On Sun, Aug 28, 2022 at 07:40:46PM +0700, Bagas Sanjaya wrote: > On Sun, Aug 28, 2022 at 12:50:03PM +0800, Ming Lei wrote: > > ublk document is missed when merging ublk driver, so add it now. > > > > Better say "Add documentation for ublk subsystem. It was supposed to be > documented when merging the driver, but missing at that time." OK. > > > diff --git a/Documentation/block/ublk.rst b/Documentation/block/ublk.rst > > new file mode 100644 > > index 000000000000..9e8f7ba518a3 > > --- /dev/null > > +++ b/Documentation/block/ublk.rst > > @@ -0,0 +1,203 @@ > > +.. SPDX-License-Identifier: GPL-2.0 > > + > > +========================================== > > +Userspace block device driver(ublk driver) > > +========================================== > > + > > +Overview > > +======== > > + > > +ublk is one generic framework for implementing block device logic from > > +userspace. It is very helpful to move virtual block drivers into userspace, > > +such as loop, nbd and similar block drivers. It can help to implement new > > +virtual block device, such as ublk-qcow2, and there was several attempts > > +of implementing qcow2 driver in kernel. > > + > > +ublk block device(``/dev/ublkb*``) is added by ublk driver. Any IO request > > +submitted to ublk device will be forwarded to ublk's userspace part( > > +ublksrv [1]), and after the IO is handled by ublksrv, the result is > > +committed back to ublk driver, then ublk IO request can be completed. With > > +this way, any specific IO handling logic is totally done inside ublksrv, > > +and ublk driver doe _not_ handle any device specific IO logic, such as > > +loop's IO handling, NBD's IO communication, or qcow2's IO mapping, ... > > + > > +/dev/ublkbN is driven by blk-mq request based driver, each request is > > +assigned by one queue wide unique tag. ublksrv assigns unique tag to each > > +IO too, which is 1:1 mapped with IO of /dev/ublkb*. > > + > > +Both the IO request forward and IO handling result committing are done via > > +io_uring passthrough command, that is why ublk is also one io_uring based > > +block driver. It has been observed that io_uring passthrough command can get > > +better IOPS than block IO. So ublk is one high performance implementation > > +of userspace block device. Not only IO request communication is done by > > +io_uring, but also the preferred IO handling in ublksrv is io_uring based > > +approach too. > > + > > +ublk provides control interface to set/get ublk block device parameters, and > > +the interface is extendable and kabi compatible, so basically any ublk request > > +queue's parameter or ublk generic feature parameters can be set/get via this > > +extendable interface. So ublk is generic userspace block device framework, such > > +as, it is easy to setup one ublk device with specified block parameters from > > +userspace. > > + > > +How to use ublk > > +=============== > > + > > +After building ublksrv[1], ublk block device(``/dev/ublkb*``) can be added > > +and deleted by the utility, then existed block IO applications can talk with > > +it. > > + > > +See usage details in README[2] of ublksrv, for example of ublk-loop: > > + > > +- add ublk device: > > + ublk add -t loop -f ublk-loop.img > > + > > +- use it: > > + mkfs.xfs /dev/ublkb0 > > + mount /dev/ublkb0 /mnt > > + .... # all IOs are handled by io_uring!!! > > + umount /mnt > > + > > +- get ublk dev info: > > + ublk list > > + > > +- delete ublk device > > + ublk del -a > > + ublk del -n $ublk_dev_id > > + > > +Design > > +====== > > + > > +Control plane > > +------------- > > + > > +ublk driver provides global misc device node(``/dev/ublk-control``) for > > +managing and controlling ublk devices with help of several control commands: > > + > > +- UBLK_CMD_ADD_DEV > > + Add one ublk char device(``/dev/ublkc*``) which is talked with ublksrv wrt. > > + IO command communication. Basic device info is sent together with this > > + command, see UAPI structure of ublksrv_ctrl_dev_info, such as nr_hw_queues, > > + queue_depth, and max IO request buffer size, which info is negotiated with > > + ublk driver and sent back to ublksrv. After this command is completed, the > > + basic device info can't be changed any more. > > + > > +- UBLK_CMD_SET_PARAMS / UBLK_CMD_GET_PARAMS > > + Set or get ublk device's parameters, which can be generic feature related, > > + or request queue limit related, but can't be IO logic specific, cause ublk > > + driver does not handle any IO logic. This command has to be sent before > > + sending UBLK_CMD_START_DEV. > > + > > +- UBLK_CMD_START_DEV > > + After ublksrv prepares userspace resource such as, creating per-queue > > + pthread & io_ruing for handling ublk IO, this command is set for ublk > > + driver to allocate & expose /dev/ublkb*. Parameters set via > > + UBLK_CMD_SET_PARAMS are applied for creating /dev/ublkb*. > > + > > +- UBLK_CMD_STOP_DEV > > + Quiesce IO on /dev/ublkb* and delete the disk. After this command returns, > > + ublksrv can release resource, such as destroy per-queue pthread & io_uring > > + for handling io command. > > + > > +- UBLK_CMD_DEL_DEV > > + Delete /dev/ublkc*. After this command returns, the allocated ublk device > > + number can be reused. > > + > > +- UBLK_CMD_GET_QUEUE_AFFINITY > > + After /dev/ublkc is added, ublk driver creates block layer tagset, so each > > + queue's affinity info is available, ublksrv sends UBLK_CMD_GET_QUEUE_AFFINITY > > + to retrieve queue affinity info, so ublksrv can setup the per-queue context > > + efficiently, such as bind affine CPUs with IO pthread, and try to allocate > > + buffers in IO thread context. > > + > > +- UBLK_CMD_GET_DEV_INFO > > + For retrieve device info of ublksrv_ctrl_dev_info. And it is ublksrv's > > + responsibility to save IO target specific info in userspace. > > + > > +Data plane > > +---------- > > + > > +ublksrv needs to create per-queue IO pthread & io_uring for handling IO > > +command (io_uring passthrough command), and the per-queue IO pthread > > +focuses on IO handling and shouldn't handle any control & management > > +task. > > + > > +ublksrv's IO is assigned by one unique tag, which is 1:1 mapping with IO > > +request of /dev/ublkb*. > > + > > +UAPI structure of ublksrv_io_desc is defined for describing each IO from > > +ublk driver. One fixed mmaped area(array) on /dev/ublkc* is provided for > > +exporting IO info to ublksrv, such as IO offset, length, OP/flags and > > +buffer address. Each ublksrv_io_desc instance can be indexed via queue id > > +and IO tag directly. > > + > > +Following IO commands are communicated via io_uring passthrough command, > > +and each command is only for forwarding ublk IO and committing IO result > > +with specified IO tag in the command data: > > + > > +- UBLK_IO_FETCH_REQ > > + Sent from ublksrv IO pthread for fetching future coming IO request > > + issued to /dev/ublkb*. This command is just sent once from ublksrv IO > > + pthread for ublk driver to setup IO forward environment. > > + > > +- UBLK_IO_COMMIT_AND_FETCH_REQ > > + After one IO request is issued to /dev/ublkb*, ublk driver stores this > > + IO's ublksrv_io_desc to the specified mapped area, then the previous > > + received IO command of this IO tag, either UBLK_IO_FETCH_REQ or > > + UBLK_IO_COMMIT_AND_FETCH_REQ, is completed, so ulksrv gets the IO > > + notification via io_uring. > > + > > + After ublksrv handles this IO, this IO's result is committed back to ublk > > + driver by sending UBLK_IO_COMMIT_AND_FETCH_REQ back. Once ublkdrv received > > + this command, it parses the IO result and complete the IO request to > > + /dev/ublkb*. Meantime setup environment for fetching future IO request > > + with this IO tag. So UBLK_IO_COMMIT_AND_FETCH_REQ is reused for both > > + fetching request and committing back IO result. > > + > > +- UBLK_IO_NEED_GET_DATA > > + ublksrv pre-allocates IO buffer for each IO at default, any new project > > + should use this IO buffer to communicate with ublk driver. But existed > > + project may not work or be changed to in this way, so add this command > > + to provide chance for userspace to use its existed buffer for handling > > + IO. > > + > > +- data copy between ublkserv IO buffer and ublk block IO request > > + ublk driver needs to copy ublk block IO request pages into ublksrv buffer > > + (pages) first for WRITE before notifying ublksrv of the coming IO, so > > + ublksrv can hanldle WRITE request. > > + > > + After ublksrv handles READ request and sends UBLK_IO_COMMIT_AND_FETCH_REQ > > + to ublksrv, ublkdrv needs to copy read ublksrv buffer(pages) to the ublk > > + IO request pages. > > + > > +Future development > > +================== > > + > > +Container-ware ublk deivice > > +--------------------------- > > + > > +ublk driver doesn't handle any IO logic, and its function is well defined > > +so far, and very limited userspace interfaces are needed, and each one is > > +well defined too, then it is very likely to make ublk device one > > +container-ware block device in future, as Stefan Hajnoczi suggested[3], by > > +removing ADMIN privilege. > > + > > +Zero copy > > +--------- > > + > > +Wrt. zero copy support, it is one generic requirement for nbd, fuse or > > +similar drivers, one problem Xiaoguang mentioned is that pages mapped to > > +userspace can't be remapped any more in kernel with existed mm interfaces, > > +and it can be involved when submitting direct IO to /dev/ublkb*. Also > > +Xiaoguang reported that big request may benefit from zero copy a lot, > > +such as >= 256KB IO. > > + > > At what thread on lore the problem is mentioned? https://lore.kernel.org/linux-mm/20220318095531.15479-1-xiaoguang.wang@xxxxxxxxxxxxxxxxx/ > > > + > > +References > > +========== > > + > > +[1] https://github.com/ming1/ubdsrv > > + > > +[2] https://github.com/ming1/ubdsrv/blob/master/README > > + > > +[3] https://lore.kernel.org/linux-block/YoOr6jBfgVm8GvWg@stefanha-x1.localdomain/ > > The documentation can be improved: > > - Its slug should be added to table of contents (index.rst) > - Use footnote syntax for external references > - The wordings are weird, almost to the point that I can't comprehend > it (barely understand the meaning). > > Here's the improv: OK, I will integrate it in V2. thanks, Ming
