Re: [RFC 07/11] mm/memremap: Add folio_split support

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

Hi,

On 3/6/25 06:42, 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().
>
> Signed-off-by: Balbir Singh <balbirs@nvidia.com>
> ---
>  include/linux/memremap.h |  7 +++++++
>  include/linux/mm.h       |  1 +
>  lib/test_hmm.c           | 35 +++++++++++++++++++++++++++++++++++
>  mm/huge_memory.c         |  5 +++++
>  mm/page_alloc.c          |  1 +
>  5 files changed, 49 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 98a67488b5fe..3d0e91e0a923 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -1415,6 +1415,7 @@ 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 a81d2f8a0426..18b6a7b061d7 100644
> --- a/lib/test_hmm.c
> +++ b/lib/test_hmm.c
> @@ -1640,10 +1640,45 @@ 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 folio *new_rfolio;
> +	struct folio *rfolio;
> +	unsigned long offset = 0;
> +
> +	if (!rpage) {
> +		folio_page(tail, 0)->zone_device_data = NULL;
> +		return;
> +	}
> +
> +	offset = folio_pfn(tail) - folio_pfn(head);
> +	rfolio = page_folio(rpage);
> +	new_rfolio = page_folio(folio_page(rfolio, offset));
> +
> +	folio_page(tail, 0)->zone_device_data = folio_page(new_rfolio, 0);
> +

> +	if (folio_pfn(tail) - folio_pfn(head) == 1) {
> +		if (folio_order(head))
> +			prep_compound_page(folio_page(rfolio, 0),
> +						folio_order(head));
> +		folio_set_count(rfolio, 1);
> +	}

I think this might need at least a comment. Also, isn't the
folio_order(head) always 0, tail and head are splitted folios and if pfn
difference == 1?
If the intention is to adjust the backing folio's head page to the new
order, shouldn't there be clear_compound_head() also for backing head
page if split to zero order?


> +	clear_compound_head(folio_page(new_rfolio, 0));
> +	if (folio_order(tail))
> +		prep_compound_page(folio_page(new_rfolio, 0),
> +						folio_order(tail));
> +	folio_set_count(new_rfolio, 1);
> +	folio_page(new_rfolio, 0)->mapping = folio_page(rfolio, 0)->mapping;
> +	tail->pgmap = head->pgmap;
> +}
> +
>  static const struct dev_pagemap_ops dmirror_devmem_ops = {
>  	.page_free	= dmirror_devmem_free,
>  	.migrate_to_ram	= dmirror_devmem_fault,
>  	.page_free	= dmirror_devmem_free,
> +	.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 995ac8be5709..518a70d1b58a 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -3655,6 +3655,11 @@ static int __split_unmapped_folio(struct folio *folio, int new_order,
>  						MTHP_STAT_NR_ANON, 1);
>  			}
>  
> +			if (folio_is_device_private(origin_folio) &&
> +					origin_folio->pgmap->ops->folio_split)
> +				origin_folio->pgmap->ops->folio_split(
> +					origin_folio, release);
> +
>  			/*
>  			 * Unfreeze refcount first. Additional reference from
>  			 * page cache.
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 17ea8fb27cbf..563f7e39aa79 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -573,6 +573,7 @@ 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)
>  {

--Mika