in programming and
> > > > I forget the other one. But I didn't want to just say I don't like
> > > > free_zone_device_folio_prepare() without offering an alternative, I'd be open
> > > > to others.
> > > > 
> > > 
> > > zone_device_folio_reinitialise is good with me.
> > > 
> > > > > 
> > > > > > roll this into a zone_device_folio_init() type function (similar to
> > > > > > zone_device_page_init()) that just deals with everything at allocation time?
> > > > > > 
> > > > > 
> > > > > I don’t think doing this at allocation actually works without a big lock
> > > > > per pgmap. Consider the case where a VRAM allocator allocates two
> > > > > distinct subsets of a large folio and you have a multi-threaded GPU page
> > > > > fault handler (Xe does). It’s possible two threads could call
> > > > > zone_device_folio_reinitialise at the same time, racing and causing all
> > > > > sorts of issues. My plan is to just call this function in the driver’s
> > > > > ->folio_free() prior to returning the VRAM allocation to my driver pool.
> > > > 
> > > > This doesn't make sense to me (at least as someone who doesn't know DRM SVM
> > > > intimately) - the folio metadata initialisation should only happen after the
> > > > VRAM allocation has occured.
> > > > 
> > > > IOW the VRAM allocator needs to deal with the locking, once you have the VRAM
> > > > physical range you just initialise the folio/pages associated with that range
> > > > with zone_device_folio_(re)initialise() and you're done.
> > > > 
> > > 
> > > Our VRAM allocator does have locking (via DRM buddy), but that layer
> > 
> > I mean I assumed it did :-)
> > 
> > > doesn’t have visibility into the folio or its pages. By the time we
> > > handle the folio/pages in the GPU fault handler, there are no global
> > > locks preventing two GPU faults from each having, say, 16 pages from the
> > > same order-9 folio. I believe if both threads call
> > > zone_device_folio_reinitialise/init at the same time, bad things could
> > > happen.
> > 
> > This is confusing to me. If you are getting a GPU fault it implies no page is
> > mapped at a particular virtual address. The normal process (or at least the
> > process I'm familiar with) for handling this is to allocate and map a page at
> > the faulting virtual address. So in the scenario of two GPUs faulting on the
> > same VA each thread would allocate VRAM using DRM buddy, presumably getting
> 
> Different VAs.
> 
> > different physical pages, and so the zone_device_folio_init() call would be to
> 
> Yes, different physical pages but same folio which is possible if it
> hasn't been split yet (i.e., both threads a different subset of pages in
> the same folio, try to split at the same time and boom something bad
> happens).

So is you're concern something like this:

1) There is a free folio A of order 9, starting at physical address 0.
2) You have two GPU faults, both call into DRM Buddy to get a 4K page .
3) GPU 1 gets allocated physical address 0 (ie. folio_page(folio_A, 0))
4) GPU 2 gets allocated physical address 0x1000 (ie. folio_page(folio_A, 1))
5) Both call zone_device_folio_init() which splits the folio, meaning the
   previous step would touch folio_page(folio_A, 0) even though it has not been
   allocated physical address 0.

If that's the concern then what I'm saying (and what I think Jason was getting
at) is that (5) above is wrong - the driver doesn't (and shouldn't) update the
compound head (ie. folio_page(folio_a, 0)) - zone_device_folio_init() should
just overwrite all the metadata in the struct pages it has been allocated. We're
not really splitting folios, because it makes no sense to talk of splitting a
free folio which I think is why some core-mm people took notice.

Also It doesn't matter that you are leaving the previous compound head struct
pages in some weird state, the core-mm doesn't care about them anymore and the
struct page/folio is only used by core-mm not drivers. They will get properly
(re)initialised when needed for the core-mm in zone_device_folio_init() which in
this case would happen in step 3.

 - Alistair

> > different folios/pages.
> > 
> > Then eventually one thread would succeed in creating the mapping from VA->VRAM
> > and the losing thread would free the VRAM allocation back to DRM buddy.
> > 
> > So I'm a bit confused by the above statement that two GPUs faults could each
> > have the same pages or be calling zone_device_folio_init() on the same pages.
> > How would that happen?
> > 
> 
> See above. I hope my above statements make this clear.
> 
> Matt
> 
> > > > Is the concern that reinitialisation would touch pages outside of the allocated
> > > > VRAM range if it was previously a large folio?
> > > 
> > > No just two threads call zone_device_folio_reinitialise/init at the same
> > > time, on the same folio.
> > > 
> > > If we call zone_device_folio_reinitialise in ->folio_free this problem
> > > goes away. We could solve this with split_lock or something but I'd
> > > prefer not to add lock for this (although some of prior revs did do
> > > this, maybe we will revist this later).
> > > 
> > > Anyways - this falls in driver detail / choice IMO.
> > 
> > Agreed.
> > 
> >  - Alistair
> > 
> > > Matt
> > > 
> > > > 
> > > > > > > > > +{
> > > > > > > > > +	struct dev_pagemap *pgmap = page_pgmap(&folio->page);
> > > > > > > > > +	int order, i;
> > > > > > > > > +
> > > > > > > > > +	VM_WARN_ON_FOLIO(!folio_is_zone_device(folio), folio);
> > > > > > > > > +
> > > > > > > > > +	folio->mapping = NULL;
> > > > > > > > > +	order = folio_order(folio);
> > > > > > > > > +	if (!order)
> > > > > > > > > +		return;
> > > > > > > > > +
> > > > > > > > > +	folio_reset_order(folio);
> > > > > > > > > +
> > > > > > > > > +	for (i = 0; i < (1UL << order); i++) {
> > > > > > > > > +		struct page *page = folio_page(folio, i);
> > > > > > > > > +		struct folio *new_folio = (struct folio *)page;
> > > > > > > > > +
> > > > > > > > > +		ClearPageHead(page);
> > > > > > > > > +		clear_compound_head(page);
> > > > > > > > > +
> > > > > > > > > +		new_folio->mapping = NULL;
> > > > > > > > > +		/*
> > > > > > > > > +		 * Reset pgmap which was over-written by
> > > > > > > > > +		 * prep_compound_page().
> > > > > > > > > +		 */
> > > > > > > > > +		new_folio->pgmap = pgmap;
> > > > > > > > > +		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);
> > > > > > > > 
> > > > > > > > Does calling the free_folio() callback on new_folio solve the issue you are facing, or is
> > > > > > > > that PMD_ORDER more frees than we'd like?
> > > > > > > > 
> > > > > > > 
> > > > > > > No, calling free_folio() more often doesn’t solve anything—in fact, that
> > > > > > > would make my implementation explode. I explained this in detail here [1]
> > > > > > > to Zi.
> > > > > > > 
> > > > > > > To recap [1], my memory allocator has no visibility into individual
> > > > > > > pages or folios; it is DRM Buddy layered on top of TTM BO. This design
> > > > > > > allows VRAM to be allocated or evicted for both traditional GPU
> > > > > > > allocations (GEMs) and SVM allocations.
> > > > > > > 
> > > > > > > Now, to recap the actual issue: if device folios are not split upon free
> > > > > > > and are later reallocated with a different order in
> > > > > > > zone_device_page_init, the implementation breaks. This problem is not
> > > > > > > specific to Xe—Nouveau happens to always allocate at the same order, so
> > > > > > > it works by coincidence. Reallocating at a different order is valid
> > > > > > > behavior and must be supported.
> > > > > > 
> > > > > > I agree it's probably by coincidence but it is a perfectly valid design to
> > > > > > always just (re)allocate at the same order and not worry about having to
> > > > > > reinitialise things to different orders.
> > > > > > 
> > > > > 
> > > > > I would agree with this statement too — it’s perfectly valid if a driver
> > > > > always wants to (re)allocate at the same order.
> > > > > 
> > > > > Matt
> > > > > 
> > > > > >  - Alistair
> > > > > > 
> > > > > > > Matt
> > > > > > > 
> > > > > > > [1] https://patchwork.freedesktop.org/patch/697710/?series=159119&rev=3#comment_1282413
> > > > > > > 
> > > > > > > > > +	}
> > > > > > > > > +}
> > > > > > > > > +EXPORT_SYMBOL_GPL(free_zone_device_folio_prepare);
> > > > > > > > > +
> > > > > > > > >  void free_zone_device_folio(struct folio *folio)
> > > > > > > > >  {
> > > > > > > > >  	struct dev_pagemap *pgmap = folio->pgmap;
> > > > > > > > > @@ -454,6 +508,7 @@ void free_zone_device_folio(struct folio *folio)
> > > > > > > > >  	case MEMORY_DEVICE_COHERENT:
> > > > > > > > >  		if (WARN_ON_ONCE(!pgmap->ops || !pgmap->ops->folio_free))
> > > > > > > > >  			break;
> > > > > > > > > +		free_zone_device_folio_prepare(folio);
> > > > > > > > >  		pgmap->ops->folio_free(folio, order);
> > > > > > > > >  		percpu_ref_put_many(&folio->pgmap->ref, nr);
> > > > > > > > >  		break;
> > > > > > > > 
> > > > > > > > Balbir

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