linux-kernel - Re: [PATCH 1/2] mm: avoid spurious 'bad pmd' warning messages

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Message-ID: <9c45c769-2f5e-9327-c39e-1df7744fa633@intel.com>
Date:   Wed, 17 May 2017 10:33:58 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     Ross Zwisler <ross.zwisler@...ux.intel.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        linux-kernel@...r.kernel.org
Cc:     "Darrick J. Wong" <darrick.wong@...cle.com>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Christoph Hellwig <hch@....de>,
        Dan Williams <dan.j.williams@...el.com>,
        Jan Kara <jack@...e.cz>,
        Matthew Wilcox <mawilcox@...rosoft.com>,
        linux-fsdevel@...r.kernel.org, linux-mm@...ck.org,
        linux-nvdimm@...ts.01.org,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        Pawel Lebioda <pawel.lebioda@...el.com>,
        Dave Jiang <dave.jiang@...el.com>,
        Xiong Zhou <xzhou@...hat.com>, Eryu Guan <eguan@...hat.com>,
        stable@...r.kernel.org
Subject: Re: [PATCH 1/2] mm: avoid spurious 'bad pmd' warning messages

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?