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() > 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. > > > > > When I was researching pipe/splice code for supporting ublk zero copy[1], I > > > have got idea for async copy_file_range(), such as: io uring based > > > direct splice, user backed intermediate buffer, still zero copy, if these > > > ideas are finally implemented, we could get super-fast generic offload copy, > > > and bdev copy is really covered too. > > > > > > [1] https://lore.kernel.org/linux-block/20221103085004.1029763-1-ming.lei@xxxxxxxxxx/ > > > > > > > Seems interesting, We will take a look into this. > > BTW, that is probably one direction of ublk's async zero copy IO too. > > > Thanks, > Ming > > Thanks, Nitesh
