From: Christoph Hellwig <hch-jcswGhMUV9g@public.gmane.org>
Subject: Re: [PATCH v5 1/5] mm: add vm_insert_mixed_mkwrite()
Date: Tue, 25 Jul 2017 14:15:22 +0200
Message-ID: <20170725121522.GA13457@lst.de>
References: <20170724170616.25810-1-ross.zwisler@linux.intel.com>
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 <20170724221400.pcq5zvke7w2yfkxi@node.shutemov.name>
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Cc: Theodore Ts'o <tytso-3s7WtUTddSA@public.gmane.org>, Andrew Morton <akpm-de/tnXTf+JLsfHDXvbKv3WD2FQJk+8+b@public.gmane.org>,
 "Darrick J. Wong" <darrick.wong-QHcLZuEGTsvQT0dZR+AlfA@public.gmane.org>,
 Dave Chinner <david-FqsqvQoI3Ljby3iVrkZq2A@public.gmane.org>, Jonathan Corbet <corbet-T1hC0tSOHrs@public.gmane.org>,
 linux-kernel-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, Steven Rostedt <rostedt-nx8X9YLhiw1AfugRpC6u6w@public.gmane.org>,
 linux-doc-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, linux-xfs-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, linux-mm-Bw31MaZKKs3YtjvyW6yDsg@public.gmane.org,
 Ingo Molnar <mingo-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>, Andreas Dilger <adilger.kernel-m1MBpc4rdrD3fQ9qLvQP4Q@public.gmane.org>,
 Alexander Viro <viro-RmSDqhL/yNMiFSDQTTA3OLVCufUGDwFn@public.gmane.org>, linux-fsdevel-u79uwXL29TY76Z2rM5mHXA@public.gmane.org,
 "Kirill A. Shutemov" <kirill-oKw7cIdHH8eLwutG50LtGA@public.gmane.org>, linux-ext4-u79uwXL29TY76Z2rM5mHXA@public.gmane.org,
 Christoph Hellwig <hch-jcswGhMUV9g@public.gmane.org>, Matthew Wilcox <mawilcox-0li6OtcxBFHby3iVrkZq2A@public.gmane.org>,
 linux-nvdimm-hn68Rpc1hR1g9hUCZPvPmw@public.gmane.org
To: Jan Kara <jack-AlSwsSmVLrQ@public.gmane.org>
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On Tue, Jul 25, 2017 at 11:35:08AM +0200, Jan Kara wrote:
> On Tue 25-07-17 10:01:58, Christoph Hellwig wrote:
> > On Tue, Jul 25, 2017 at 01:14:00AM +0300, Kirill A. Shutemov wrote:
> > > 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.
> > 
> > Or XFS with reflinks for that matter.  Which currently can't be
> > combined with DAX, but I had a somewhat working version a few month
> > ago.
> 
> But in cases like COW when the block mapping changes, the process
> must run unmap_mapping_range() before installing the new PTE so that all
> processes mapping this file offset actually refault and see the new
> mapping. So this would go through pte_none() case. Am I missing something?

Yes, for DAX COW mappings we'd probably need something like this, unlike
the pagecache COW handling for which only the underlying block change,
but not the page.