Re: [PATCH V1 1/9] mm/gup: repin_folio_unhugely

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

cc'ing linux-mm for review of this one patch of the series.

This proposes a new KAPI function repin_folio_unhugely(), for use in this
patch of the iommu_ioas_map_file series:

   iommufd: IOMMU_IOAS_MAP_FILE implementation
   https://lore.kernel.org/linux-iommu/1726319158-283074-7-git-send-email-steven.sistare@oracle.com

- Steve

On 9/14/2024 9:05 AM, Steve Sistare wrote:
> Export a function that repins a huge-page folio at small-page granularity.
> 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
> repin_folio_unhugely could be unpinned via unpin_user_page(s).
> 
> Suggested-by: Jason Gunthorpe <jgg@nvidia.com>
> Signed-off-by: Steve Sistare <steven.sistare@oracle.com>
> ---
>   include/linux/mm.h |  1 +
>   mm/gup.c           | 18 ++++++++++++++++++
>   2 files changed, 19 insertions(+)
> 
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index 1470736..ba8344f 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -2514,6 +2514,7 @@ 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);
> +void repin_folio_unhugely(struct folio *folio, unsigned long npin);
>   
>   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 947881ff..f8f3f2a 100644
> --- a/mm/gup.c
> +++ b/mm/gup.c
> @@ -3720,3 +3720,21 @@ long memfd_pin_folios(struct file *memfd, loff_t start, loff_t end,
>   	return ret;
>   }
>   EXPORT_SYMBOL_GPL(memfd_pin_folios);
> +
> +/**
> + * repin_folio_unhugely() - repin a folio at small page granularity
> + * @folio: the folio to repin
> + * @npin:  the number of pages pinned in the folio
> + *
> + * Given a huge page folio that is already pinned, and the number of small
> + * pages that are pinned in it, adjust the pincount to reflect small-page
> + * granularity.  Each small page can later be unpinned individually.
> + */
> +void repin_folio_unhugely(struct folio *folio, unsigned long npin)
> +{
> +	if (!folio_test_large(folio) || is_huge_zero_folio(folio) || npin == 1)
> +		return;
> +	atomic_add(npin - 1, &folio->_refcount);
> +	atomic_add(npin - 1, &folio->_pincount);
> +}
> +EXPORT_SYMBOL_GPL(repin_folio_unhugely);