mirror of
https://github.com/edk2-porting/linux-next.git
synced 2024-12-25 05:34:00 +08:00
670e9f34ee
Remove many duplicated words under Documentation/ and do other small
cleanups.
Examples:
"and and" --> "and"
"in in" --> "in"
"the the" --> "the"
"the the" --> "to the"
...
Signed-off-by: Paolo Ornati <ornati@fastwebnet.it>
Signed-off-by: Adrian Bunk <bunk@stusta.de>
203 lines
8.7 KiB
Plaintext
203 lines
8.7 KiB
Plaintext
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.
|