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 longe in-use. Future code extensions are expect to handle - 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. - maintaining last-access times on cache entries - clean out old entries when the caches become full The code for performing a cache lookup is also common, but in the form of a template. i.e. a #define. Each cache defines a lookup function by using the DefineCacheLookup macro, or the simpler DefineSimpleCacheLookup macro Creating a Cache ---------------- 1/ A cache needs a datum to cache. 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, and some parameters for cache management. 3/ A cache needs a lookup function. This is created using the DefineCacheLookup macro. This lookup function is used both to find entries and to update entries. The normal mode for updating an entry is to replace the old entry with a new entry. However it is possible to allow update-in-place for those caches where it makes sense (no atomicity issues or indirect reference counting issue) 4/ A cache needs to be registered using cache_register(). This includes in on a list of caches that will be regularly cleaned to discard old data. For this to work, some thread must periodically call cache_clean Using a cache ------------- To find a value in a cache, call the lookup function passing it a the datum which contains key, and possibly content, and a flag saying whether to update the cache with new data from the datum. Depending on how the cache lookup function was defined, it may take an extra argument to identify the particular cache in question. Except in cases of kmalloc failure, the lookup function will return a new datum which will store the key and may contain valid content, or may not. This datum is typically passed to cache_check which determines the validity of the datum and may later initiate an upcall to fill in the data. 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. 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 fail, or when it suceeds but finds an entry that may soon expiry, 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 be 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 looks 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 are 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 quotes. 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, newlines nul, and '\' must be quoted in this way.