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Subject: Re: [PATCH 1/2] mm: avoid spurious 'bad pmd' warning messages
To: Ross Zwisler <ross.zwisler@linux.intel.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        linux-kernel@vger.kernel.org
References: <20170517171639.14501-1-ross.zwisler@linux.intel.com>
Cc: "Darrick J. Wong" <darrick.wong@oracle.com>,
        Alexander Viro <viro@zeniv.linux.org.uk>,
        Christoph Hellwig <hch@lst.de>,
        Dan Williams <dan.j.williams@intel.com>, Jan Kara <jack@suse.cz>,
        Matthew Wilcox <mawilcox@microsoft.com>, linux-fsdevel@vger.kernel.org,
        linux-mm@kvack.org, linux-nvdimm@ml01.01.org,
        "Kirill A . Shutemov" <kirill.shutemov@linux.intel.com>,
        Pawel Lebioda <pawel.lebioda@intel.com>,
        Dave Jiang <dave.jiang@intel.com>, Xiong Zhou <xzhou@redhat.com>,
        Eryu Guan <eguan@redhat.com>, stable@vger.kernel.org
From: Dave Hansen <dave.hansen@intel.com>
Message-ID: <9c45c769-2f5e-9327-c39e-1df7744fa633@intel.com>
Date: Wed, 17 May 2017 10:33:58 -0700
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On 05/17/2017 10:16 AM, Ross Zwisler wrote:
> @@ -3061,7 +3061,7 @@ static int pte_alloc_one_map(struct vm_fault *vmf)
>  	 * through an atomic read in C, which is what pmd_trans_unstable()
>  	 * provides.
>  	 */
> -	if (pmd_trans_unstable(vmf->pmd) || pmd_devmap(*vmf->pmd))
> +	if (pmd_devmap(*vmf->pmd) || pmd_trans_unstable(vmf->pmd))
>  		return VM_FAULT_NOPAGE;

I'm worried we are very unlikely to get this right in the future.  It's
totally not obvious what the ordering requirement is here.

Could we move pmd_devmap() and pmd_trans_unstable() into a helper that
gets the ordering right and also spells out the ordering requirement?