Re: [Xen-devel] [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  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.

___
Xen-devel mailing list
Xen-devel@lists.xen.org
https://lists.xen.org/xen-devel

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

[Christoph Hellwig]

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.

___
Xen-devel mailing list
Xen-devel@lists.xen.org
https://lists.xen.org/xen-devel

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

[Dan Williams]

[ adding Xen ]

On Thu, Aug 24, 2017 at 9:11 AM, Christoph Hellwig  wrote:
> I still can't make any sense of this description.  What is an external
> agent?  Userspace obviously can't ever see a change in the extent
> map, so it can't be meant.

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

> It would help a lot if you could come up with a concrete user for this,
> including example code.

Will do.

___
Xen-devel mailing list
Xen-devel@lists.xen.org
https://lists.xen.org/xen-devel