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
