linux-kernel - Re: [PATCH v2 5/5] vfio/type1: optimize vfio_unpin_pages

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Message-ID: <77d99da0-10eb-4a4d-8ad9-c6ec83cb4540@redhat.com>
Date: Fri, 4 Jul 2025 10:47:00 +0200
From: David Hildenbrand <david@...hat.com>
To: lizhe.67@...edance.com, alex.williamson@...hat.com,
 akpm@...ux-foundation.org, peterx@...hat.com, jgg@...pe.ca
Cc: kvm@...r.kernel.org, linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH v2 5/5] vfio/type1: optimize vfio_unpin_pages_remote()

On 04.07.25 08:26, lizhe.67@...edance.com wrote:
> From: Li Zhe <lizhe.67@...edance.com>
> 
> When vfio_unpin_pages_remote() is called with a range of addresses that
> includes large folios, the function currently performs individual
> put_pfn() operations for each page. This can lead to significant
> performance overheads, especially when dealing with large ranges of pages.
> 
> It would be very rare for reserved PFNs and non reserved will to be mixed
> within the same range. So this patch utilizes the has_rsvd variable
> introduced in the previous patch to determine whether batch put_pfn()
> operations can be performed. Moreover, compared to put_pfn(),
> unpin_user_page_range_dirty_lock() is capable of handling large folio
> scenarios more efficiently.
> 
> The performance test results for completing the 16G VFIO IOMMU DMA
> unmapping are as follows.
> 
> Base(v6.16-rc4):
> ./vfio-pci-mem-dma-map 0000:03:00.0 16
> ------- AVERAGE (MADV_HUGEPAGE) --------
> VFIO UNMAP DMA in 0.135 s (118.6 GB/s)
> ------- AVERAGE (MAP_POPULATE) --------
> VFIO UNMAP DMA in 0.312 s (51.3 GB/s)
> ------- AVERAGE (HUGETLBFS) --------
> VFIO UNMAP DMA in 0.136 s (117.3 GB/s)
> 
> With this patchset:
> ------- AVERAGE (MADV_HUGEPAGE) --------
> VFIO UNMAP DMA in 0.045 s (357.0 GB/s)
> ------- AVERAGE (MAP_POPULATE) --------
> VFIO UNMAP DMA in 0.288 s (55.6 GB/s)
> ------- AVERAGE (HUGETLBFS) --------
> VFIO UNMAP DMA in 0.045 s (353.9 GB/s)
> 
> For large folio, we achieve an over 66% performance improvement in
> the VFIO UNMAP DMA item. For small folios, the performance test
> results appear to show a slight improvement.
> 
> Suggested-by: Jason Gunthorpe <jgg@...pe.ca>
> Signed-off-by: Li Zhe <lizhe.67@...edance.com>
> ---
>   drivers/vfio/vfio_iommu_type1.c | 20 ++++++++++++++++----
>   1 file changed, 16 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/vfio/vfio_iommu_type1.c b/drivers/vfio/vfio_iommu_type1.c
> index 13c5667d431c..3971539b0d67 100644
> --- a/drivers/vfio/vfio_iommu_type1.c
> +++ b/drivers/vfio/vfio_iommu_type1.c
> @@ -792,17 +792,29 @@ static long vfio_pin_pages_remote(struct vfio_dma *dma, unsigned long vaddr,
>   	return pinned;
>   }
>   
> +static inline void put_valid_unreserved_pfns(unsigned long start_pfn,
> +		unsigned long npage, int prot)
> +{
> +	unpin_user_page_range_dirty_lock(pfn_to_page(start_pfn), npage,
> +					 prot & IOMMU_WRITE);
> +}
> +
>   static long vfio_unpin_pages_remote(struct vfio_dma *dma, dma_addr_t iova,
>   				    unsigned long pfn, unsigned long npage,
>   				    bool do_accounting)
>   {
>   	long unlocked = 0, locked = vpfn_pages(dma, iova, npage);
> -	long i;
>   
> -	for (i = 0; i < npage; i++)
> -		if (put_pfn(pfn++, dma->prot))
> -			unlocked++;
> +	if (dma->has_rsvd) {
> +		long i;

No need to move "long i" here, but also doesn't really matter.

>   
> +		for (i = 0; i < npage; i++)
> +			if (put_pfn(pfn++, dma->prot))
> +				unlocked++;
> +	} else {
> +		put_valid_unreserved_pfns(pfn, npage, dma->prot);
> +		unlocked = npage;
> +	}
>   	if (do_accounting)
>   		vfio_lock_acct(dma, locked - unlocked, true);
>   

Reviewed-by: David Hildenbrand <david@...hat.com>

-- 
Cheers,

David / dhildenb