From: "Kirill A. Shutemov" <kirill@shutemov.name>
Subject: Re: [PATCH v5 1/5] mm: add vm_insert_mixed_mkwrite()
Date: Tue, 25 Jul 2017 01:14:00 +0300
Message-ID: <20170724221400.pcq5zvke7w2yfkxi@node.shutemov.name>
References: <20170724170616.25810-1-ross.zwisler@linux.intel.com>
 <20170724170616.25810-2-ross.zwisler@linux.intel.com>
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Cc: Andrew Morton <akpm@linux-foundation.org>, linux-kernel@vger.kernel.org,
	"Darrick J. Wong" <darrick.wong@oracle.com>,
	Theodore Ts'o <tytso@mit.edu>,
	Alexander Viro <viro@zeniv.linux.org.uk>,
	Andreas Dilger <adilger.kernel@dilger.ca>,
	Christoph Hellwig <hch@lst.de>,
	Dan Williams <dan.j.williams@intel.com>,
	Dave Chinner <david@fromorbit.com>, Ingo Molnar <mingo@redhat.com>,
	Jan Kara <jack@suse.cz>, Jonathan Corbet <corbet@lwn.net>,
	Matthew Wilcox <mawilcox@microsoft.com>,
	Steven Rostedt <rostedt@goodmis.org>, linux-doc@vger.kernel.org,
	linux-ext4@vger.kernel.org, linux-fsdevel@vger.kernel.org,
	linux-mm@kvack.org, linux-nvdimm@lists.01.org,
	linux-xfs@vger.kernel.org
To: Ross Zwisler <ross.zwisler@linux.intel.com>
Return-path: <owner-linux-mm@kvack.org>
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In-Reply-To: <20170724170616.25810-2-ross.zwisler@linux.intel.com>
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On Mon, Jul 24, 2017 at 11:06:12AM -0600, Ross Zwisler wrote:
> @@ -1658,14 +1658,35 @@ static int insert_pfn(struct vm_area_struct *vma, unsigned long addr,
>  	if (!pte)
>  		goto out;
>  	retval = -EBUSY;
> -	if (!pte_none(*pte))
> -		goto out_unlock;
> +	if (!pte_none(*pte)) {
> +		if (mkwrite) {
> +			/*
> +			 * For read faults on private mappings the PFN passed
> +			 * in may not match the PFN we have mapped if the
> +			 * mapped PFN is a writeable COW page.  In the mkwrite
> +			 * case we are creating a writable PTE for a shared
> +			 * mapping and we expect the PFNs to match.
> +			 */

Can we?

I guess it's up to filesystem if it wants to reuse the same spot to write
data or not. I think your assumptions works for ext4 and xfs. I wouldn't
be that sure for btrfs or other filesystems with CoW support.

-- 
 Kirill A. Shutemov

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