linux-kernel - Re: [PATCH v4 10/12] dax: add struct iomap based DAX PMD support

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Message-ID: <20161003163754.GB1496@lst.de>
Date:   Mon, 3 Oct 2016 18:37:54 +0200
From:   Christoph Hellwig <hch@....de>
To:     Jan Kara <jack@...e.cz>
Cc:     Ross Zwisler <ross.zwisler@...ux.intel.com>,
        linux-kernel@...r.kernel.org, Theodore Ts'o <tytso@....edu>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Andreas Dilger <adilger.kernel@...ger.ca>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Christoph Hellwig <hch@....de>,
        Dan Williams <dan.j.williams@...el.com>,
        Dave Chinner <david@...morbit.com>, Jan Kara <jack@...e.com>,
        Matthew Wilcox <mawilcox@...rosoft.com>,
        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 v4 10/12] dax: add struct iomap based DAX PMD support

On Mon, Oct 03, 2016 at 12:59:49PM +0200, Jan Kara wrote:
> I'm not quite sure if it is OK to call ->iomap_begin() without ever calling
> ->iomap_end. Specifically the comment before iomap_apply() says:
> 
> "It is assumed that the filesystems will lock whatever resources they
> require in the iomap_begin call, and release them in the iomap_end call."
> 
> so what you do could result in unbalanced allocations / locks / whatever.
> Christoph?

Indeed.  For XFS we only rely on iomap_end for error handling at the
moment, but it is intended to be paired for locking, as cluster file
systems like gfs2 requested this.