1 .. SPDX-License-Identifier: GPL-2.0 1 .. SPDX-License-Identifier: GPL-2.0
2 2
3 =========================================== 3 ===========================================
4 Userspace block device driver (ublk driver) 4 Userspace block device driver (ublk driver)
5 =========================================== 5 ===========================================
6 6
7 Overview 7 Overview
8 ======== 8 ========
9 9
10 ublk is a generic framework for implementing b 10 ublk is a generic framework for implementing block device logic from userspace.
11 The motivation behind it is that moving virtua 11 The motivation behind it is that moving virtual block drivers into userspace,
12 such as loop, nbd and similar can be very help 12 such as loop, nbd and similar can be very helpful. It can help to implement
13 new virtual block device such as ublk-qcow2 (t 13 new virtual block device such as ublk-qcow2 (there are several attempts of
14 implementing qcow2 driver in kernel). 14 implementing qcow2 driver in kernel).
15 15
16 Userspace block devices are attractive because 16 Userspace block devices are attractive because:
17 17
18 - They can be written many programming languag 18 - They can be written many programming languages.
19 - They can use libraries that are not availabl 19 - They can use libraries that are not available in the kernel.
20 - They can be debugged with tools familiar to 20 - They can be debugged with tools familiar to application developers.
21 - Crashes do not kernel panic the machine. 21 - Crashes do not kernel panic the machine.
22 - Bugs are likely to have a lower security imp 22 - Bugs are likely to have a lower security impact than bugs in kernel
23 code. 23 code.
24 - They can be installed and updated independen 24 - They can be installed and updated independently of the kernel.
25 - They can be used to simulate block device ea 25 - They can be used to simulate block device easily with user specified
26 parameters/setting for test/debug purpose 26 parameters/setting for test/debug purpose
27 27
28 ublk block device (``/dev/ublkb*``) is added b 28 ublk block device (``/dev/ublkb*``) is added by ublk driver. Any IO request
29 on the device will be forwarded to ublk usersp 29 on the device will be forwarded to ublk userspace program. For convenience,
30 in this document, ``ublk server`` refers to ge 30 in this document, ``ublk server`` refers to generic ublk userspace
31 program. ``ublksrv`` [#userspace]_ is one of s 31 program. ``ublksrv`` [#userspace]_ is one of such implementation. It
32 provides ``libublksrv`` [#userspace_lib]_ libr 32 provides ``libublksrv`` [#userspace_lib]_ library for developing specific
33 user block device conveniently, while also gen 33 user block device conveniently, while also generic type block device is
34 included, such as loop and null. Richard W.M. 34 included, such as loop and null. Richard W.M. Jones wrote userspace nbd device
35 ``nbdublk`` [#userspace_nbdublk]_ based on `` 35 ``nbdublk`` [#userspace_nbdublk]_ based on ``libublksrv`` [#userspace_lib]_.
36 36
37 After the IO is handled by userspace, the resu 37 After the IO is handled by userspace, the result is committed back to the
38 driver, thus completing the request cycle. Thi 38 driver, thus completing the request cycle. This way, any specific IO handling
39 logic is totally done by userspace, such as lo 39 logic is totally done by userspace, such as loop's IO handling, NBD's IO
40 communication, or qcow2's IO mapping. 40 communication, or qcow2's IO mapping.
41 41
42 ``/dev/ublkb*`` is driven by blk-mq request-ba 42 ``/dev/ublkb*`` is driven by blk-mq request-based driver. Each request is
43 assigned by one queue wide unique tag. ublk se 43 assigned by one queue wide unique tag. ublk server assigns unique tag to each
44 IO too, which is 1:1 mapped with IO of ``/dev/ 44 IO too, which is 1:1 mapped with IO of ``/dev/ublkb*``.
45 45
46 Both the IO request forward and IO handling re 46 Both the IO request forward and IO handling result committing are done via
47 ``io_uring`` passthrough command; that is why 47 ``io_uring`` passthrough command; that is why ublk is also one io_uring based
48 block driver. It has been observed that using 48 block driver. It has been observed that using io_uring passthrough command can
49 give better IOPS than block IO; which is why u 49 give better IOPS than block IO; which is why ublk is one of high performance
50 implementation of userspace block device: not 50 implementation of userspace block device: not only IO request communication is
51 done by io_uring, but also the preferred IO ha 51 done by io_uring, but also the preferred IO handling in ublk server is io_uring
52 based approach too. 52 based approach too.
53 53
54 ublk provides control interface to set/get ubl 54 ublk provides control interface to set/get ublk block device parameters.
55 The interface is extendable and kabi compatibl 55 The interface is extendable and kabi compatible: basically any ublk request
56 queue's parameter or ublk generic feature para 56 queue's parameter or ublk generic feature parameters can be set/get via the
57 interface. Thus, ublk is generic userspace blo 57 interface. Thus, ublk is generic userspace block device framework.
58 For example, it is easy to setup a ublk device 58 For example, it is easy to setup a ublk device with specified block
59 parameters from userspace. 59 parameters from userspace.
60 60
61 Using ublk 61 Using ublk
62 ========== 62 ==========
63 63
64 ublk requires userspace ublk server to handle 64 ublk requires userspace ublk server to handle real block device logic.
65 65
66 Below is example of using ``ublksrv`` to provi 66 Below is example of using ``ublksrv`` to provide ublk-based loop device.
67 67
68 - add a device:: 68 - add a device::
69 69
70 ublk add -t loop -f ublk-loop.img 70 ublk add -t loop -f ublk-loop.img
71 71
72 - format with xfs, then use it:: 72 - format with xfs, then use it::
73 73
74 mkfs.xfs /dev/ublkb0 74 mkfs.xfs /dev/ublkb0
75 mount /dev/ublkb0 /mnt 75 mount /dev/ublkb0 /mnt
76 # do anything. all IOs are handled by io_ 76 # do anything. all IOs are handled by io_uring
77 ... 77 ...
78 umount /mnt 78 umount /mnt
79 79
80 - list the devices with their info:: 80 - list the devices with their info::
81 81
82 ublk list 82 ublk list
83 83
84 - delete the device:: 84 - delete the device::
85 85
86 ublk del -a 86 ublk del -a
87 ublk del -n $ublk_dev_id 87 ublk del -n $ublk_dev_id
88 88
89 See usage details in README of ``ublksrv`` [#u 89 See usage details in README of ``ublksrv`` [#userspace_readme]_.
90 90
91 Design 91 Design
92 ====== 92 ======
93 93
94 Control plane 94 Control plane
95 ------------- 95 -------------
96 96
97 ublk driver provides global misc device node ( 97 ublk driver provides global misc device node (``/dev/ublk-control``) for
98 managing and controlling ublk devices with hel 98 managing and controlling ublk devices with help of several control commands:
99 99
100 - ``UBLK_CMD_ADD_DEV`` 100 - ``UBLK_CMD_ADD_DEV``
101 101
102 Add a ublk char device (``/dev/ublkc*``) whi 102 Add a ublk char device (``/dev/ublkc*``) which is talked with ublk server
103 WRT IO command communication. Basic device i 103 WRT IO command communication. Basic device info is sent together with this
104 command. It sets UAPI structure of ``ublksrv 104 command. It sets UAPI structure of ``ublksrv_ctrl_dev_info``,
105 such as ``nr_hw_queues``, ``queue_depth``, a 105 such as ``nr_hw_queues``, ``queue_depth``, and max IO request buffer size,
106 for which the info is negotiated with the dr 106 for which the info is negotiated with the driver and sent back to the server.
107 When this command is completed, the basic de 107 When this command is completed, the basic device info is immutable.
108 108
109 - ``UBLK_CMD_SET_PARAMS`` / ``UBLK_CMD_GET_PAR 109 - ``UBLK_CMD_SET_PARAMS`` / ``UBLK_CMD_GET_PARAMS``
110 110
111 Set or get parameters of the device, which c 111 Set or get parameters of the device, which can be either generic feature
112 related, or request queue limit related, but 112 related, or request queue limit related, but can't be IO logic specific,
113 because the driver does not handle any IO lo 113 because the driver does not handle any IO logic. This command has to be
114 sent before sending ``UBLK_CMD_START_DEV``. 114 sent before sending ``UBLK_CMD_START_DEV``.
115 115
116 - ``UBLK_CMD_START_DEV`` 116 - ``UBLK_CMD_START_DEV``
117 117
118 After the server prepares userspace resource 118 After the server prepares userspace resources (such as creating per-queue
119 pthread & io_uring for handling ublk IO), th 119 pthread & io_uring for handling ublk IO), this command is sent to the
120 driver for allocating & exposing ``/dev/ublk 120 driver for allocating & exposing ``/dev/ublkb*``. Parameters set via
121 ``UBLK_CMD_SET_PARAMS`` are applied for crea 121 ``UBLK_CMD_SET_PARAMS`` are applied for creating the device.
122 122
123 - ``UBLK_CMD_STOP_DEV`` 123 - ``UBLK_CMD_STOP_DEV``
124 124
125 Halt IO on ``/dev/ublkb*`` and remove the de 125 Halt IO on ``/dev/ublkb*`` and remove the device. When this command returns,
126 ublk server will release resources (such as 126 ublk server will release resources (such as destroying per-queue pthread &
127 io_uring). 127 io_uring).
128 128
129 - ``UBLK_CMD_DEL_DEV`` 129 - ``UBLK_CMD_DEL_DEV``
130 130
131 Remove ``/dev/ublkc*``. When this command re 131 Remove ``/dev/ublkc*``. When this command returns, the allocated ublk device
132 number can be reused. 132 number can be reused.
133 133
134 - ``UBLK_CMD_GET_QUEUE_AFFINITY`` 134 - ``UBLK_CMD_GET_QUEUE_AFFINITY``
135 135
136 When ``/dev/ublkc`` is added, the driver cre 136 When ``/dev/ublkc`` is added, the driver creates block layer tagset, so
137 that each queue's affinity info is available 137 that each queue's affinity info is available. The server sends
138 ``UBLK_CMD_GET_QUEUE_AFFINITY`` to retrieve 138 ``UBLK_CMD_GET_QUEUE_AFFINITY`` to retrieve queue affinity info. It can
139 set up the per-queue context efficiently, su 139 set up the per-queue context efficiently, such as bind affine CPUs with IO
140 pthread and try to allocate buffers in IO th 140 pthread and try to allocate buffers in IO thread context.
141 141
142 - ``UBLK_CMD_GET_DEV_INFO`` 142 - ``UBLK_CMD_GET_DEV_INFO``
143 143
144 For retrieving device info via ``ublksrv_ctr 144 For retrieving device info via ``ublksrv_ctrl_dev_info``. It is the server's
145 responsibility to save IO target specific in 145 responsibility to save IO target specific info in userspace.
146 146
147 - ``UBLK_CMD_GET_DEV_INFO2`` 147 - ``UBLK_CMD_GET_DEV_INFO2``
148 Same purpose with ``UBLK_CMD_GET_DEV_INFO``, 148 Same purpose with ``UBLK_CMD_GET_DEV_INFO``, but ublk server has to
149 provide path of the char device of ``/dev/ub 149 provide path of the char device of ``/dev/ublkc*`` for kernel to run
150 permission check, and this command is added 150 permission check, and this command is added for supporting unprivileged
151 ublk device, and introduced with ``UBLK_F_UN 151 ublk device, and introduced with ``UBLK_F_UNPRIVILEGED_DEV`` together.
152 Only the user owning the requested device ca 152 Only the user owning the requested device can retrieve the device info.
153 153
154 How to deal with userspace/kernel compatibil 154 How to deal with userspace/kernel compatibility:
155 155
156 1) if kernel is capable of handling ``UBLK_F 156 1) if kernel is capable of handling ``UBLK_F_UNPRIVILEGED_DEV``
157 157
158 If ublk server supports ``UBLK_F_UNPRIVILE 158 If ublk server supports ``UBLK_F_UNPRIVILEGED_DEV``:
159 159
160 ublk server should send ``UBLK_CMD_GET_DEV 160 ublk server should send ``UBLK_CMD_GET_DEV_INFO2``, given anytime
161 unprivileged application needs to query de 161 unprivileged application needs to query devices the current user owns,
162 when the application has no idea if ``UBLK 162 when the application has no idea if ``UBLK_F_UNPRIVILEGED_DEV`` is set
163 given the capability info is stateless, an 163 given the capability info is stateless, and application should always
164 retrieve it via ``UBLK_CMD_GET_DEV_INFO2`` 164 retrieve it via ``UBLK_CMD_GET_DEV_INFO2``
165 165
166 If ublk server doesn't support ``UBLK_F_UN 166 If ublk server doesn't support ``UBLK_F_UNPRIVILEGED_DEV``:
167 167
168 ``UBLK_CMD_GET_DEV_INFO`` is always sent t 168 ``UBLK_CMD_GET_DEV_INFO`` is always sent to kernel, and the feature of
169 UBLK_F_UNPRIVILEGED_DEV isn't available fo 169 UBLK_F_UNPRIVILEGED_DEV isn't available for user
170 170
171 2) if kernel isn't capable of handling ``UBL 171 2) if kernel isn't capable of handling ``UBLK_F_UNPRIVILEGED_DEV``
172 172
173 If ublk server supports ``UBLK_F_UNPRIVILE 173 If ublk server supports ``UBLK_F_UNPRIVILEGED_DEV``:
174 174
175 ``UBLK_CMD_GET_DEV_INFO2`` is tried first, 175 ``UBLK_CMD_GET_DEV_INFO2`` is tried first, and will be failed, then
176 ``UBLK_CMD_GET_DEV_INFO`` needs to be retr 176 ``UBLK_CMD_GET_DEV_INFO`` needs to be retried given
177 ``UBLK_F_UNPRIVILEGED_DEV`` can't be set 177 ``UBLK_F_UNPRIVILEGED_DEV`` can't be set
178 178
179 If ublk server doesn't support ``UBLK_F_UN 179 If ublk server doesn't support ``UBLK_F_UNPRIVILEGED_DEV``:
180 180
181 ``UBLK_CMD_GET_DEV_INFO`` is always sent t 181 ``UBLK_CMD_GET_DEV_INFO`` is always sent to kernel, and the feature of
182 ``UBLK_F_UNPRIVILEGED_DEV`` isn't availabl 182 ``UBLK_F_UNPRIVILEGED_DEV`` isn't available for user
183 183
184 - ``UBLK_CMD_START_USER_RECOVERY`` 184 - ``UBLK_CMD_START_USER_RECOVERY``
185 185
186 This command is valid if ``UBLK_F_USER_RECOV 186 This command is valid if ``UBLK_F_USER_RECOVERY`` feature is enabled. This
187 command is accepted after the old process ha 187 command is accepted after the old process has exited, ublk device is quiesced
188 and ``/dev/ublkc*`` is released. User should 188 and ``/dev/ublkc*`` is released. User should send this command before he starts
189 a new process which re-opens ``/dev/ublkc*`` 189 a new process which re-opens ``/dev/ublkc*``. When this command returns, the
190 ublk device is ready for the new process. 190 ublk device is ready for the new process.
191 191
192 - ``UBLK_CMD_END_USER_RECOVERY`` 192 - ``UBLK_CMD_END_USER_RECOVERY``
193 193
194 This command is valid if ``UBLK_F_USER_RECOV 194 This command is valid if ``UBLK_F_USER_RECOVERY`` feature is enabled. This
195 command is accepted after ublk device is qui 195 command is accepted after ublk device is quiesced and a new process has
196 opened ``/dev/ublkc*`` and get all ublk queu 196 opened ``/dev/ublkc*`` and get all ublk queues be ready. When this command
197 returns, ublk device is unquiesced and new I 197 returns, ublk device is unquiesced and new I/O requests are passed to the
198 new process. 198 new process.
199 199
200 - user recovery feature description 200 - user recovery feature description
201 201
202 Two new features are added for user recovery 202 Two new features are added for user recovery: ``UBLK_F_USER_RECOVERY`` and
203 ``UBLK_F_USER_RECOVERY_REISSUE``. 203 ``UBLK_F_USER_RECOVERY_REISSUE``.
204 204
205 With ``UBLK_F_USER_RECOVERY`` set, after one 205 With ``UBLK_F_USER_RECOVERY`` set, after one ubq_daemon(ublk server's io
206 handler) is dying, ublk does not delete ``/d 206 handler) is dying, ublk does not delete ``/dev/ublkb*`` during the whole
207 recovery stage and ublk device ID is kept. I 207 recovery stage and ublk device ID is kept. It is ublk server's
208 responsibility to recover the device context 208 responsibility to recover the device context by its own knowledge.
209 Requests which have not been issued to users 209 Requests which have not been issued to userspace are requeued. Requests
210 which have been issued to userspace are abor 210 which have been issued to userspace are aborted.
211 211
212 With ``UBLK_F_USER_RECOVERY_REISSUE`` set, a 212 With ``UBLK_F_USER_RECOVERY_REISSUE`` set, after one ubq_daemon(ublk
213 server's io handler) is dying, contrary to ` 213 server's io handler) is dying, contrary to ``UBLK_F_USER_RECOVERY``,
214 requests which have been issued to userspace 214 requests which have been issued to userspace are requeued and will be
215 re-issued to the new process after handling 215 re-issued to the new process after handling ``UBLK_CMD_END_USER_RECOVERY``.
216 ``UBLK_F_USER_RECOVERY_REISSUE`` is designed 216 ``UBLK_F_USER_RECOVERY_REISSUE`` is designed for backends who tolerate
217 double-write since the driver may issue the 217 double-write since the driver may issue the same I/O request twice. It
218 might be useful to a read-only FS or a VM ba 218 might be useful to a read-only FS or a VM backend.
219 219
220 Unprivileged ublk device is supported by passi 220 Unprivileged ublk device is supported by passing ``UBLK_F_UNPRIVILEGED_DEV``.
221 Once the flag is set, all control commands can 221 Once the flag is set, all control commands can be sent by unprivileged
222 user. Except for command of ``UBLK_CMD_ADD_DEV 222 user. Except for command of ``UBLK_CMD_ADD_DEV``, permission check on
223 the specified char device(``/dev/ublkc*``) is 223 the specified char device(``/dev/ublkc*``) is done for all other control
224 commands by ublk driver, for doing that, path 224 commands by ublk driver, for doing that, path of the char device has to
225 be provided in these commands' payload from ub 225 be provided in these commands' payload from ublk server. With this way,
226 ublk device becomes container-ware, and device 226 ublk device becomes container-ware, and device created in one container
227 can be controlled/accessed just inside this co 227 can be controlled/accessed just inside this container.
228 228
229 Data plane 229 Data plane
230 ---------- 230 ----------
231 231
232 ublk server needs to create per-queue IO pthre 232 ublk server needs to create per-queue IO pthread & io_uring for handling IO
233 commands via io_uring passthrough. The per-que 233 commands via io_uring passthrough. The per-queue IO pthread
234 focuses on IO handling and shouldn't handle an 234 focuses on IO handling and shouldn't handle any control & management
235 tasks. 235 tasks.
236 236
237 The's IO is assigned by a unique tag, which is 237 The's IO is assigned by a unique tag, which is 1:1 mapping with IO
238 request of ``/dev/ublkb*``. 238 request of ``/dev/ublkb*``.
239 239
240 UAPI structure of ``ublksrv_io_desc`` is defin 240 UAPI structure of ``ublksrv_io_desc`` is defined for describing each IO from
241 the driver. A fixed mmapped area (array) on `` 241 the driver. A fixed mmapped area (array) on ``/dev/ublkc*`` is provided for
242 exporting IO info to the server; such as IO of 242 exporting IO info to the server; such as IO offset, length, OP/flags and
243 buffer address. Each ``ublksrv_io_desc`` insta 243 buffer address. Each ``ublksrv_io_desc`` instance can be indexed via queue id
244 and IO tag directly. 244 and IO tag directly.
245 245
246 The following IO commands are communicated via 246 The following IO commands are communicated via io_uring passthrough command,
247 and each command is only for forwarding the IO 247 and each command is only for forwarding the IO and committing the result
248 with specified IO tag in the command data: 248 with specified IO tag in the command data:
249 249
250 - ``UBLK_IO_FETCH_REQ`` 250 - ``UBLK_IO_FETCH_REQ``
251 251
252 Sent from the server IO pthread for fetching 252 Sent from the server IO pthread for fetching future incoming IO requests
253 destined to ``/dev/ublkb*``. This command is 253 destined to ``/dev/ublkb*``. This command is sent only once from the server
254 IO pthread for ublk driver to setup IO forwa 254 IO pthread for ublk driver to setup IO forward environment.
255 255
256 - ``UBLK_IO_COMMIT_AND_FETCH_REQ`` 256 - ``UBLK_IO_COMMIT_AND_FETCH_REQ``
257 257
258 When an IO request is destined to ``/dev/ubl 258 When an IO request is destined to ``/dev/ublkb*``, the driver stores
259 the IO's ``ublksrv_io_desc`` to the specifie 259 the IO's ``ublksrv_io_desc`` to the specified mapped area; then the
260 previous received IO command of this IO tag 260 previous received IO command of this IO tag (either ``UBLK_IO_FETCH_REQ``
261 or ``UBLK_IO_COMMIT_AND_FETCH_REQ)`` is comp 261 or ``UBLK_IO_COMMIT_AND_FETCH_REQ)`` is completed, so the server gets
262 the IO notification via io_uring. 262 the IO notification via io_uring.
263 263
264 After the server handles the IO, its result 264 After the server handles the IO, its result is committed back to the
265 driver by sending ``UBLK_IO_COMMIT_AND_FETCH 265 driver by sending ``UBLK_IO_COMMIT_AND_FETCH_REQ`` back. Once ublkdrv
266 received this command, it parses the result 266 received this command, it parses the result and complete the request to
267 ``/dev/ublkb*``. In the meantime setup envir 267 ``/dev/ublkb*``. In the meantime setup environment for fetching future
268 requests with the same IO tag. That is, ``UB 268 requests with the same IO tag. That is, ``UBLK_IO_COMMIT_AND_FETCH_REQ``
269 is reused for both fetching request and comm 269 is reused for both fetching request and committing back IO result.
270 270
271 - ``UBLK_IO_NEED_GET_DATA`` 271 - ``UBLK_IO_NEED_GET_DATA``
272 272
273 With ``UBLK_F_NEED_GET_DATA`` enabled, the W 273 With ``UBLK_F_NEED_GET_DATA`` enabled, the WRITE request will be firstly
274 issued to ublk server without data copy. The 274 issued to ublk server without data copy. Then, IO backend of ublk server
275 receives the request and it can allocate dat 275 receives the request and it can allocate data buffer and embed its addr
276 inside this new io command. After the kernel 276 inside this new io command. After the kernel driver gets the command,
277 data copy is done from request pages to this 277 data copy is done from request pages to this backend's buffer. Finally,
278 backend receives the request again with data 278 backend receives the request again with data to be written and it can
279 truly handle the request. 279 truly handle the request.
280 280
281 ``UBLK_IO_NEED_GET_DATA`` adds one additiona 281 ``UBLK_IO_NEED_GET_DATA`` adds one additional round-trip and one
282 io_uring_enter() syscall. Any user thinks th 282 io_uring_enter() syscall. Any user thinks that it may lower performance
283 should not enable UBLK_F_NEED_GET_DATA. ublk 283 should not enable UBLK_F_NEED_GET_DATA. ublk server pre-allocates IO
284 buffer for each IO by default. Any new proje 284 buffer for each IO by default. Any new project should try to use this
285 buffer to communicate with ublk driver. Howe 285 buffer to communicate with ublk driver. However, existing project may
286 break or not able to consume the new buffer 286 break or not able to consume the new buffer interface; that's why this
287 command is added for backwards compatibility 287 command is added for backwards compatibility so that existing projects
288 can still consume existing buffers. 288 can still consume existing buffers.
289 289
290 - data copy between ublk server IO buffer and 290 - data copy between ublk server IO buffer and ublk block IO request
291 291
292 The driver needs to copy the block IO reques 292 The driver needs to copy the block IO request pages into the server buffer
293 (pages) first for WRITE before notifying the 293 (pages) first for WRITE before notifying the server of the coming IO, so
294 that the server can handle WRITE request. 294 that the server can handle WRITE request.
295 295
296 When the server handles READ request and sen 296 When the server handles READ request and sends
297 ``UBLK_IO_COMMIT_AND_FETCH_REQ`` to the serv 297 ``UBLK_IO_COMMIT_AND_FETCH_REQ`` to the server, ublkdrv needs to copy
298 the server buffer (pages) read to the IO req 298 the server buffer (pages) read to the IO request pages.
299 299
300 Future development 300 Future development
301 ================== 301 ==================
302 302
303 Zero copy 303 Zero copy
304 --------- 304 ---------
305 305
306 Zero copy is a generic requirement for nbd, fu 306 Zero copy is a generic requirement for nbd, fuse or similar drivers. A
307 problem [#xiaoguang]_ Xiaoguang mentioned is t 307 problem [#xiaoguang]_ Xiaoguang mentioned is that pages mapped to userspace
308 can't be remapped any more in kernel with exis 308 can't be remapped any more in kernel with existing mm interfaces. This can
309 occurs when destining direct IO to ``/dev/ublk 309 occurs when destining direct IO to ``/dev/ublkb*``. Also, he reported that
310 big requests (IO size >= 256 KB) may benefit a 310 big requests (IO size >= 256 KB) may benefit a lot from zero copy.
311 311
312 312
313 References 313 References
314 ========== 314 ==========
315 315
316 .. [#userspace] https://github.com/ming1/ubdsr 316 .. [#userspace] https://github.com/ming1/ubdsrv
317 317
318 .. [#userspace_lib] https://github.com/ming1/u 318 .. [#userspace_lib] https://github.com/ming1/ubdsrv/tree/master/lib
319 319
320 .. [#userspace_nbdublk] https://gitlab.com/rwm 320 .. [#userspace_nbdublk] https://gitlab.com/rwmjones/libnbd/-/tree/nbdublk
321 321
322 .. [#userspace_readme] https://github.com/ming 322 .. [#userspace_readme] https://github.com/ming1/ubdsrv/blob/master/README
323 323
324 .. [#stefan] https://lore.kernel.org/linux-blo 324 .. [#stefan] https://lore.kernel.org/linux-block/YoOr6jBfgVm8GvWg@stefanha-x1.localdomain/
325 325
326 .. [#xiaoguang] https://lore.kernel.org/linux- 326 .. [#xiaoguang] https://lore.kernel.org/linux-block/YoOr6jBfgVm8GvWg@stefanha-x1.localdomain/
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