2.6.25

 Documentation

diff --git a/Documentation/rpc-cache.txt b/Documentation/rpc-cache.txt
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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.
-
-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.