Re: [PATCH V3 1/9] mm/gup: folio_add_pins

[Steven Sistare <steven.sistare@oracle.com>]

cc mm folks.

On 10/4/2024 2:48 PM, Steve Sistare wrote:
> Export a function that adds pins to an already-pinned huge-page folio.
> This allows any range of small pages within the folio to be unpinned later.
> For example, pages pinned via memfd_pin_folios and modified by
> folio_add_pins could be unpinned via unpin_user_page(s).
> 
> Suggested-by: Jason Gunthorpe <jgg@nvidia.com>
> Suggested-by: David Hildenbrand <david@redhat.com>
> Signed-off-by: Steve Sistare <steven.sistare@oracle.com>
> ---
>   include/linux/mm.h |  1 +
>   mm/gup.c           | 24 ++++++++++++++++++++++++
>   2 files changed, 25 insertions(+)
> 
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index 13bff7c..70d5293 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -2521,6 +2521,7 @@ long pin_user_pages_unlocked(unsigned long start, unsigned long nr_pages,
>   long memfd_pin_folios(struct file *memfd, loff_t start, loff_t end,
>   		      struct folio **folios, unsigned int max_folios,
>   		      pgoff_t *offset);
> +int folio_add_pins(struct folio *folio, unsigned int pins);
>   
>   int get_user_pages_fast(unsigned long start, int nr_pages,
>   			unsigned int gup_flags, struct page **pages);
> diff --git a/mm/gup.c b/mm/gup.c
> index fcd602b..11c5f27 100644
> --- a/mm/gup.c
> +++ b/mm/gup.c
> @@ -3733,3 +3733,27 @@ long memfd_pin_folios(struct file *memfd, loff_t start, loff_t end,
>   	return ret;
>   }
>   EXPORT_SYMBOL_GPL(memfd_pin_folios);
> +
> +/**
> + * folio_add_pins() - add pins to an already-pinned folio
> + * @folio: the folio to add more pins to
> + * @pins: number of pins to add
> + *
> + * Try to add more pins to an already-pinned folio. The semantics
> + * of the pin (e.g., FOLL_WRITE) follow any existing pin and cannot
> + * be changed.
> + *
> + * This function is helpful when having obtained a pin on a large folio
> + * using memfd_pin_folios(), but wanting to logically unpin parts
> + * (e.g., individual pages) of the folio later, for example, using
> + * unpin_user_page_range_dirty_lock().
> + *
> + * This is not the right interface to initially pin a folio.
> + */
> +int folio_add_pins(struct folio *folio, unsigned int pins)
> +{
> +	VM_WARN_ON_ONCE(!folio_maybe_dma_pinned(folio));
> +
> +	return try_grab_folio(folio, pins, FOLL_PIN);
> +}
> +EXPORT_SYMBOL_GPL(folio_add_pins);