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+This document gives a brief introduction to the caching
+mechanisms in the sunrpc layer that is used, in particular,
+for NFS authentication.
+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
+Thus a user-space helper is likely to:
+ open the channel.
+ select for readable
+ read a request
+ write a response
+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
+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'.
+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
+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.