Re: [PATCH v5 02/10] block: Add copy offload support infrastructure

[Ming Lei]

On Wed, Dec 07, 2022 at 11:24:00AM +0530, Nitesh Shetty wrote:
> On Tue, Nov 29, 2022 at 05:14:28PM +0530, Nitesh Shetty wrote:
> > On Thu, Nov 24, 2022 at 08:03:56AM +0800, Ming Lei wrote:
> > > On Wed, Nov 23, 2022 at 03:37:12PM +0530, Nitesh Shetty wrote:
> > > > On Wed, Nov 23, 2022 at 04:04:18PM +0800, Ming Lei wrote:
> > > > > On Wed, Nov 23, 2022 at 11:28:19AM +0530, Nitesh Shetty wrote:
> > > > > > Introduce blkdev_issue_copy which supports source and destination bdevs,
> > > > > > and an array of (source, destination and copy length) tuples.
> > > > > > Introduce REQ_COPY copy offload operation flag. Create a read-write
> > > > > > bio pair with a token as payload and submitted to the device in order.
> > > > > > Read request populates token with source specific information which
> > > > > > is then passed with write request.
> > > > > > This design is courtesy Mikulas Patocka's token based copy
> > > > > 
> > > > > I thought this patchset is just for enabling copy command which is
> > > > > supported by hardware. But turns out it isn't, because blk_copy_offload()
> > > > > still submits read/write bios for doing the copy.
> > > > > 
> > > > > I am just wondering why not let copy_file_range() cover this kind of copy,
> > > > > and the framework has been there.
> > > > > 
> > > > 
> > > > Main goal was to enable copy command, but community suggested to add
> > > > copy emulation as well.
> > > > 
> > > > blk_copy_offload - actually issues copy command in driver layer.
> > > > The way read/write BIOs are percieved is different for copy offload.
> > > > In copy offload we check REQ_COPY flag in NVMe driver layer to issue
> > > > copy command. But we did missed it to add in other driver's, where they
> > > > might be treated as normal READ/WRITE.
> > > > 
> > > > blk_copy_emulate - is used if we fail or if device doesn't support native
> > > > copy offload command. Here we do READ/WRITE. Using copy_file_range for
> > > > emulation might be possible, but we see 2 issues here.
> > > > 1. We explored possibility of pulling dm-kcopyd to block layer so that we 
> > > > can readily use it. But we found it had many dependecies from dm-layer.
> > > > So later dropped that idea.
> > > 
> > > Is it just because dm-kcopyd supports async copy? If yes, I believe we
> > > can reply on io_uring for implementing async copy_file_range, which will
> > > be generic interface for async copy, and could get better perf.
> > >
> > 
> > It supports both sync and async. But used only inside dm-layer.
> > Async version of copy_file_range can help, using io-uring can be helpful
> > for user , but in-kernel users can't use uring.
> > 
> > > > 2. copy_file_range, for block device atleast we saw few check's which fail
> > > > it for raw block device. At this point I dont know much about the history of
> > > > why such check is present.
> > > 
> > > Got it, but IMO the check in generic_copy_file_checks() can be
> > > relaxed to cover blkdev cause splice does support blkdev.
> > > 
> > > Then your bdev offload copy work can be simplified into:
> > > 
> > > 1) implement .copy_file_range for def_blk_fops, suppose it is
> > > blkdev_copy_file_range()
> > > 
> > > 2) inside blkdev_copy_file_range()
> > > 
> > > - if the bdev supports offload copy, just submit one bio to the device,
> > > and this will be converted to one pt req to device
> > > 
> > > - otherwise, fallback to generic_copy_file_range()
> > >
> > 
> 
> Actually we sent initial version with single bio, but later community
> suggested two bio's is must for offload, main reasoning being

Is there any link which holds the discussion?

> dm-layer,Xcopy,copy across namespace compatibilty.

But dm kcopy has supported bdev copy already, so once your patch is
ready, dm kcopy can just sends one bio with REQ_COPY if the device
supports offload command, otherwise the current dm kcopy code can work
as before.

> 
> > We will check the feasibilty and try to implement the scheme in next versions.
> > It would be helpful, if someone in community know's why such checks were
> > present ? We see copy_file_range accepts only regular file. Was it
> > designed only for regular files or can we extend it to regular block
> > device.
> >
> 
> As you suggested we were able to integrate def_blk_ops and
> run with user application, but we see one main issue with this approach.
> Using blkdev_copy_file_range requires having 2 file descriptors, which
> is not possible for in kernel users such as fabrics/dm-kcopyd which has
> only bdev descriptors.
> Do you have any plumbing suggestions here ?

What is the fabrics kernel user? Any kernel target code(such as nvme target)
has file/bdev path available, vfs_copy_file_range() should be fine.

Also IMO, kernel copy user shouldn't be important long term, especially if
io_uring copy_file_range() can be supported, forwarding to userspace not
only gets better performance, but also cleanup kernel related copy code
much.


thanks, 
Ming