| This document gives a brief introduction to the caching |
| mechanisms in the sunrpc layer that is used, in particular, |
| for NFS authentication. |
| |
| CACHES |
| ====== |
| The caching replaces the old exports table and allows for |
| a wide variety of values to be caches. |
| |
| There are a number of caches that are similar in structure though |
| quite possibly very different in content and use. There is a corpus |
| of common code for managing these caches. |
| |
| Examples of caches that are likely to be needed are: |
| - mapping from IP address to client name |
| - mapping from client name and filesystem to export options |
| - mapping from UID to list of GIDs, to work around NFS's limitation |
| of 16 gids. |
| - mappings between local UID/GID and remote UID/GID for sites that |
| do not have uniform uid assignment |
| - mapping from network identify to public key for crypto authentication. |
| |
| The common code handles such things as: |
| - general cache lookup with correct locking |
| - supporting 'NEGATIVE' as well as positive entries |
| - allowing an EXPIRED time on cache items, and removing |
| items after they expire, and are no longer in-use. |
| - making requests to user-space to fill in cache entries |
| - allowing user-space to directly set entries in the cache |
| - delaying RPC requests that depend on as-yet incomplete |
| cache entries, and replaying those requests when the cache entry |
| is complete. |
| - clean out old entries as they expire. |
| |
| Creating a Cache |
| ---------------- |
| |
| 1/ A cache needs a datum to store. This is in the form of a |
| structure definition that must contain a |
| struct cache_head |
| as an element, usually the first. |
| It will also contain a key and some content. |
| Each cache element is reference counted and contains |
| expiry and update times for use in cache management. |
| 2/ A cache needs a "cache_detail" structure that |
| describes the cache. This stores the hash table, some |
| parameters for cache management, and some operations detailing how |
| to work with particular cache items. |
| The operations requires are: |
| struct cache_head *alloc(void) |
| This simply allocates appropriate memory and returns |
| a pointer to the cache_detail embedded within the |
| structure |
| void cache_put(struct kref *) |
| This is called when the last reference to an item is |
| dropped. The pointer passed is to the 'ref' field |
| in the cache_head. cache_put should release any |
| references create by 'cache_init' and, if CACHE_VALID |
| is set, any references created by cache_update. |
| It should then release the memory allocated by |
| 'alloc'. |
| int match(struct cache_head *orig, struct cache_head *new) |
| test if the keys in the two structures match. Return |
| 1 if they do, 0 if they don't. |
| void init(struct cache_head *orig, struct cache_head *new) |
| Set the 'key' fields in 'new' from 'orig'. This may |
| include taking references to shared objects. |
| void update(struct cache_head *orig, struct cache_head *new) |
| Set the 'content' fileds in 'new' from 'orig'. |
| int cache_show(struct seq_file *m, struct cache_detail *cd, |
| struct cache_head *h) |
| Optional. Used to provide a /proc file that lists the |
| contents of a cache. This should show one item, |
| usually on just one line. |
| int cache_request(struct cache_detail *cd, struct cache_head *h, |
| char **bpp, int *blen) |
| Format a request to be send to user-space for an item |
| to be instantiated. *bpp is a buffer of size *blen. |
| bpp should be moved forward over the encoded message, |
| and *blen should be reduced to show how much free |
| space remains. Return 0 on success or <0 if not |
| enough room or other problem. |
| int cache_parse(struct cache_detail *cd, char *buf, int len) |
| A message from user space has arrived to fill out a |
| cache entry. It is in 'buf' of length 'len'. |
| cache_parse should parse this, find the item in the |
| cache with sunrpc_cache_lookup, and update the item |
| with sunrpc_cache_update. |
| |
| |
| 3/ A cache needs to be registered using cache_register(). This |
| includes it on a list of caches that will be regularly |
| cleaned to discard old data. |
| |
| Using a cache |
| ------------- |
| |
| To find a value in a cache, call sunrpc_cache_lookup passing a pointer |
| to the cache_head in a sample item with the 'key' fields filled in. |
| This will be passed to ->match to identify the target entry. If no |
| entry is found, a new entry will be create, added to the cache, and |
| marked as not containing valid data. |
| |
| The item returned is typically passed to cache_check which will check |
| if the data is valid, and may initiate an up-call to get fresh data. |
| cache_check will return -ENOENT in the entry is negative or if an up |
| call is needed but not possible, -EAGAIN if an upcall is pending, |
| or 0 if the data is valid; |
| |
| cache_check can be passed a "struct cache_req *". This structure is |
| typically embedded in the actual request and can be used to create a |
| deferred copy of the request (struct cache_deferred_req). This is |
| done when the found cache item is not uptodate, but the is reason to |
| believe that userspace might provide information soon. When the cache |
| item does become valid, the deferred copy of the request will be |
| revisited (->revisit). It is expected that this method will |
| reschedule the request for processing. |
| |
| The value returned by sunrpc_cache_lookup can also be passed to |
| sunrpc_cache_update to set the content for the item. A second item is |
| passed which should hold the content. If the item found by _lookup |
| has valid data, then it is discarded and a new item is created. This |
| saves any user of an item from worrying about content changing while |
| it is being inspected. If the item found by _lookup does not contain |
| valid data, then the content is copied across and CACHE_VALID is set. |
| |
| Populating a cache |
| ------------------ |
| |
| Each cache has a name, and when the cache is registered, a directory |
| with that name is created in /proc/net/rpc |
| |
| This directory contains a file called 'channel' which is a channel |
| for communicating between kernel and user for populating the cache. |
| This directory may later contain other files of interacting |
| with the cache. |
| |
| The 'channel' works a bit like a datagram socket. Each 'write' is |
| passed as a whole to the cache for parsing and interpretation. |
| Each cache can treat the write requests differently, but it is |
| expected that a message written will contain: |
| - a key |
| - an expiry time |
| - a content. |
| with the intention that an item in the cache with the give key |
| should be create or updated to have the given content, and the |
| expiry time should be set on that item. |
| |
| Reading from a channel is a bit more interesting. When a cache |
| lookup fails, or when it succeeds but finds an entry that may soon |
| expire, a request is lodged for that cache item to be updated by |
| user-space. These requests appear in the channel file. |
| |
| Successive reads will return successive requests. |
| If there are no more requests to return, read will return EOF, but a |
| select or poll for read will block waiting for another request to be |
| added. |
| |
| Thus a user-space helper is likely to: |
| open the channel. |
| select for readable |
| read a request |
| write a response |
| loop. |
| |
| If it dies and needs to be restarted, any requests that have not been |
| answered will still appear in the file and will be read by the new |
| instance of the helper. |
| |
| Each cache should define a "cache_parse" method which takes a message |
| written from user-space and processes it. It should return an error |
| (which propagates back to the write syscall) or 0. |
| |
| Each cache should also define a "cache_request" method which |
| takes a cache item and encodes a request into the buffer |
| provided. |
| |
| Note: If a cache has no active readers on the channel, and has had not |
| active readers for more than 60 seconds, further requests will not be |
| added to the channel but instead all lookups that do not find a valid |
| entry will fail. This is partly for backward compatibility: The |
| previous nfs exports table was deemed to be authoritative and a |
| failed lookup meant a definite 'no'. |
| |
| request/response format |
| ----------------------- |
| |
| While each cache is free to use it's own format for requests |
| and responses over channel, the following is recommended as |
| appropriate and support routines are available to help: |
| Each request or response record should be printable ASCII |
| with precisely one newline character which should be at the end. |
| Fields within the record should be separated by spaces, normally one. |
| If spaces, newlines, or nul characters are needed in a field they |
| much be quoted. two mechanisms are available: |
| 1/ If a field begins '\x' then it must contain an even number of |
| hex digits, and pairs of these digits provide the bytes in the |
| field. |
| 2/ otherwise a \ in the field must be followed by 3 octal digits |
| which give the code for a byte. Other characters are treated |
| as them selves. At the very least, space, newline, nul, and |
| '\' must be quoted in this way. |