Re: [PATCH] gup: optimize longterm pin_user_pages() for large folio

[Dev Jain <dev.jain@arm.com>]

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On 30/05/25 2:53 pm, lizhe.67@bytedance.com wrote:
> From: Li Zhe<lizhe.67@bytedance.com>
>
> In the current implementation of the longterm pin_user_pages() function,
> we invoke the collect_longterm_unpinnable_folios() function. This function
> iterates through the list to check whether each folio belongs to the
> "longterm_unpinnabled" category. The folios in this list essentially
> correspond to a contiguous region of user-space addresses, with each folio
> representing a physical address in increments of PAGESIZE. If this
> user-space address range is mapped with large folio, we can optimize the
> performance of function pin_user_pages() by reducing the number of if-else
> branches and the frequency of memory accesses using READ_ONCE. This patch
> leverages this approach to achieve performance improvements.
>
> The performance test results obtained through the gup_test tool from the
> kernel source tree are as follows. We achieve an improvement of over 75%
> for large folio with pagesize=2M. For normal page, we have only observed
> a very slight degradation in performance.


Thanks for the patch! I have no idea on GUP but in my limited understanding
the patch looks fine, let's wait for more comments.

[----]

>   }
>   
> +static struct folio *pofs_next_folio(struct folio *folio,
> +				struct pages_or_folios *pofs, long *index_ptr)
> +{
> +	long i = *index_ptr + 1;
> +	unsigned long nr_pages = folio_nr_pages(folio);
> +
> +	if (!pofs->has_folios)
> +		while ((i < pofs->nr_entries) &&
> +			/* Is this page part of this folio? */
> +			(folio_page_idx(folio, pofs->pages[i]) < nr_pages))
> +			i++;
> +
> +	if (unlikely(i == pofs->nr_entries))
> +		return NULL;
> +	*index_ptr = i;
> +
> +	return pofs_get_folio(pofs, i);
> +}
> +
>   /*
>    * Returns the number of collected folios. Return value is always >= 0.
>    */
> @@ -2324,16 +2343,12 @@ static void collect_longterm_unpinnable_folios(
>   		struct list_head *movable_folio_list,
>   		struct pages_or_folios *pofs)
>   {
> -	struct folio *prev_folio = NULL;
>   	bool drain_allow = true;
> -	unsigned long i;
> -
> -	for (i = 0; i < pofs->nr_entries; i++) {
> -		struct folio *folio = pofs_get_folio(pofs, i);
> +	long i = 0;


Why not unsigned long?


> +	struct folio *folio;
>   
> -		if (folio == prev_folio)
> -			continue;
> -		prev_folio = folio;
> +	for (folio = pofs_get_folio(pofs, 0); folio;
> +			folio = pofs_next_folio(folio, pofs, &i)) {
>   
>   		if (folio_is_longterm_pinnable(folio))
>   			continue;

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