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

[David Hildenbrand <david@redhat.com>]

On 06.03.25 05:42, 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

Confusing. When a folio is split, it is split into multiple folios.

So when will this be invoked?

> +	 */
> +	void (*folio_split)(struct folio *head, struct folio *tail);

head and tail are really suboptimal termonology. They refer to head and 
tail pages, which is not really the case with folios (in the long run).

>   };
>   
>   #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);
> +	}
> +	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;

Most of this doesn't look like it should be the responsibility of this 
callback.

Setting up a new folio after the split (messing with compound pages etc) 
really should not be the responsibility of this callback.

So no, this looks misplaced.

> +}
> +
>   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);

Absolutely ugly. I think we need a wrapper for the

> +
>   			/*
>   			 * 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);

Hmmm, that is questionable. We don't want arbitrary modules to make use 
of that.

Another sign that you are exposing the wrong functionality/interface 
(folio_split) to modules.

-- 
Cheers,

David / dhildenb