linux-kernel - Re: [PATCH] vfio/type1: avoid redundant PageReserved checking

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Date:   Tue, 27 Aug 2019 12:40:41 -0600
From:   Alex Williamson <alex.williamson@...hat.com>
To:     Ben Luo <luoben@...ux.alibaba.com>
Cc:     cohuck@...hat.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] vfio/type1: avoid redundant PageReserved checking

On Tue, 27 Aug 2019 20:49:48 +0800
Ben Luo <luoben@...ux.alibaba.com> wrote:

> currently, if the page is not a tail of compound page, it will be
> checked twice for the same thing.
> 
> Signed-off-by: Ben Luo <luoben@...ux.alibaba.com>
> ---
>  drivers/vfio/vfio_iommu_type1.c | 5 ++---
>  1 file changed, 2 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/vfio/vfio_iommu_type1.c b/drivers/vfio/vfio_iommu_type1.c
> index 054391f..cbe0d88 100644
> --- a/drivers/vfio/vfio_iommu_type1.c
> +++ b/drivers/vfio/vfio_iommu_type1.c
> @@ -291,11 +291,10 @@ static int vfio_lock_acct(struct vfio_dma *dma, long npage, bool async)
>  static bool is_invalid_reserved_pfn(unsigned long pfn)
>  {
>  	if (pfn_valid(pfn)) {
> -		bool reserved;
>  		struct page *tail = pfn_to_page(pfn);
>  		struct page *head = compound_head(tail);
> -		reserved = !!(PageReserved(head));
>  		if (head != tail) {
> +			bool reserved = !!(PageReserved(head));
>  			/*
>  			 * "head" is not a dangling pointer
>  			 * (compound_head takes care of that)
> @@ -310,7 +309,7 @@ static bool is_invalid_reserved_pfn(unsigned long pfn)
>  			if (PageTail(tail))
>  				return reserved;
>  		}
> -		return PageReserved(tail);
> +		return !!(PageReserved(tail));
>  	}
>  
>  	return true;

Logic seems fine to me, though I'd actually prefer to get rid of the !!
in the first use than duplicate it at the second use.  Thanks,

Alex