Re: [PATCH 0/3] iopmem : A block device for PCIe memory

[Stephen Bates]

On Tue, October 25, 2016 3:19 pm, Dave Chinner wrote:
> On Tue, Oct 25, 2016 at 05:50:43AM -0600, Stephen Bates wrote:
>>
>> Dave are you saying that even for local mappings of files on a DAX
>> capable system it is possible for the mappings to move on you unless the
>> FS supports locking?
>>
>
> Yes.
>
>
>> Does that not mean DAX on such FS is
>> inherently broken?
>
> No. DAX is accessed through a virtual mapping layer that abstracts
> the physical location from userspace applications.
>
> Example: think copy-on-write overwrites. It occurs atomically from
> the perspective of userspace and starts by invalidating any current
> mappings userspace has of that physical location. The location is changes,
> the data copied in, and then when the locks are released userspace can
> fault in a new page table mapping on the next access

Dave

Thanks for the good input and for correcting some of my DAX
misconceptions! We will certainly be taking this into account as we
consider v1.

>
 And at least for XFS we have such a mechanism :)  E.g. I have a
 prototype of a pNFS layout that uses XFS+DAX to allow clients to do
 RDMA directly to XFS files, with the same locking mechanism we use
 for the current block and scsi layout in xfs_pnfs.c.
>>
>> Thanks for fixing this issue on XFS Christoph! I assume this problem
>> continues to exist on the other DAX capable FS?
>
> Yes, but it they implement the exportfs API that supplies this
> capability, they'll be able to use pNFS, too.
>
>> One more reason to consider a move to /dev/dax I guess ;-)...
>>
>
> That doesn't get rid of the need for sane access control arbitration
> across all machines that are directly accessing the storage. That's the
> problem pNFS solves, regardless of whether your direct access target is a
> filesystem, a block device or object storage...

Fair point. I am still hoping for a bit more discussion on the best choice
of user-space interface for this work. If/When that happens we will take
it into account when we look at spinning the patchset.


Stephen

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Re: [PATCH 0/3] iopmem : A block device for PCIe memory

[Sagi Grimberg]

You do realise that local filesystems can silently change the
location of file data at any point in time, so there is no such
thing as a "stable mapping" of file data to block device addresses
in userspace?

If you want remote access to the blocks owned and controlled by a
filesystem, then you need to use a filesystem with a remote locking
mechanism to allow co-ordinated, coherent access to the data in
those blocks. Anything else is just asking for ongoing, unfixable
filesystem corruption or data leakage problems (i.e.  security
issues).


And at least for XFS we have such a mechanism :)  E.g. I have a
prototype of a pNFS layout that uses XFS+DAX to allow clients to do
RDMA directly to XFS files, with the same locking mechanism we use
for the current block and scsi layout in xfs_pnfs.c.


Christoph, did you manage to leap to the future and solve the
RDMA persistency hole? :)

e.g. what happens with O_DSYNC in this model? Or you did
a message exchange for commits?
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Re: [PATCH 0/3] iopmem : A block device for PCIe memory

[Christoph Hellwig]

On Thu, Oct 27, 2016 at 01:22:49PM +0300, Sagi Grimberg wrote:
> Christoph, did you manage to leap to the future and solve the
> RDMA persistency hole? :)
> 
> e.g. what happens with O_DSYNC in this model? Or you did
> a message exchange for commits?

Yes, pNFS calls this the layoutcommit.  That being said once we get a RDMA
commit or flush operation we could easily make the layoutcommit optional
for some operations.  There already is a precedence for the in the
flexfiles layout specification.
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Re: [PATCH 0/3] iopmem : A block device for PCIe memory

[Dan Williams]

On Wed, Oct 26, 2016 at 1:24 AM, Haggai Eran  wrote:
[..]
>> I wonder if we could (ab)use a
>> software-defined 'pasid' as the requester id for a peer-to-peer
>> mapping that needs address translation.
> Why would you need that? Isn't it enough to map the peer-to-peer
> addresses correctly in the iommu driver?
>

You're right, we might already have enough...

We would just need to audit iommu drivers to undo any assumptions that
the page being mapped is always in host memory and apply any bus
address translations between source device and target device.
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Re: [PATCH 0/3] iopmem : A block device for PCIe memory

[Haggai Eran]

On 10/19/2016 6:51 AM, Dan Williams wrote:
> On Tue, Oct 18, 2016 at 2:42 PM, Stephen Bates  wrote:
>> 1. Address Translation. Suggestions have been made that in certain
>> architectures and topologies the dma_addr_t passed to the DMA master
>> in a peer-2-peer transfer will not correctly route to the IO memory
>> intended. However in our testing to date we have not seen this to be
>> an issue, even in systems with IOMMUs and PCIe switches. It is our
>> understanding that an IOMMU only maps system memory and would not
>> interfere with device memory regions.
I'm not sure that's the case. I think it works because with ZONE_DEVICE,
the iommu driver will simply treat a dma_map_page call as any other PFN,
and create a mapping as it does for any memory page.

>> (It certainly has no opportunity
>> to do so if the transfer gets routed through a switch).
It can still go through the IOMMU if you enable ACS upstream forwarding.

> There may still be platforms where peer-to-peer cycles are routed up
> through the root bridge and then back down to target device, but we
> can address that when / if it happens.
I agree.

> I wonder if we could (ab)use a
> software-defined 'pasid' as the requester id for a peer-to-peer
> mapping that needs address translation.
Why would you need that? Isn't it enough to map the peer-to-peer
addresses correctly in the iommu driver?

Haggai
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Re: [PATCH 0/3] iopmem : A block device for PCIe memory

[Dave Chinner]

On Tue, Oct 25, 2016 at 05:50:43AM -0600, Stephen Bates wrote:
> Hi Dave and Christoph
> 
> On Fri, Oct 21, 2016 at 10:12:53PM +1100, Dave Chinner wrote:
> > On Fri, Oct 21, 2016 at 02:57:14AM -0700, Christoph Hellwig wrote:
> > > On Fri, Oct 21, 2016 at 10:22:39AM +1100, Dave Chinner wrote:
> > > > You do realise that local filesystems can silently change the
> > > > location of file data at any point in time, so there is no such
> > > > thing as a "stable mapping" of file data to block device addresses
> > > > in userspace?
> > > >
> > > > If you want remote access to the blocks owned and controlled by a
> > > > filesystem, then you need to use a filesystem with a remote locking
> > > > mechanism to allow co-ordinated, coherent access to the data in
> > > > those blocks. Anything else is just asking for ongoing, unfixable
> > > > filesystem corruption or data leakage problems (i.e.  security
> > > > issues).
> > >
> 
> Dave are you saying that even for local mappings of files on a DAX
> capable system it is possible for the mappings to move on you unless
> the FS supports locking?

Yes.

> Does that not mean DAX on such FS is
> inherently broken?

No. DAX is accessed through a virtual mapping layer that abstracts
the physical location from userspace applications.

Example: think copy-on-write overwrites. It occurs atomically from
the perspective of userspace and starts by invalidating any current
mappings userspace has of that physical location. The location is
changes, the data copied in, and then when the locks are released
userspace can fault in a new page table mapping on the next
access

> > > And at least for XFS we have such a mechanism :)  E.g. I have a
> > > prototype of a pNFS layout that uses XFS+DAX to allow clients to do
> > > RDMA directly to XFS files, with the same locking mechanism we use
> > > for the current block and scsi layout in xfs_pnfs.c.
> 
> Thanks for fixing this issue on XFS Christoph! I assume this problem
> continues to exist on the other DAX capable FS?

Yes, but it they implement the exportfs API that supplies this
capability, they'll be able to use pNFS, too.

> One more reason to consider a move to /dev/dax I guess ;-)...

That doesn't get rid of the need for sane access control arbitration
across all machines that are directly accessing the storage. That's
the problem pNFS solves, regardless of whether your direct access
target is a filesystem, a block device or object storage...

Cheers,

Dave.
-- 
Dave Chinner
da...@fromorbit.com
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Re: [PATCH 0/3] iopmem : A block device for PCIe memory

[Dave Chinner]

On Fri, Oct 21, 2016 at 02:57:14AM -0700, Christoph Hellwig wrote:
> On Fri, Oct 21, 2016 at 10:22:39AM +1100, Dave Chinner wrote:
> > You do realise that local filesystems can silently change the
> > location of file data at any point in time, so there is no such
> > thing as a "stable mapping" of file data to block device addresses
> > in userspace?
> > 
> > If you want remote access to the blocks owned and controlled by a
> > filesystem, then you need to use a filesystem with a remote locking
> > mechanism to allow co-ordinated, coherent access to the data in
> > those blocks. Anything else is just asking for ongoing, unfixable
> > filesystem corruption or data leakage problems (i.e.  security
> > issues).
> 
> And at least for XFS we have such a mechanism :)  E.g. I have a
> prototype of a pNFS layout that uses XFS+DAX to allow clients to do
> RDMA directly to XFS files, with the same locking mechanism we use
> for the current block and scsi layout in xfs_pnfs.c.

Oh, that's good to know - pNFS over XFS was exactly what I was
thinking of when I wrote my earlier reply. A few months ago someone
else was trying to use file mappings in userspace for direct remote
client access on fabric connected devices. I told them "pNFS on XFS
and write an efficient transport for you hardware"

Now that I know we've got RDMA support for pNFS on XFS in the
pipeline, I can just tell them "just write an rdma driver for your
hardware" instead. :P

Cheers,

Dave.
-- 
Dave Chinner
da...@fromorbit.com
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Re: [PATCH 0/3] iopmem : A block device for PCIe memory

[Christoph Hellwig]

On Fri, Oct 21, 2016 at 10:22:39AM +1100, Dave Chinner wrote:
> You do realise that local filesystems can silently change the
> location of file data at any point in time, so there is no such
> thing as a "stable mapping" of file data to block device addresses
> in userspace?
> 
> If you want remote access to the blocks owned and controlled by a
> filesystem, then you need to use a filesystem with a remote locking
> mechanism to allow co-ordinated, coherent access to the data in
> those blocks. Anything else is just asking for ongoing, unfixable
> filesystem corruption or data leakage problems (i.e.  security
> issues).

And at least for XFS we have such a mechanism :)  E.g. I have a
prototype of a pNFS layout that uses XFS+DAX to allow clients to do
RDMA directly to XFS files, with the same locking mechanism we use
for the current block and scsi layout in xfs_pnfs.c.
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Re: [PATCH 0/3] iopmem : A block device for PCIe memory

[Dan Williams]

[ adding Ashok and David for potential iommu comments ]

On Tue, Oct 18, 2016 at 2:42 PM, Stephen Bates  wrote:
> This patch follows from an RFC we did earlier this year [1]. This
> patchset applies cleanly to v4.9-rc1.
>
> Updates since RFC
> -
>   Rebased.
>   Included the iopmem driver in the submission.
>
> History
> ---
>
> There have been several attempts to upstream patchsets that enable
> DMAs between PCIe peers. These include Peer-Direct [2] and DMA-Buf
> style patches [3]. None have been successful to date. Haggai Eran
> gives a nice overview of the prior art in this space in his cover
> letter [3].
>
> Motivation and Use Cases
> 
>
> PCIe IO devices are getting faster. It is not uncommon now to find PCIe
> network and storage devices that can generate and consume several GB/s.
> Almost always these devices have either a high performance DMA engine, a
> number of exposed PCIe BARs or both.
>
> Until this patch, any high-performance transfer of information between
> two PICe devices has required the use of a staging buffer in system
> memory. With this patch the bandwidth to system memory is not compromised
> when high-throughput transfers occurs between PCIe devices. This means
> that more system memory bandwidth is available to the CPU cores for data
> processing and manipulation. In addition, in systems where the two PCIe
> devices reside behind a PCIe switch the datapath avoids the CPU
> entirely.

I agree with the motivation and the need for a solution, but I have
some questions about this implementation.

>
> Consumers
> -
>
> We provide a PCIe device driver in an accompanying patch that can be
> used to map any PCIe BAR into a DAX capable block device. For
> non-persistent BARs this simply serves as an alternative to using
> system memory bounce buffers. For persistent BARs this can serve as an
> additional storage device in the system.

Why block devices?  I wonder if iopmem was initially designed back
when we were considering enabling DAX for raw block devices.  However,
that support has since been ripped out / abandoned.  You currently
need a filesystem on top of a block-device to get DAX operation.
Putting xfs or ext4 on top of PCI-E memory mapped range seems awkward
if all you want is a way to map the bar for another PCI-E device in
the topology.

If you're only using the block-device as a entry-point to create
dax-mappings then a device-dax (drivers/dax/) character-device might
be a better fit.

>
> Testing and Performance
> ---
>
> We have done a moderate about of testing of this patch on a QEMU
> environment and on real hardware. On real hardware we have observed
> peer-to-peer writes of up to 4GB/s and reads of up to 1.2 GB/s. In
> both cases these numbers are limitations of our consumer hardware. In
> addition, we have observed that the CPU DRAM bandwidth is not impacted
> when using IOPMEM which is not the case when a traditional path
> through system memory is taken.
>
> For more information on the testing and performance results see the
> GitHub site [4].
>
> Known Issues
> 
>
> 1. Address Translation. Suggestions have been made that in certain
> architectures and topologies the dma_addr_t passed to the DMA master
> in a peer-2-peer transfer will not correctly route to the IO memory
> intended. However in our testing to date we have not seen this to be
> an issue, even in systems with IOMMUs and PCIe switches. It is our
> understanding that an IOMMU only maps system memory and would not
> interfere with device memory regions. (It certainly has no opportunity
> to do so if the transfer gets routed through a switch).
>

There may still be platforms where peer-to-peer cycles are routed up
through the root bridge and then back down to target device, but we
can address that when / if it happens.  I wonder if we could (ab)use a
software-defined 'pasid' as the requester id for a peer-to-peer
mapping that needs address translation.

> 2. Memory Segment Spacing. This patch has the same limitations that
> ZONE_DEVICE does in that memory regions must be spaces at least
> SECTION_SIZE bytes part. On x86 this is 128MB and there are cases where
> BARs can be placed closer together than this. Thus ZONE_DEVICE would not
> be usable on neighboring BARs. For our purposes, this is not an issue as
> we'd only be looking at enabling a single BAR in a given PCIe device.
> More exotic use cases may have problems with this.

I'm working on patches for 4.10 to allow mixing multiple
devm_memremap_pages() allocations within the same physical section.
Hopefully this won't be a problem going forward.

> 3. Coherency Issues. When IOMEM is written from both the CPU and a PCIe
> peer there is potential for coherency issues and for writes to occur out
> of order. This is something that users of this feature need to be
> cognizant of. Though really, this isn't much different than the
> existing situation