Re: [PATCH 1/3] fs, xfs: introduce S_IOMAP_IMMUTABLE

From: Christoph Hellwig
Date: Sat Aug 05 2017 - 05:46:16 EST

On Tue, Aug 01, 2017 at 12:42:18PM +1000, Dave Chinner wrote:
> I've outlined other use cases in previous discussions. To repeat
> myself, every so often we get someone with, say, a new high
> speed camera that want to dma the camera frames direct to the
> storage because they can't push 500,000 frames/s through the CPU
> to storage. Hence they want to bypass the OS and DMA the data direct
> to the storage. To do this they need a mechanism to freeze and unfreeze
> the block map of the file so that nothing modifies the block map
> while the camera hardware is dumping data direct to the storage.
> Immutable extent maps provide the functionality they need to
> implement this safely.

And we have such a mechanism already: it's called the iolock during
I/O, and dirct I/O. I've worked on plenty such schemes and the proper way
works perfectly fine. Just because people ask for stupid ways to
archives that doesn't mean they understand what they are doing.

> There's also other similar use cases for RDMA targets on PMEM
> (regardless of whether DAX is enabled or not), and I've come across
> a couple of requests for mechanisms to allow fabric based nvme
> storage to do direct data transfers between storage devices, too.
> All of these use cases can be safely implemented if there is a
> mechanism to mark extent maps as immutable for the duration of
> the operation they need to perform.

As someone who spent most of them time on the last 2 years in this
area: we have a massive problem discoverability and addressing
(lack of struct page) for p2p devices. We have absolutely no problem
with the direct I/O model with them.

> DAX isn't the driver of that functionality, it's the other use cases
> that need it, and why the proposed "only remove flag if len == 0"
> API is a non-starter....

The other "use" cases are even more bullshit than the DAX one.