From: "Kirill A. Shutemov" <kirill@shutemov.name>
Subject: Re: [PATCH v5 1/5] mm: add vm_insert_mixed_mkwrite()
Date: Tue, 25 Jul 2017 17:31:20 +0300
Message-ID: <20170725143120.fyjcxwsoff34vqoj@node.shutemov.name>
References: <20170724170616.25810-1-ross.zwisler@linux.intel.com>
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 <20170724221400.pcq5zvke7w2yfkxi@node.shutemov.name>
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 <20170725125037.GH19943@quack2.suse.cz>
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Cc: Christoph Hellwig <hch@lst.de>,
        Ross Zwisler <ross.zwisler@linux.intel.com>,
        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>,
        Dan Williams <dan.j.williams@intel.com>,
        Dave Chinner <david@fromorbit.com>,
        Ingo Molnar <mingo@redhat.com>,
        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: Jan Kara <jack@suse.cz>
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On Tue, Jul 25, 2017 at 02:50:37PM +0200, Jan Kara wrote:
> On Tue 25-07-17 14:15:22, Christoph Hellwig wrote:
> > 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.
> 
> Right. So again nothing where the WARN_ON should trigger.

Yes. I was confused on how COW is handled.

Acked-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>

-- 
 Kirill A. Shutemov