[PATCH v8 07/20] cachefiles: document on-demand read mode

[Jeffle Xu <jefflexu@linux.alibaba.com>]

Document new user interface introduced by on-demand read mode.

Signed-off-by: Jeffle Xu <jefflexu@linux.alibaba.com>
---
 .../filesystems/caching/cachefiles.rst        | 165 ++++++++++++++++++
 1 file changed, 165 insertions(+)

diff --git a/Documentation/filesystems/caching/cachefiles.rst b/Documentation/filesystems/caching/cachefiles.rst
index 8bf396b76359..386801135027 100644
--- a/Documentation/filesystems/caching/cachefiles.rst
+++ b/Documentation/filesystems/caching/cachefiles.rst
@@ -28,6 +28,8 @@ Cache on Already Mounted Filesystem
 
  (*) Debugging.
 
+ (*) On-demand Read.
+
 
 
 Overview
@@ -482,3 +484,166 @@ the control file.  For example::
 	echo $((1|4|8)) >/sys/module/cachefiles/parameters/debug
 
 will turn on all function entry debugging.
+
+
+On-demand Read
+==============
+
+When working in original mode, cachefiles mainly serves as a local cache for
+remote networking fs, while in on-demand read mode, cachefiles can boost the
+scenario where on-demand read semantics is needed, e.g. container image
+distribution.
+
+The essential difference between these two modes is that, in original mode,
+when cache miss, netfs itself will fetch data from remote, and then write the
+fetched data into cache file. While in on-demand read mode, a user daemon is
+responsible for fetching data and then writing to the cache file.
+
+``CONFIG_CACHEFILES_ONDEMAND`` shall be enabled to support on-demand read mode.
+
+
+Protocol Communication
+----------------------
+
+The on-demand read mode relies on a simple protocol used for communication
+between kernel and user daemon. The model is like::
+
+	kernel --[request]--> user daemon --[reply]--> kernel
+
+The cachefiles kernel module will send requests to user daemon when needed.
+User daemon needs to poll on the devnode ('/dev/cachefiles') to check if
+there's pending request to be processed. A POLLIN event will be returned
+when there's pending request.
+
+Then user daemon needs to read the devnode to fetch one request and process it
+accordingly. It is worth nothing that each read only gets one request. When
+finished processing the request, user daemon needs to write the reply to the
+devnode.
+
+Each request is started with a message header like::
+
+	struct cachefiles_msg {
+		__u32 id;
+		__u32 opcode;
+		__u32 len;
+		__u8  data[];
+	};
+
+	* ``id`` is a unique ID identifying this request among all pending
+	  requests.
+
+	* ``opcode`` identifies the type of this request.
+
+	* ``data`` identifies the payload of this request.
+
+	* ``len`` identifies the whole length of this request, including the
+	  header and following type specific payload.
+
+
+Turn on On-demand Mode
+----------------------
+
+An optional parameter is added to "bind" command::
+
+	bind [ondemand]
+
+When "bind" command takes without argument, it defaults to the original mode.
+When "bind" command takes with "ondemand" argument, i.e. "bind ondemand",
+on-demand read mode will be enabled.
+
+
+OPEN Request
+------------
+
+When netfs opens a cache file for the first time, a request with
+CACHEFILES_OP_OPEN opcode, a.k.a OPEN request will be sent to user daemon. The
+payload format is like::
+
+	struct cachefiles_open {
+		__u32 volume_key_size;
+		__u32 cookie_key_size;
+		__u32 fd;
+		__u32 flags;
+		__u8  data[];
+	};
+
+	* ``data`` contains volume_key and cookie_key in sequence.
+
+	* ``volume_key_size`` identifies the size of volume key of the cache
+	  file, in bytes. volume_key is of string format, with a suffix '\0'.
+
+	* ``cookie_key_size`` identifies the size of cookie key of the cache
+	  file, in bytes. cookie_key is of binary format, which is netfs
+	  specific.
+
+	* ``fd`` identifies the anonymous fd of the cache file, with which user
+	  daemon can perform write/llseek file operations on the cache file.
+
+
+OPEN request contains (volume_key, cookie_key, anon_fd) triple for corresponding
+cache file. With this triple, user daemon could fetch and write data into the
+cache file in the background, even when kernel has not triggered the cache miss
+yet. User daemon is able to distinguish the requested cache file with the given
+(volume_key, cookie_key), and write the fetched data into cache file with the
+given anon_fd.
+
+After recording the (volume_key, cookie_key, anon_fd) triple, user daemon shall
+reply with "copen" (complete open) command::
+
+	copen <id>,<cache_size>
+
+	* ``id`` is exactly the id field of the previous OPEN request.
+
+	* When >= 0, ``cache_size`` identifies the size of the cache file;
+	  when < 0, ``cache_size`` identifies the error code ecountered by the
+	  user daemon.
+
+
+CLOSE Request
+-------------
+When cookie withdrawed, a request with CACHEFILES_OP_CLOSE opcode, a.k.a CLOSE
+request, will be sent to user daemon. It will notify user daemon to close the
+attached anon_fd. The payload format is like::
+
+	struct cachefiles_close {
+		__u32 fd;
+	};
+
+	* ``fd`` identifies the anon_fd to be closed, which is exactly the same
+	  with that in OPEN request.
+
+
+READ Request
+------------
+
+When on-demand read mode is turned on, and cache miss encountered, kernel will
+send a request with CACHEFILES_OP_READ opcode, a.k.a READ request, to user
+daemon. It will notify user daemon to fetch data in the requested file range.
+The payload format is like::
+
+	struct cachefiles_read {
+		__u64 off;
+		__u64 len;
+		__u32 fd;
+	};
+
+	* ``off`` identifies the starting offset of the requested file range.
+
+	* ``len`` identifies the length of the requested file range.
+
+	* ``fd`` identifies the anonymous fd of the requested cache file. It is
+	  guaranteed that it shall be the same with the fd field in the previous
+	  OPEN request.
+
+When receiving one READ request, user daemon needs to fetch data of the
+requested file range, and then write the fetched data into cache file with the
+given anonymous fd.
+
+When finished processing the READ request, user daemon needs to reply with
+CACHEFILES_IOC_CREAD ioctl on the corresponding anon_fd::
+
+	ioctl(fd, CACHEFILES_IOC_CREAD, id);
+
+	* ``fd`` is exactly the fd field of the previous READ request.
+
+	* ``id`` is exactly the id field of the previous READ request.
-- 
2.27.0