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Date:   Thu, 3 Oct 2019 12:41:33 -0400
From:   Andrea Arcangeli <aarcange@redhat.com>
To:     Ben Luo <luoben@linux.alibaba.com>
Cc:     alex.williamson@redhat.com, linux-kernel@vger.kernel.org
Subject: Re: [PATCH] vfio/type1: remove hugepage checks in
 is_invalid_reserved_pfn()
Message-ID: <20191003164133.GG13922@redhat.com>
References: <1d6b7e1c40783f2db4c6cb15bf679a94222ec6a3.1570073993.git.luoben@linux.alibaba.com>
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On Thu, Oct 03, 2019 at 11:49:42AM +0800, Ben Luo wrote:
> Currently, no hugepage split code can transfer the reserved bit
> from head to tail during the split, so checking the head can't make
> a difference in a racing condition with hugepage spliting.
> 
> The buddy wouldn't allow a driver to allocate an hugepage if any
> subpage is reserved in the e820 map at boot, if any driver sets the
> reserved bit of head page before mapping the hugepage in userland,
> it needs to set the reserved bit in all subpages to be safe.
> 
> Signed-off-by: Ben Luo <luoben@linux.alibaba.com>

Reviewed-by: Andrea Arcangeli <aarcange@redhat.com>


> ---
>  drivers/vfio/vfio_iommu_type1.c | 26 ++++----------------------
>  1 file changed, 4 insertions(+), 22 deletions(-)
> 
> diff --git a/drivers/vfio/vfio_iommu_type1.c b/drivers/vfio/vfio_iommu_type1.c
> index 054391f..e2019ba 100644
> --- a/drivers/vfio/vfio_iommu_type1.c
> +++ b/drivers/vfio/vfio_iommu_type1.c
> @@ -287,31 +287,13 @@ static int vfio_lock_acct(struct vfio_dma *dma, long npage, bool async)
>   * Some mappings aren't backed by a struct page, for example an mmap'd
>   * MMIO range for our own or another device.  These use a different
>   * pfn conversion and shouldn't be tracked as locked pages.
> + * For compound pages, any driver that sets the reserved bit in head
> + * page needs to set the reserved bit in all subpages to be safe.
>   */
>  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) {
> -			/*
> -			 * "head" is not a dangling pointer
> -			 * (compound_head takes care of that)
> -			 * but the hugepage may have been split
> -			 * from under us (and we may not hold a
> -			 * reference count on the head page so it can
> -			 * be reused before we run PageReferenced), so
> -			 * we've to check PageTail before returning
> -			 * what we just read.
> -			 */
> -			smp_rmb();
> -			if (PageTail(tail))
> -				return reserved;
> -		}
> -		return PageReserved(tail);
> -	}
> +	if (pfn_valid(pfn))
> +		return PageReserved(pfn_to_page(pfn));
>  
>  	return true;
>  }
> -- 
> 1.8.3.1
>