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

[Dan Williams]

On Thu, Aug 24, 2017 at 9:39 AM, Christoph Hellwig <hch@xxxxxx> 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.