1========= 2RPC Cache 3========= 4 5This document gives a brief introduction to the caching 6mechanisms in the sunrpc layer that is used, in particular, 7for NFS authentication. 8 9Caches 10====== 11 12The caching replaces the old exports table and allows for 13a wide variety of values to be caches. 14 15There are a number of caches that are similar in structure though 16quite possibly very different in content and use. There is a corpus 17of common code for managing these caches. 18 19Examples of caches that are likely to be needed are: 20 21 - mapping from IP address to client name 22 - mapping from client name and filesystem to export options 23 - mapping from UID to list of GIDs, to work around NFS's limitation 24 of 16 gids. 25 - mappings between local UID/GID and remote UID/GID for sites that 26 do not have uniform uid assignment 27 - mapping from network identify to public key for crypto authentication. 28 29The common code handles such things as: 30 31 - general cache lookup with correct locking 32 - supporting 'NEGATIVE' as well as positive entries 33 - allowing an EXPIRED time on cache items, and removing 34 items after they expire, and are no longer in-use. 35 - making requests to user-space to fill in cache entries 36 - allowing user-space to directly set entries in the cache 37 - delaying RPC requests that depend on as-yet incomplete 38 cache entries, and replaying those requests when the cache entry 39 is complete. 40 - clean out old entries as they expire. 41 42Creating a Cache 43---------------- 44 45- A cache needs a datum to store. This is in the form of a 46 structure definition that must contain a struct cache_head 47 as an element, usually the first. 48 It will also contain a key and some content. 49 Each cache element is reference counted and contains 50 expiry and update times for use in cache management. 51- A cache needs a "cache_detail" structure that 52 describes the cache. This stores the hash table, some 53 parameters for cache management, and some operations detailing how 54 to work with particular cache items. 55 56 The operations are: 57 58 struct cache_head \*alloc(void) 59 This simply allocates appropriate memory and returns 60 a pointer to the cache_detail embedded within the 61 structure 62 63 void cache_put(struct kref \*) 64 This is called when the last reference to an item is 65 dropped. The pointer passed is to the 'ref' field 66 in the cache_head. cache_put should release any 67 references create by 'cache_init' and, if CACHE_VALID 68 is set, any references created by cache_update. 69 It should then release the memory allocated by 70 'alloc'. 71 72 int match(struct cache_head \*orig, struct cache_head \*new) 73 test if the keys in the two structures match. Return 74 1 if they do, 0 if they don't. 75 76 void init(struct cache_head \*orig, struct cache_head \*new) 77 Set the 'key' fields in 'new' from 'orig'. This may 78 include taking references to shared objects. 79 80 void update(struct cache_head \*orig, struct cache_head \*new) 81 Set the 'content' fields in 'new' from 'orig'. 82 83 int cache_show(struct seq_file \*m, struct cache_detail \*cd, struct cache_head \*h) 84 Optional. Used to provide a /proc file that lists the 85 contents of a cache. This should show one item, 86 usually on just one line. 87 88 int cache_request(struct cache_detail \*cd, struct cache_head \*h, char \*\*bpp, int \*blen) 89 Format a request to be send to user-space for an item 90 to be instantiated. \*bpp is a buffer of size \*blen. 91 bpp should be moved forward over the encoded message, 92 and \*blen should be reduced to show how much free 93 space remains. Return 0 on success or <0 if not 94 enough room or other problem. 95 96 int cache_parse(struct cache_detail \*cd, char \*buf, int len) 97 A message from user space has arrived to fill out a 98 cache entry. It is in 'buf' of length 'len'. 99 cache_parse should parse this, find the item in the 100 cache with sunrpc_cache_lookup_rcu, and update the item 101 with sunrpc_cache_update. 102 103 104- A cache needs to be registered using cache_register(). This 105 includes it on a list of caches that will be regularly 106 cleaned to discard old data. 107 108Using a cache 109------------- 110 111To find a value in a cache, call sunrpc_cache_lookup_rcu passing a pointer 112to the cache_head in a sample item with the 'key' fields filled in. 113This will be passed to ->match to identify the target entry. If no 114entry is found, a new entry will be create, added to the cache, and 115marked as not containing valid data. 116 117The item returned is typically passed to cache_check which will check 118if the data is valid, and may initiate an up-call to get fresh data. 119cache_check will return -ENOENT in the entry is negative or if an up 120call is needed but not possible, -EAGAIN if an upcall is pending, 121or 0 if the data is valid; 122 123cache_check can be passed a "struct cache_req\*". This structure is 124typically embedded in the actual request and can be used to create a 125deferred copy of the request (struct cache_deferred_req). This is 126done when the found cache item is not uptodate, but the is reason to 127believe that userspace might provide information soon. When the cache 128item does become valid, the deferred copy of the request will be 129revisited (->revisit). It is expected that this method will 130reschedule the request for processing. 131 132The value returned by sunrpc_cache_lookup_rcu can also be passed to 133sunrpc_cache_update to set the content for the item. A second item is 134passed which should hold the content. If the item found by _lookup 135has valid data, then it is discarded and a new item is created. This 136saves any user of an item from worrying about content changing while 137it is being inspected. If the item found by _lookup does not contain 138valid data, then the content is copied across and CACHE_VALID is set. 139 140Populating a cache 141------------------ 142 143Each cache has a name, and when the cache is registered, a directory 144with that name is created in /proc/net/rpc 145 146This directory contains a file called 'channel' which is a channel 147for communicating between kernel and user for populating the cache. 148This directory may later contain other files of interacting 149with the cache. 150 151The 'channel' works a bit like a datagram socket. Each 'write' is 152passed as a whole to the cache for parsing and interpretation. 153Each cache can treat the write requests differently, but it is 154expected that a message written will contain: 155 156 - a key 157 - an expiry time 158 - a content. 159 160with the intention that an item in the cache with the give key 161should be create or updated to have the given content, and the 162expiry time should be set on that item. 163 164Reading from a channel is a bit more interesting. When a cache 165lookup fails, or when it succeeds but finds an entry that may soon 166expire, a request is lodged for that cache item to be updated by 167user-space. These requests appear in the channel file. 168 169Successive reads will return successive requests. 170If there are no more requests to return, read will return EOF, but a 171select or poll for read will block waiting for another request to be 172added. 173 174Thus a user-space helper is likely to:: 175 176 open the channel. 177 select for readable 178 read a request 179 write a response 180 loop. 181 182If it dies and needs to be restarted, any requests that have not been 183answered will still appear in the file and will be read by the new 184instance of the helper. 185 186Each cache should define a "cache_parse" method which takes a message 187written from user-space and processes it. It should return an error 188(which propagates back to the write syscall) or 0. 189 190Each cache should also define a "cache_request" method which 191takes a cache item and encodes a request into the buffer 192provided. 193 194.. note:: 195 If a cache has no active readers on the channel, and has had not 196 active readers for more than 60 seconds, further requests will not be 197 added to the channel but instead all lookups that do not find a valid 198 entry will fail. This is partly for backward compatibility: The 199 previous nfs exports table was deemed to be authoritative and a 200 failed lookup meant a definite 'no'. 201 202request/response format 203----------------------- 204 205While each cache is free to use its own format for requests 206and responses over channel, the following is recommended as 207appropriate and support routines are available to help: 208Each request or response record should be printable ASCII 209with precisely one newline character which should be at the end. 210Fields within the record should be separated by spaces, normally one. 211If spaces, newlines, or nul characters are needed in a field they 212much be quoted. two mechanisms are available: 213 214- If a field begins '\x' then it must contain an even number of 215 hex digits, and pairs of these digits provide the bytes in the 216 field. 217- otherwise a \ in the field must be followed by 3 octal digits 218 which give the code for a byte. Other characters are treated 219 as them selves. At the very least, space, newline, nul, and 220 '\' must be quoted in this way. 221