Re: [PATCH V2 8/9] iommufd: optimize file mapping

[Jason Gunthorpe <jgg@nvidia.com>]

On Tue, Sep 24, 2024 at 08:05:37AM -0700, Steve Sistare wrote:
> @@ -622,6 +630,67 @@ static void batch_from_pages(struct pfn_batch *batch, struct page **pages,
>  			break;
>  }
>  
> +static void batch_from_folios(struct pfn_batch *batch, struct folio **folios,
> +			      unsigned long offset, unsigned long npages)
> +{
> +	unsigned long nr, pfn, i = 0;
> +	struct folio *folio;
> +
> +	while (npages) {
> +		folio = folios[i++];
> +		nr = folio_nr_pages(folio) - offset;
> +		nr = min_t(unsigned long, nr, npages);
> +		pfn = page_to_pfn(folio_page(folio, offset));
> +		batch_add_pfn_num(batch, pfn, nr);

I think we technically need to cap nr here as well?

> +		offset = 0;
> +		npages -= nr;
> +	}
> +}
> +
> +/*
> + * Return the folio containing the page which is @start_index pages beyond
> + * page number @offset in folios[0].  Return the index of that page in
> + * @offset_out.
> + */
> +static struct folio **folios_start(struct folio **folios, unsigned long offset,
> +				   unsigned long start_index,
> +				   unsigned long *offset_out)
> +{
> +	unsigned long nr = folio_nr_pages(*folios);
> +
> +	while (offset + start_index > nr) {
> +		start_index -= (nr - offset);
> +		offset = 0;
> +		folios++;
> +		nr = folio_nr_pages(*folios);
> +	}
> +
> +	*offset_out = offset + start_index;
> +	return folios;
> +}

Why does this exist? I would expect you'd call memfd_pin_folios with
the correct offset so that the first folio was the right folio, this
is how the pin_user_pages path works?

> +static void folios_unpin_partial(struct folio **folios, unsigned long offset,
> +				 unsigned long npages)
> +{
> +	unsigned long nr, j, i = 0;
> +	struct folio *folio;
> +
> +	while (npages) {
> +		folio = folios[i++];
> +		nr = folio_nr_pages(folio);
> +		if (offset == 0 && nr < npages) {
> +			unpin_folio(folio);
> +		} else {
> +			nr = min_t(unsigned long, npages, nr - offset);
> +			for (j = 0; j < nr; j++)
> +				unpin_user_page(folio_page(folio, offset + j));

Don't loop, we have unpin_user_pages() for this

Jason