Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
Date:   Mon, 27 Apr 2020 05:28:36 -0700
From:   Matthew Wilcox <willy@infradead.org>
To:     Shiyang Ruan <ruansy.fnst@cn.fujitsu.com>
Cc:     linux-kernel@vger.kernel.org, linux-xfs@vger.kernel.org,
        linux-nvdimm@lists.01.org, linux-mm@kvack.org,
        linux-fsdevel@vger.kernel.org, darrick.wong@oracle.com,
        dan.j.williams@intel.com, david@fromorbit.com, hch@lst.de,
        rgoldwyn@suse.de, qi.fuli@fujitsu.com, y-goto@fujitsu.com
Subject: Re: [RFC PATCH 0/8] dax: Add a dax-rmap tree to support reflink
Message-ID: <20200427122836.GD29705@bombadil.infradead.org>
References: <20200427084750.136031-1-ruansy.fnst@cn.fujitsu.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20200427084750.136031-1-ruansy.fnst@cn.fujitsu.com>
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

On Mon, Apr 27, 2020 at 04:47:42PM +0800, Shiyang Ruan wrote:
> This patchset is a try to resolve the shared 'page cache' problem for
> fsdax.
> 
> In order to track multiple mappings and indexes on one page, I
> introduced a dax-rmap rb-tree to manage the relationship.  A dax entry
> will be associated more than once if is shared.  At the second time we
> associate this entry, we create this rb-tree and store its root in
> page->private(not used in fsdax).  Insert (->mapping, ->index) when
> dax_associate_entry() and delete it when dax_disassociate_entry().

Do we really want to track all of this on a per-page basis?  I would
have thought a per-extent basis was more useful.  Essentially, create
a new address_space for each shared extent.  Per page just seems like
a huge overhead.