Re: [v1 resend 07/12] mm/memremap: add folio_split support

["Mika Penttilä" <mpenttil@redhat.com>]

On 7/4/25 02:35, Balbir Singh wrote:
> When a zone device page is split (via huge pmd folio split). The
> driver callback for folio_split is invoked to let the device driver
> know that the folio size has been split into a smaller order.
>
> The HMM test driver has been updated to handle the split, since the
> test driver uses backing pages, it requires a mechanism of reorganizing
> the backing pages (backing pages are used to create a mirror device)
> again into the right sized order pages. This is supported by exporting
> prep_compound_page().
>
> Cc: Karol Herbst <kherbst@redhat.com>
> Cc: Lyude Paul <lyude@redhat.com>
> Cc: Danilo Krummrich <dakr@kernel.org>
> Cc: David Airlie <airlied@gmail.com>
> Cc: Simona Vetter <simona@ffwll.ch>
> Cc: "Jérôme Glisse" <jglisse@redhat.com>
> Cc: Shuah Khan <shuah@kernel.org>
> Cc: David Hildenbrand <david@redhat.com>
> Cc: Barry Song <baohua@kernel.org>
> Cc: Baolin Wang <baolin.wang@linux.alibaba.com>
> Cc: Ryan Roberts <ryan.roberts@arm.com>
> Cc: Matthew Wilcox <willy@infradead.org>
> Cc: Peter Xu <peterx@redhat.com>
> Cc: Zi Yan <ziy@nvidia.com>
> Cc: Kefeng Wang <wangkefeng.wang@huawei.com>
> Cc: Jane Chu <jane.chu@oracle.com>
> Cc: Alistair Popple <apopple@nvidia.com>
> Cc: Donet Tom <donettom@linux.ibm.com>
>
> Signed-off-by: Balbir Singh <balbirs@nvidia.com>
> ---
>  include/linux/memremap.h |  7 +++++++
>  include/linux/mm.h       |  1 +
>  lib/test_hmm.c           | 42 ++++++++++++++++++++++++++++++++++++++++
>  mm/huge_memory.c         | 14 ++++++++++++++
>  mm/page_alloc.c          |  1 +
>  5 files changed, 65 insertions(+)
>
> diff --git a/include/linux/memremap.h b/include/linux/memremap.h
> index 11d586dd8ef1..2091b754f1da 100644
> --- a/include/linux/memremap.h
> +++ b/include/linux/memremap.h
> @@ -100,6 +100,13 @@ struct dev_pagemap_ops {
>  	 */
>  	int (*memory_failure)(struct dev_pagemap *pgmap, unsigned long pfn,
>  			      unsigned long nr_pages, int mf_flags);
> +
> +	/*
> +	 * Used for private (un-addressable) device memory only.
> +	 * This callback is used when a folio is split into
> +	 * a smaller folio
> +	 */
> +	void (*folio_split)(struct folio *head, struct folio *tail);
>  };
>  
>  #define PGMAP_ALTMAP_VALID	(1 << 0)
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index ef40f68c1183..f7bda8b1e46c 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -1183,6 +1183,7 @@ static inline struct folio *virt_to_folio(const void *x)
>  void __folio_put(struct folio *folio);
>  
>  void split_page(struct page *page, unsigned int order);
> +void prep_compound_page(struct page *page, unsigned int order);
>  void folio_copy(struct folio *dst, struct folio *src);
>  int folio_mc_copy(struct folio *dst, struct folio *src);
>  
> diff --git a/lib/test_hmm.c b/lib/test_hmm.c
> index 95b4276a17fd..e20021fb7c69 100644
> --- a/lib/test_hmm.c
> +++ b/lib/test_hmm.c
> @@ -1646,9 +1646,51 @@ static vm_fault_t dmirror_devmem_fault(struct vm_fault *vmf)
>  	return ret;
>  }
>  
> +static void dmirror_devmem_folio_split(struct folio *head, struct folio *tail)
> +{
> +	struct page *rpage = BACKING_PAGE(folio_page(head, 0));
> +	struct page *rpage_tail;
> +	struct folio *rfolio;
> +	unsigned long offset = 0;
> +	unsigned int tail_order;
> +	unsigned int head_order = folio_order(head);
> +
> +	if (!rpage) {
> +		tail->page.zone_device_data = NULL;
> +		return;
> +	}
> +
> +	rfolio = page_folio(rpage);
> +
> +	if (tail == NULL) {
> +		folio_reset_order(rfolio);
> +		rfolio->mapping = NULL;
> +		if (head_order)
> +			prep_compound_page(rpage, head_order);
> +		folio_set_count(rfolio, 1 << head_order);
> +		return;
> +	}
> +
> +	offset = folio_pfn(tail) - folio_pfn(head);
> +
> +	rpage_tail = folio_page(rfolio, offset);
> +	tail->page.zone_device_data = rpage_tail;
> +	clear_compound_head(rpage_tail);
> +	rpage_tail->mapping = NULL;
> +
> +	tail_order = folio_order(tail);
> +	if (tail_order)
> +		prep_compound_page(rpage_tail, tail_order);
> +
> +	folio_page(tail, 0)->mapping = folio_page(head, 0)->mapping;
> +	tail->pgmap = head->pgmap;
> +	folio_set_count(page_folio(rpage_tail), 1 << tail_order);
> +}
> +
>  static const struct dev_pagemap_ops dmirror_devmem_ops = {
>  	.page_free	= dmirror_devmem_free,
>  	.migrate_to_ram	= dmirror_devmem_fault,
> +	.folio_split	= dmirror_devmem_folio_split,
>  };
>  
>  static int dmirror_device_init(struct dmirror_device *mdevice, int id)
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index f29add796931..d55e36ae0c39 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -3630,6 +3630,11 @@ static int __split_unmapped_folio(struct folio *folio, int new_order,
>  			if (release == origin_folio)
>  				continue;
>  
> +			if (folio_is_device_private(origin_folio) &&
> +					origin_folio->pgmap->ops->folio_split)
> +				origin_folio->pgmap->ops->folio_split(
> +					origin_folio, release);

Should folio split fail if pgmap->ops->folio_split() is not defined? If not then at least the >pgmap pointer copy should be in the common code.

> +
>  			folio_ref_unfreeze(release, 1 +
>  					((mapping || swap_cache) ?
>  						folio_nr_pages(release) : 0));
> @@ -3661,6 +3666,15 @@ static int __split_unmapped_folio(struct folio *folio, int new_order,
>  		}
>  	}
>  
> +	/*
> +	 * Mark the end of the split, so that the driver can setup origin_folio's
> +	 * order and other metadata
> +	 */
> +	if (folio_is_device_private(origin_folio) &&
> +			origin_folio->pgmap->ops->folio_split)
> +		origin_folio->pgmap->ops->folio_split(
> +			origin_folio, NULL);
> +
>  	/*
>  	 * Unfreeze origin_folio only after all page cache entries, which used
>  	 * to point to it, have been updated with new folios. Otherwise,
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 4f55f8ed65c7..0a538e9c24bd 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -722,6 +722,7 @@ void prep_compound_page(struct page *page, unsigned int order)
>  
>  	prep_compound_head(page, order);
>  }
> +EXPORT_SYMBOL_GPL(prep_compound_page);
>  
>  static inline void set_buddy_order(struct page *page, unsigned int order)
>  {