linux-kernel - Re: [PATCH] Revert "vfio/type1: Unpin zero pages"

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Date: Fri, 1 Mar 2024 11:53:01 +0100
From: David Hildenbrand <david@...hat.com>
To: Alex Williamson <alex.williamson@...hat.com>
Cc: kvm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] Revert "vfio/type1: Unpin zero pages"

On 29.02.24 23:35, Alex Williamson wrote:
> This reverts commit 873aefb376bbc0ed1dd2381ea1d6ec88106fdbd4.
> 
> This was a heinous workaround and it turns out it's been fixed in mm
> twice since it was introduced.  Most recently, commit c8070b787519
> ("mm: Don't pin ZERO_PAGE in pin_user_pages()") would have prevented
> running up the zeropage refcount, but even before that commit
> 84209e87c696 ("mm/gup: reliable R/O long-term pinning in COW mappings")
> avoids the vfio use case from pinning the zeropage at all, instead
> replacing it with exclusive anonymous pages.
> 
> Remove this now useless overhead.
> 
> Suggested-by: David Hildenbrand <david@...hat.com>
> Signed-off-by: Alex Williamson <alex.williamson@...hat.com>
> ---
>   drivers/vfio/vfio_iommu_type1.c | 12 ------------
>   1 file changed, 12 deletions(-)
> 
> diff --git a/drivers/vfio/vfio_iommu_type1.c b/drivers/vfio/vfio_iommu_type1.c
> index b2854d7939ce..b5c15fe8f9fc 100644
> --- a/drivers/vfio/vfio_iommu_type1.c
> +++ b/drivers/vfio/vfio_iommu_type1.c
> @@ -567,18 +567,6 @@ static int vaddr_get_pfns(struct mm_struct *mm, unsigned long vaddr,
>   	ret = pin_user_pages_remote(mm, vaddr, npages, flags | FOLL_LONGTERM,
>   				    pages, NULL);
>   	if (ret > 0) {
> -		int i;
> -
> -		/*
> -		 * The zero page is always resident, we don't need to pin it
> -		 * and it falls into our invalid/reserved test so we don't
> -		 * unpin in put_pfn().  Unpin all zero pages in the batch here.
> -		 */
> -		for (i = 0 ; i < ret; i++) {
> -			if (unlikely(is_zero_pfn(page_to_pfn(pages[i]))))
> -				unpin_user_page(pages[i]);
> -		}
> -
>   		*pfn = page_to_pfn(pages[0]);
>   		goto done;
>   	}

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

-- 
Cheers,

David / dhildenb