On Tue, Jan 13, 2026 at 10:58:27AM +1100, Alistair Popple wrote:
> On 2026-01-12 at 12:16 +1100, Matthew Brost <[email protected]> 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 <[email protected]>
> > > >
> > > > 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 <[email protected]>
> > > > Cc: David Hildenbrand <[email protected]>
> > > > Cc: Oscar Salvador <[email protected]>
> > > > Cc: Andrew Morton <[email protected]>
> > > > Cc: Balbir Singh <[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: Alistair Popple <[email protected]>
> > > > Cc: [email protected]
> > > > Cc: [email protected]
> > > > Cc: [email protected]
> > > > Suggested-by: Alistair Popple <[email protected]>
> > > > Signed-off-by: Matthew Brost <[email protected]>
> > > > Signed-off-by: Francois Dugast <[email protected]>
> > > > ---
> > > > 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.
> 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.
> > > > +{
> > > > + 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
