linux-ext4 - Re: [PATCH v5 1/5] mm: add vm_insert_mixed

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Message-ID: <20170725121522.GA13457@lst.de>
Date:   Tue, 25 Jul 2017 14:15:22 +0200
From:   Christoph Hellwig <hch@....de>
To:     Jan Kara <jack@...e.cz>
Cc:     Christoph Hellwig <hch@....de>,
        "Kirill A. Shutemov" <kirill@...temov.name>,
        Ross Zwisler <ross.zwisler@...ux.intel.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        linux-kernel@...r.kernel.org,
        "Darrick J. Wong" <darrick.wong@...cle.com>,
        Theodore Ts'o <tytso@....edu>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Andreas Dilger <adilger.kernel@...ger.ca>,
        Dan Williams <dan.j.williams@...el.com>,
        Dave Chinner <david@...morbit.com>,
        Ingo Molnar <mingo@...hat.com>,
        Jonathan Corbet <corbet@....net>,
        Matthew Wilcox <mawilcox@...rosoft.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        linux-doc@...r.kernel.org, linux-ext4@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, linux-mm@...ck.org,
        linux-nvdimm@...ts.01.org, linux-xfs@...r.kernel.org
Subject: Re: [PATCH v5 1/5] mm: add vm_insert_mixed_mkwrite()

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.