linux-kernel - Re: [PATCH v6 4/5] fs, xfs: introduce MAP_DIRECT for creating block-map-atomic file ranges

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Date:   Thu, 24 Aug 2017 13:26:55 -0700
From:   Dan Williams <dan.j.williams@...el.com>
To:     Christoph Hellwig <hch@....de>
Cc:     Andrew Morton <akpm@...ux-foundation.org>, Jan Kara <jack@...e.cz>,
        "Darrick J. Wong" <darrick.wong@...cle.com>,
        Linux API <linux-api@...r.kernel.org>,
        "linux-nvdimm@...ts.01.org" <linux-nvdimm@...ts.01.org>,
        Dave Chinner <david@...morbit.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        linux-xfs@...r.kernel.org, Linux MM <linux-mm@...ck.org>,
        Jeff Moyer <jmoyer@...hat.com>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Andy Lutomirski <luto@...nel.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Ross Zwisler <ross.zwisler@...ux.intel.com>,
        xen-devel@...ts.xen.org
Subject: Re: [PATCH v6 4/5] fs, xfs: introduce MAP_DIRECT for creating
 block-map-atomic file ranges

On Thu, Aug 24, 2017 at 9:39 AM, Christoph Hellwig <hch@....de> wrote:
> On Thu, Aug 24, 2017 at 09:31:17AM -0700, Dan Williams wrote:
>> External agent is a DMA device, or a hypervisor like Xen. In the DMA
>> case perhaps we can use the fcntl lease mechanism, I'll investigate.
>> In the Xen case it actually would need to use fiemap() to discover the
>> physical addresses that back the file to setup their M2P tables.
>> Here's the discussion where we discovered that physical address
>> dependency:
>>
>>     https://lists.xen.org/archives/html/xen-devel/2017-04/msg00419.html
>
> fiemap does not work to discover physical addresses.  If they want
> to do anything involving physical address they will need a kernel
> driver.

True, it's broken with respect to multi-device filesystems and these
patches do nothing to fix that problem. Ok, I'm fine to let that use
case depend on a kernel driver and just focus on fixing the DMA case.