mirror of
https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git
synced 2024-09-12 05:48:40 +00:00
![Linus Torvalds](/assets/img/avatar_default.png)
Initial git repository build. I'm not bothering with the full history, even though we have it. We can create a separate "historical" git archive of that later if we want to, and in the meantime it's about 3.2GB when imported into git - space that would just make the early git days unnecessarily complicated, when we don't have a lot of good infrastructure for it. Let it rip!
171 lines
7.1 KiB
Text
171 lines
7.1 KiB
Text
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.
|
|
|