linux-ext4 - RE: [PATCH v2 2/9] ext2: tell DAX the size of allocation holes

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Message-ID: <DM2PR21MB0089C20EF469AA91A916867CCBFD0@DM2PR21MB0089.namprd21.prod.outlook.com>
Date:   Sat, 10 Sep 2016 07:52:53 +0000
From:   Matthew Wilcox <mawilcox@...rosoft.com>
To:     Christoph Hellwig <hch@...radead.org>
CC:     Ross Zwisler <ross.zwisler@...ux.intel.com>,
        Theodore Ts'o <tytso@....edu>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "linux-nvdimm@...1.01.org" <linux-nvdimm@...1.01.org>,
        Dave Chinner <david@...morbit.com>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "Andreas Dilger" <adilger.kernel@...ger.ca>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Jan Kara <jack@...e.com>,
        "linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
        "linux-ext4@...r.kernel.org" <linux-ext4@...r.kernel.org>
Subject: RE: [PATCH v2 2/9] ext2: tell DAX the size of allocation holes

From: Christoph Hellwig [mailto:hch@...radead.org]
> On Sat, Sep 10, 2016 at 07:33:18AM +0000, Matthew Wilcox wrote:
> > > caller specific is unaceptable.  That being said your idea doesn't
> > > sounds unreasonable, but will require a bit more work and has no
> > > real short-term need.
> >
> > So your proposal is to remove buffer_heads from ext2?
> 
> No, the proposal is to remove buffer_heads from XFS first, then GFS2 and then
> maybe others like ext4.  I'd like to remove buffer_heads from the DAX path for
> ext2 and ext4 entitrely for sure (and direct I/O next).

That's ... what I propose.  The only use of buffer_head in my proposal is to
communicate a single extent from the filesystem to the DAX core, and that
can equally well use an iomap.  Ross seems to think that converting the current
DAX code over to using iomap requires converting all of ext2 away from
buffer_head; are you saying he's wrong?

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