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U2FsdGVkX1+y5Ij3BW5faWkyViWeSF+A2x6DIzyz7iVV6i31EswNpoHAf20mwhPTQ3/Do95729N5VCniYzCUJr0hv7t0TgGg+ojKpMgDWwV3Xxk8FW06yS9LTaoVtmOyKUfVYcyjKNgxHkaGU3426DoNNq1kaBQgP67S+2DIBmsI5nGEnmzxUGTWR/YY7hU/Z2kj9ZEN9lUm8ceZczKfqYrR4m50pAD3cszh1FllRq5yh3CPXQJ7zKvY5Z8JD8hVrM6YouYRjaBDZ4j5YAZgZplU/Ljrq/8Qedl/23JbgBO8fdnNZ1NE9Vvue2bQuBiw+8GyDYsvAE31fXv3aFyJV6Ls7j54Phgky6Nt81r+cY/SkmDZRG3fa/3cSpYaW72l5jQFP1w2ea6K5CVNSkalisX3J9rYKAqD3mjV+MgL7nDXZkLcuQYkoJBw1k/3XlT+OGwIme+0luXSEdTi4w+darQ4nGxXf2elmiAjzFjT990lgHn2xt5kS7QmfUYBtr9Gsa3FTMKL5Kg/YKFC6dco8jCbcylKEgAnTRHh0+dzo/SwJC92rUVDQDdmNepnkHhcP/wv6sRm4zWDafsop+Z+goLszrTO0Uo00L1O9lvlMP2zQWOHYYZJFgZfEoY4fllBbdzEOBQ3mYhqPMKWtYKxirPHqHDh7FGjCwhfPB9UDSL/bzTaaCBOAZgvBhtk4Y83Vvw/MQ0EhgwXfmqLfvy8PKcKGKjb/SOyfTRnqxbLh5JsOnFbwXl9149OYj4Pu7tERmGthXOCtuUlKS79tod4kUnLLOrj4pR6fohcKeeq8AHgiVS3v9EeVHkqBhmU0u9/Sy2oeTCoZTmQheXPQAlyl0Pvz7CPVlStfMWyrwuwPajdtDMfVv0/GWJ+t5wnrvuH2GbLxESNJspQsqSJVl71N1/eR1QJ6FoBvefz7SIse8m6rlMqIu+4QjnVkp6E/5E3QRMcVx2343Mql7O0R6M HU13t9jC 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Singh wrote: > On 1/8/26 08:03, Zi Yan wrote: > > On 7 Jan 2026, at 16:15, Matthew Brost wrote: > > > >> On Wed, Jan 07, 2026 at 03:38:35PM -0500, Zi Yan wrote: > >>> On 7 Jan 2026, at 15:20, Zi Yan wrote: > >>> > >>>> +THP folks > >>> > >>> +willy, since he commented in another thread. > >>> > >>>> > >>>> On 16 Dec 2025, at 15:10, Francois Dugast wrote: > >>>> > >>>>> From: Matthew Brost > >>>>> > >>>>> Introduce migrate_device_split_page() to split a device page into > >>>>> lower-order pages. Used when a folio allocated as higher-order is freed > >>>>> and later reallocated at a smaller order by the driver memory manager. > >>>>> > >>>>> Cc: Andrew Morton > >>>>> Cc: Balbir Singh > >>>>> Cc: dri-devel@lists.freedesktop.org > >>>>> Cc: linux-mm@kvack.org > >>>>> Signed-off-by: Matthew Brost > >>>>> Signed-off-by: Francois Dugast > >>>>> --- > >>>>> include/linux/huge_mm.h | 3 +++ > >>>>> include/linux/migrate.h | 1 + > >>>>> mm/huge_memory.c | 6 ++--- > >>>>> mm/migrate_device.c | 49 +++++++++++++++++++++++++++++++++++++++++ > >>>>> 4 files changed, 56 insertions(+), 3 deletions(-) > >>>>> > >>>>> diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h > >>>>> index a4d9f964dfde..6ad8f359bc0d 100644 > >>>>> --- a/include/linux/huge_mm.h > >>>>> +++ b/include/linux/huge_mm.h > >>>>> @@ -374,6 +374,9 @@ int __split_huge_page_to_list_to_order(struct page *page, struct list_head *list > >>>>> int folio_split_unmapped(struct folio *folio, unsigned int new_order); > >>>>> unsigned int min_order_for_split(struct folio *folio); > >>>>> int split_folio_to_list(struct folio *folio, struct list_head *list); > >>>>> +int __split_unmapped_folio(struct folio *folio, int new_order, > >>>>> + struct page *split_at, struct xa_state *xas, > >>>>> + struct address_space *mapping, enum split_type split_type); > >>>>> int folio_check_splittable(struct folio *folio, unsigned int new_order, > >>>>> enum split_type split_type); > >>>>> int folio_split(struct folio *folio, unsigned int new_order, struct page *page, > >>>>> diff --git a/include/linux/migrate.h b/include/linux/migrate.h > >>>>> index 26ca00c325d9..ec65e4fd5f88 100644 > >>>>> --- a/include/linux/migrate.h > >>>>> +++ b/include/linux/migrate.h > >>>>> @@ -192,6 +192,7 @@ void migrate_device_pages(unsigned long *src_pfns, unsigned long *dst_pfns, > >>>>> unsigned long npages); > >>>>> void migrate_device_finalize(unsigned long *src_pfns, > >>>>> unsigned long *dst_pfns, unsigned long npages); > >>>>> +int migrate_device_split_page(struct page *page); > >>>>> > >>>>> #endif /* CONFIG_MIGRATION */ > >>>>> > >>>>> diff --git a/mm/huge_memory.c b/mm/huge_memory.c > >>>>> index 40cf59301c21..7ded35a3ecec 100644 > >>>>> --- a/mm/huge_memory.c > >>>>> +++ b/mm/huge_memory.c > >>>>> @@ -3621,9 +3621,9 @@ static void __split_folio_to_order(struct folio *folio, int old_order, > >>>>> * Return: 0 - successful, <0 - failed (if -ENOMEM is returned, @folio might be > >>>>> * split but not to @new_order, the caller needs to check) > >>>>> */ > >>>>> -static int __split_unmapped_folio(struct folio *folio, int new_order, > >>>>> - struct page *split_at, struct xa_state *xas, > >>>>> - struct address_space *mapping, enum split_type split_type) > >>>>> +int __split_unmapped_folio(struct folio *folio, int new_order, > >>>>> + struct page *split_at, struct xa_state *xas, > >>>>> + struct address_space *mapping, enum split_type split_type) > >>>>> { > >>>>> const bool is_anon = folio_test_anon(folio); > >>>>> int old_order = folio_order(folio); > >>>>> diff --git a/mm/migrate_device.c b/mm/migrate_device.c > >>>>> index 23379663b1e1..eb0f0e938947 100644 > >>>>> --- a/mm/migrate_device.c > >>>>> +++ b/mm/migrate_device.c > >>>>> @@ -775,6 +775,49 @@ int migrate_vma_setup(struct migrate_vma *args) > >>>>> EXPORT_SYMBOL(migrate_vma_setup); > >>>>> > >>>>> #ifdef CONFIG_ARCH_ENABLE_THP_MIGRATION > >>>>> +/** > >>>>> + * migrate_device_split_page() - Split device page > >>>>> + * @page: Device page to split > >>>>> + * > >>>>> + * Splits a device page into smaller pages. Typically called when reallocating a > >>>>> + * folio to a smaller size. Inherently racy—only safe if the caller ensures > >>>>> + * mutual exclusion within the page's folio (i.e., no other threads are using > >>>>> + * pages within the folio). Expected to be called a free device page and > >>>>> + * restores all split out pages to a free state. > >>>>> + */ > >>> > >>> Do you mind explaining why __split_unmapped_folio() is needed for a free device > >>> page? A free page is not supposed to be a large folio, at least from a core > >>> MM point of view. __split_unmapped_folio() is intended to work on large folios > >>> (or compound pages), even if the input folio has refcount == 0 (because it is > >>> frozen). > >>> > >> > >> Well, then maybe this is a bug in core MM where the freed page is still > >> a THP. Let me explain the scenario and why this is needed from my POV. > >> > >> Our VRAM allocator in Xe (and several other DRM drivers) is DRM buddy. > >> This is a shared pool between traditional DRM GEMs (buffer objects) and > >> SVM allocations (pages). It doesn’t have any view of the page backing—it > >> basically just hands back a pointer to VRAM space that we allocate from. > >> From that, if it’s an SVM allocation, we can derive the device pages. > >> > >> What I see happening is: a 2M buddy allocation occurs, we make the > >> backing device pages a large folio, and sometime later the folio > >> refcount goes to zero and we free the buddy allocation. Later, the buddy > >> allocation is reused for a smaller allocation (e.g., 4K or 64K), but the > >> backing pages are still a large folio. Here is where we need to split > > > > I agree with you that it might be a bug in free_zone_device_folio() based > > on my understanding. Since zone_device_page_init() calls prep_compound_page() > > for >0 orders, but free_zone_device_folio() never reverse the process. > > > > Balbir and Alistair might be able to help here. > > I agree it's an API limitation > > > > > I cherry picked the code from __free_frozen_pages() to reverse the process. > > Can you give it a try to see if it solve the above issue? Thanks. > > > > From 3aa03baa39b7e62ea079e826de6ed5aab3061e46 Mon Sep 17 00:00:00 2001 > > From: Zi Yan > > Date: Wed, 7 Jan 2026 16:49:52 -0500 > > Subject: [PATCH] mm/memremap: free device private folio fix > > Content-Type: text/plain; charset="utf-8" > > > > Signed-off-by: Zi Yan > > --- > > mm/memremap.c | 15 +++++++++++++++ > > 1 file changed, 15 insertions(+) > > > > diff --git a/mm/memremap.c b/mm/memremap.c > > index 63c6ab4fdf08..483666ff7271 100644 > > --- a/mm/memremap.c > > +++ b/mm/memremap.c > > @@ -475,6 +475,21 @@ void free_zone_device_folio(struct folio *folio) > > pgmap->ops->folio_free(folio); > > break; > > } > > + > > + if (nr > 1) { > > + struct page *head = folio_page(folio, 0); > > + > > + head[1].flags.f &= ~PAGE_FLAGS_SECOND; > > +#ifdef NR_PAGES_IN_LARGE_FOLIO > > + folio->_nr_pages = 0; > > +#endif > > + for (i = 1; i < nr; i++) { > > + (head + i)->mapping = NULL; > > + clear_compound_head(head + i); > > I see that your skipping the checks in free_page_tail_prepare()? IIUC, we should be able > to invoke it even for zone device private pages > > > + } > > + folio->mapping = NULL; > > This is already done in free_zone_device_folio() > > > + head->flags.f &= ~PAGE_FLAGS_CHECK_AT_PREP; > > I don't think this is required for zone device private folios, but I suppose it > keeps the code generic > Well, the above code doesn’t work, but I think it’s the right idea. clear_compound_head aliases to pgmap, which we don’t want to be NULL. I believe the individual pages likely need their flags cleared (?), and this step must be done before calling folio_free and include a barrier, as the page can be immediately reallocated. So here’s what I came up with, and it seems to work (for Xe, GPU SVM): mm/memremap.c | 21 +++++++++++++++++++++ 1 file changed, 21 insertions(+) diff --git a/mm/memremap.c b/mm/memremap.c index 63c6ab4fdf08..ac20abb6a441 100644 --- a/mm/memremap.c +++ b/mm/memremap.c @@ -448,6 +448,27 @@ void free_zone_device_folio(struct folio *folio) pgmap->type != MEMORY_DEVICE_GENERIC) folio->mapping = NULL; + if (nr > 1) { + struct page *head = folio_page(folio, 0); + + head[1].flags.f &= ~PAGE_FLAGS_SECOND; +#ifdef NR_PAGES_IN_LARGE_FOLIO + folio->_nr_pages = 0; +#endif + for (i = 1; i < nr; i++) { + struct folio *new_folio = (struct folio *)(head + i); + + (head + i)->mapping = NULL; + (head + i)->flags.f &= ~PAGE_FLAGS_CHECK_AT_PREP; + + /* Overwrite compound_head with pgmap */ + new_folio->pgmap = pgmap; + } + + head->flags.f &= ~PAGE_FLAGS_CHECK_AT_PREP; + smp_wmb(); /* Changes but be visable before freeing folio */ + } + switch (pgmap->type) { case MEMORY_DEVICE_PRIVATE: case MEMORY_DEVICE_COHERENT: > > + } > > } > > > > void zone_device_page_init(struct page *page, unsigned int order) > > > Otherwise, it seems like the right way to solve the issue. > My question is: why isn’t Nouveau hitting this issue, or your Nvidia out-of-tree driver (lack of testing, Xe's test suite coverage is quite good at finding corners). Also, will this change in behavior break either ofthose drivers? Matt > Balbir