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Re: [PATCH v4 5/7] fs: prioritize and separate direct_io from dax_io

To: "Verma, Vishal L" <vishal.l.verma@xxxxxxxxx>
Subject: Re: [PATCH v4 5/7] fs: prioritize and separate direct_io from dax_io
From: "hch@xxxxxxxxxxxxx" <hch@xxxxxxxxxxxxx>
Date: Sun, 8 May 2016 02:01:15 -0700
Cc: "Williams, Dan J" <dan.j.williams@xxxxxxxxx>, "hch@xxxxxxxxxxxxx" <hch@xxxxxxxxxxxxx>, "linux-kernel@xxxxxxxxxxxxxxx" <linux-kernel@xxxxxxxxxxxxxxx>, "linux-block@xxxxxxxxxxxxxxx" <linux-block@xxxxxxxxxxxxxxx>, "xfs@xxxxxxxxxxx" <xfs@xxxxxxxxxxx>, "linux-nvdimm@xxxxxxxxxxx" <linux-nvdimm@xxxxxxxxxxx>, "linux-mm@xxxxxxxxx" <linux-mm@xxxxxxxxx>, "viro@xxxxxxxxxxxxxxxxxx" <viro@xxxxxxxxxxxxxxxxxx>, "axboe@xxxxxx" <axboe@xxxxxx>, "akpm@xxxxxxxxxxxxxxxxxxxx" <akpm@xxxxxxxxxxxxxxxxxxxx>, "linux-fsdevel@xxxxxxxxxxxxxxx" <linux-fsdevel@xxxxxxxxxxxxxxx>, "linux-ext4@xxxxxxxxxxxxxxx" <linux-ext4@xxxxxxxxxxxxxxx>, "david@xxxxxxxxxxxxx" <david@xxxxxxxxxxxxx>, "jack@xxxxxxx" <jack@xxxxxxx>, "matthew@xxxxxx" <matthew@xxxxxx>
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On Thu, May 05, 2016 at 09:45:07PM +0000, Verma, Vishal L wrote:
> I'm not sure I completely understand how this will work? Can you explain
> a bit? Would we have to export rw_bytes up to layers above the pmem
> driver? Where does get_user_pages come in?

A DAX filesystem can directly use the nvdimm layer the same way btt
doe,s what's the problem?

Re get_user_pages my idea was to simply use that to lock down the user
pages so that we can call rw_bytes on it.  How else would you do it?  Do
a kmalloc, copy_from_user and then another memcpy?

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