Re: [PATCH v4] vfio/type1: optimize vfio_pin_pages_remote() for large folios

[Alex Williamson <alex.williamson@redhat.com>]

On Wed, 21 May 2025 12:25:07 +0800
lizhe.67@bytedance.com wrote:

> From: Li Zhe <lizhe.67@bytedance.com>
> 
> When vfio_pin_pages_remote() is called with a range of addresses that
> includes large folios, the function currently performs individual
> statistics counting operations for each page. This can lead to significant
> performance overheads, especially when dealing with large ranges of pages.
> 
> This patch optimize this process by batching the statistics counting
> operations.
> 
> The performance test results for completing the 8G VFIO IOMMU DMA mapping,
> obtained through trace-cmd, are as follows. In this case, the 8G virtual
> address space has been mapped to physical memory using hugetlbfs with
> pagesize=2M.
> 
> Before this patch:
> funcgraph_entry:      # 33813.703 us |  vfio_pin_map_dma();
> 
> After this patch:
> funcgraph_entry:      # 16071.378 us |  vfio_pin_map_dma();
> 
> Signed-off-by: Li Zhe <lizhe.67@bytedance.com>
> Co-developed-by: Alex Williamson <alex.williamson@redhat.com>
> Signed-off-by: Alex Williamson <alex.williamson@redhat.com>
> ---

Given the discussion on v3, this is currently a Nak.  Follow-up in that
thread if there are further ideas how to salvage this.  Thanks,

Alex

> Changelogs:
> 
> v3->v4:
> - Use min_t() to obtain the step size, rather than min().
> - Fix some issues in commit message and title.
> 
> v2->v3:
> - Code simplification.
> - Fix some issues in comments.
> 
> v1->v2:
> - Fix some issues in comments and formatting.
> - Consolidate vfio_find_vpfn_range() and vfio_find_vpfn().
> - Move the processing logic for hugetlbfs folio into the while(true) loop
>   and use a variable with a default value of 1 to indicate the number of
>   consecutive pages.
> 
> v3 patch: https://lore.kernel.org/all/20250520070020.6181-1-lizhe.67@bytedance.com/
> v2 patch: https://lore.kernel.org/all/20250519070419.25827-1-lizhe.67@bytedance.com/
> v1 patch: https://lore.kernel.org/all/20250513035730.96387-1-lizhe.67@bytedance.com/
> 
>  drivers/vfio/vfio_iommu_type1.c | 48 +++++++++++++++++++++++++--------
>  1 file changed, 37 insertions(+), 11 deletions(-)
> 
> diff --git a/drivers/vfio/vfio_iommu_type1.c b/drivers/vfio/vfio_iommu_type1.c
> index 0ac56072af9f..bd46ed9361fe 100644
> --- a/drivers/vfio/vfio_iommu_type1.c
> +++ b/drivers/vfio/vfio_iommu_type1.c
> @@ -319,15 +319,22 @@ static void vfio_dma_bitmap_free_all(struct vfio_iommu *iommu)
>  /*
>   * Helper Functions for host iova-pfn list
>   */
> -static struct vfio_pfn *vfio_find_vpfn(struct vfio_dma *dma, dma_addr_t iova)
> +
> +/*
> + * Find the first vfio_pfn that overlapping the range
> + * [iova, iova + PAGE_SIZE * npage) in rb tree.
> + */
> +static struct vfio_pfn *vfio_find_vpfn_range(struct vfio_dma *dma,
> +		dma_addr_t iova, unsigned long npage)
>  {
>  	struct vfio_pfn *vpfn;
>  	struct rb_node *node = dma->pfn_list.rb_node;
> +	dma_addr_t end_iova = iova + PAGE_SIZE * npage;
>  
>  	while (node) {
>  		vpfn = rb_entry(node, struct vfio_pfn, node);
>  
> -		if (iova < vpfn->iova)
> +		if (end_iova <= vpfn->iova)
>  			node = node->rb_left;
>  		else if (iova > vpfn->iova)
>  			node = node->rb_right;
> @@ -337,6 +344,11 @@ static struct vfio_pfn *vfio_find_vpfn(struct vfio_dma *dma, dma_addr_t iova)
>  	return NULL;
>  }
>  
> +static inline struct vfio_pfn *vfio_find_vpfn(struct vfio_dma *dma, dma_addr_t iova)
> +{
> +	return vfio_find_vpfn_range(dma, iova, 1);
> +}
> +
>  static void vfio_link_pfn(struct vfio_dma *dma,
>  			  struct vfio_pfn *new)
>  {
> @@ -681,32 +693,46 @@ static long vfio_pin_pages_remote(struct vfio_dma *dma, unsigned long vaddr,
>  		 * and rsvd here, and therefore continues to use the batch.
>  		 */
>  		while (true) {
> +			struct folio *folio = page_folio(batch->pages[batch->offset]);
> +			long nr_pages;
> +
>  			if (pfn != *pfn_base + pinned ||
>  			    rsvd != is_invalid_reserved_pfn(pfn))
>  				goto out;
>  
> +			/*
> +			 * Note: The current nr_pages does not achieve the optimal
> +			 * performance in scenarios where folio_nr_pages() exceeds
> +			 * batch->capacity. It is anticipated that future enhancements
> +			 * will address this limitation.
> +			 */
> +			nr_pages = min_t(long, batch->size, folio_nr_pages(folio) -
> +						folio_page_idx(folio, batch->pages[batch->offset]));
> +			if (nr_pages > 1 && vfio_find_vpfn_range(dma, iova, nr_pages))
> +				nr_pages = 1;
> +
>  			/*
>  			 * Reserved pages aren't counted against the user,
>  			 * externally pinned pages are already counted against
>  			 * the user.
>  			 */
> -			if (!rsvd && !vfio_find_vpfn(dma, iova)) {
> +			if (!rsvd && (nr_pages > 1 || !vfio_find_vpfn(dma, iova))) {
>  				if (!dma->lock_cap &&
> -				    mm->locked_vm + lock_acct + 1 > limit) {
> +				    mm->locked_vm + lock_acct + nr_pages > limit) {
>  					pr_warn("%s: RLIMIT_MEMLOCK (%ld) exceeded\n",
>  						__func__, limit << PAGE_SHIFT);
>  					ret = -ENOMEM;
>  					goto unpin_out;
>  				}
> -				lock_acct++;
> +				lock_acct += nr_pages;
>  			}
>  
> -			pinned++;
> -			npage--;
> -			vaddr += PAGE_SIZE;
> -			iova += PAGE_SIZE;
> -			batch->offset++;
> -			batch->size--;
> +			pinned += nr_pages;
> +			npage -= nr_pages;
> +			vaddr += PAGE_SIZE * nr_pages;
> +			iova += PAGE_SIZE * nr_pages;
> +			batch->offset += nr_pages;
> +			batch->size -= nr_pages;
>  
>  			if (!batch->size)
>  				break;