diff options
Diffstat (limited to 'Documentation/filesystems/caching')
| -rw-r--r-- | Documentation/filesystems/caching/backend-api.rst | 8 | ||||
| -rw-r--r-- | Documentation/filesystems/caching/cachefiles.rst | 178 | ||||
| -rw-r--r-- | Documentation/filesystems/caching/netfs-api.rst | 27 |
3 files changed, 195 insertions, 18 deletions
diff --git a/Documentation/filesystems/caching/backend-api.rst b/Documentation/filesystems/caching/backend-api.rst index be793c49a772..d7507becf674 100644 --- a/Documentation/filesystems/caching/backend-api.rst +++ b/Documentation/filesystems/caching/backend-api.rst @@ -73,7 +73,7 @@ busy. If successful, the cache backend can then start setting up the cache. In the event that the initialisation fails, the cache backend should call:: - void fscache_relinquish_cookie(struct fscache_cache *cache); + void fscache_relinquish_cache(struct fscache_cache *cache); to reset and discard the cookie. @@ -110,9 +110,9 @@ to withdraw them, calling:: on the cookie that each object belongs to. This schedules the specified cookie for withdrawal. This gets offloaded to a workqueue. The cache backend can -test for completion by calling:: +wait for completion by calling:: - bool fscache_are_objects_withdrawn(struct fscache_cookie *cache); + void fscache_wait_for_objects(struct fscache_cache *cache); Once all the cookies are withdrawn, a cache backend can withdraw all the volumes, calling:: @@ -125,7 +125,7 @@ outstanding accesses on the volume to complete before returning. When the the cache is completely withdrawn, fscache should be notified by calling:: - void fscache_cache_relinquish(struct fscache_cache *cache); + void fscache_relinquish_cache(struct fscache_cache *cache); to clear fields in the cookie and discard the caller's ref on it. diff --git a/Documentation/filesystems/caching/cachefiles.rst b/Documentation/filesystems/caching/cachefiles.rst index 8bf396b76359..fc7abf712315 100644 --- a/Documentation/filesystems/caching/cachefiles.rst +++ b/Documentation/filesystems/caching/cachefiles.rst @@ -28,6 +28,7 @@ Cache on Already Mounted Filesystem (*) Debugging. + (*) On-demand Read. Overview @@ -482,3 +483,180 @@ 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 its original mode, CacheFiles serves as a local cache for a +remote networking fs - while in on-demand read mode, CacheFiles can boost the +scenario where on-demand read semantics are needed, e.g. container image +distribution. + +The essential difference between these two modes is seen when a cache miss +occurs: In the original mode, the netfs will fetch the data from the remote +server and then write it to the cache file; in on-demand read mode, fetching +the data and writing it into the cache is delegated to a user daemon. + +``CONFIG_CACHEFILES_ONDEMAND`` should be enabled to support on-demand read mode. + + +Protocol Communication +---------------------- + +The on-demand read mode uses a simple protocol for communication between kernel +and user daemon. The protocol can be modeled as:: + + kernel --[request]--> user daemon --[reply]--> kernel + +CacheFiles will send requests to the user daemon when needed. The user daemon +should poll the devnode ('/dev/cachefiles') to check if there's a pending +request to be processed. A POLLIN event will be returned when there's a pending +request. + +The user daemon then reads the devnode to fetch a request to process. It should +be noted that each read only gets one request. When it has finished processing +the request, the user daemon should write the reply to the devnode. + +Each request starts with a message header of the form:: + + struct cachefiles_msg { + __u32 msg_id; + __u32 opcode; + __u32 len; + __u32 object_id; + __u8 data[]; + }; + +where: + + * ``msg_id`` is a unique ID identifying this request among all pending + requests. + + * ``opcode`` indicates the type of this request. + + * ``object_id`` is a unique ID identifying the cache file operated on. + + * ``data`` indicates the payload of this request. + + * ``len`` indicates the whole length of this request, including the + header and following type-specific payload. + + +Turning on On-demand Mode +------------------------- + +An optional parameter becomes available to the "bind" command:: + + bind [ondemand] + +When the "bind" command is given no argument, it defaults to the original mode. +When it is given the "ondemand" argument, i.e. "bind ondemand", on-demand read +mode will be enabled. + + +The OPEN Request +---------------- + +When the netfs opens a cache file for the first time, a request with the +CACHEFILES_OP_OPEN opcode, a.k.a an OPEN request will be sent to the user +daemon. The payload format is of the form:: + + struct cachefiles_open { + __u32 volume_key_size; + __u32 cookie_key_size; + __u32 fd; + __u32 flags; + __u8 data[]; + }; + +where: + + * ``data`` contains the volume_key followed directly by the cookie_key. + The volume key is a NUL-terminated string; the cookie key is binary + data. + + * ``volume_key_size`` indicates the size of the volume key in bytes. + + * ``cookie_key_size`` indicates the size of the cookie key in bytes. + + * ``fd`` indicates an anonymous fd referring to the cache file, through + which the user daemon can perform write/llseek file operations on the + cache file. + + +The user daemon can use the given (volume_key, cookie_key) pair to distinguish +the requested cache file. With the given anonymous fd, the user daemon can +fetch the data and write it to the cache file in the background, even when +kernel has not triggered a cache miss yet. + +Be noted that each cache file has a unique object_id, while it may have multiple +anonymous fds. The user daemon may duplicate anonymous fds from the initial +anonymous fd indicated by the @fd field through dup(). Thus each object_id can +be mapped to multiple anonymous fds, while the usr daemon itself needs to +maintain the mapping. + +When implementing a user daemon, please be careful of RLIMIT_NOFILE, +``/proc/sys/fs/nr_open`` and ``/proc/sys/fs/file-max``. Typically these needn't +be huge since they're related to the number of open device blobs rather than +open files of each individual filesystem. + +The user daemon should reply the OPEN request by issuing a "copen" (complete +open) command on the devnode:: + + copen <msg_id>,<cache_size> + +where: + + * ``msg_id`` must match the msg_id field of the OPEN request. + + * When >= 0, ``cache_size`` indicates the size of the cache file; + when < 0, ``cache_size`` indicates any error code encountered by the + user daemon. + + +The CLOSE Request +----------------- + +When a cookie withdrawn, a CLOSE request (opcode CACHEFILES_OP_CLOSE) will be +sent to the user daemon. This tells the user daemon to close all anonymous fds +associated with the given object_id. The CLOSE request has no extra payload, +and shouldn't be replied. + + +The READ Request +---------------- + +When a cache miss is encountered in on-demand read mode, CacheFiles will send a +READ request (opcode CACHEFILES_OP_READ) to the user daemon. This tells the user +daemon to fetch the contents of the requested file range. The payload is of the +form:: + + struct cachefiles_read { + __u64 off; + __u64 len; + }; + +where: + + * ``off`` indicates the starting offset of the requested file range. + + * ``len`` indicates the length of the requested file range. + + +When it receives a READ request, the user daemon should fetch the requested data +and write it to the cache file identified by object_id. + +When it has finished processing the READ request, the user daemon should reply +by using the CACHEFILES_IOC_READ_COMPLETE ioctl on one of the anonymous fds +associated with the object_id given in the READ request. The ioctl is of the +form:: + + ioctl(fd, CACHEFILES_IOC_READ_COMPLETE, msg_id); + +where: + + * ``fd`` is one of the anonymous fds associated with the object_id + given. + + * ``msg_id`` must match the msg_id field of the READ request. diff --git a/Documentation/filesystems/caching/netfs-api.rst b/Documentation/filesystems/caching/netfs-api.rst index 5066113acad5..1d18e9def183 100644 --- a/Documentation/filesystems/caching/netfs-api.rst +++ b/Documentation/filesystems/caching/netfs-api.rst @@ -404,22 +404,21 @@ schedule a write of that region:: And if an error occurs before that point is reached, the marks can be removed by calling:: - void fscache_clear_page_bits(struct fscache_cookie *cookie, - struct address_space *mapping, + void fscache_clear_page_bits(struct address_space *mapping, loff_t start, size_t len, bool caching) -In both of these functions, the cookie representing the cache object to be -written to and a pointer to the mapping to which the source pages are attached -are passed in; start and len indicate the size of the region that's going to be -written (it doesn't have to align to page boundaries necessarily, but it does -have to align to DIO boundaries on the backing filesystem). The caching -parameter indicates if caching should be skipped, and if false, the functions -do nothing. +In these functions, a pointer to the mapping to which the source pages are +attached is passed in and start and len indicate the size of the region that's +going to be written (it doesn't have to align to page boundaries necessarily, +but it does have to align to DIO boundaries on the backing filesystem). The +caching parameter indicates if caching should be skipped, and if false, the +functions do nothing. -The write function takes some additional parameters: i_size indicates the size -of the netfs file and term_func indicates an optional completion function, to -which term_func_priv will be passed, along with the error or amount written. +The write function takes some additional parameters: the cookie representing +the cache object to be written to, i_size indicates the size of the netfs file +and term_func indicates an optional completion function, to which +term_func_priv will be passed, along with the error or amount written. Note that the write function will always run asynchronously and will unmark all the pages upon completion before calling term_func. @@ -434,11 +433,11 @@ has done a write and then the page it wrote from has been released by the VM, after which it *has* to look in the cache. To inform fscache that a page might now be in the cache, the following function -should be called from the ``releasepage`` address space op:: +should be called from the ``release_folio`` address space op:: void fscache_note_page_release(struct fscache_cookie *cookie); -if the page has been released (ie. releasepage returned true). +if the page has been released (ie. release_folio returned true). Page release and page invalidation should also wait for any mark left on the page to say that a DIO write is underway from that page:: |