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Message-ID: <k25qmqrfmhzkls2ngdaq4wdk3xtl3epv4z3dvukon5yj2cb2hl@yci7xdpvauh7>
Date: Tue, 13 Jan 2026 10:58:27 +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 2026-01-12 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
roll this into a zone_device_folio_init() type function (similar to
zone_device_page_init()) that just deals with everything at allocation time?
> > > +{
> > > + 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.
- 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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