4. On GEOM Programming

4. On GEOM Programming
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4.1. Ggate

If maximum performance is not needed, a much simpler way of making a data transformation is to implement it in userland via the ggate (GEOM gate) facility. Unfortunately, there is no easy way to convert between, or even share code between the two approaches.

4.2. GEOM Class

GEOM classes are transformations on the data. These transformations can be combined in a tree-like fashion. Instances of GEOM classes are called geoms.

Each GEOM class has several “class methods” that get called when there is no geom instance available (or they are simply not bound to a single instance):

.init is called when GEOM becomes aware of a GEOM class (when the kernel module gets loaded.)
.fini gets called when GEOM abandons the class (when the module gets unloaded)
.taste is called next, once for each provider the system has available. If applicable, this function will usually create and start a geom instance.
.destroy_geom is called when the geom should be disbanded
.ctlconf is called when user requests reconfiguration of existing geom

Also defined are the GEOM event functions, which will get copied to the geom instance.

Field .geom in the g_class structure is a LIST of geoms instantiated from the class.

These functions are called from the g_event kernel thread.

4.3. Softc

The name “softc” is a legacy term for “driver private data”. The name most probably comes from the archaic term “software control block”. In GEOM, it is a structure (more precise: pointer to a structure) that can be attached to a geom instance to hold whatever data is private to the geom instance. Most GEOM classes have the following members:

struct g_provider *provider : The “provider” this geom instantiates
uint16_t n_disks : Number of consumer this geom consumes
struct g_consumer **disks : Array of struct g_consumer*. (It is not possible to use just single indirection because struct g_consumer* are created on our behalf by GEOM).

The softc structure contains all the state of geom instance. Every geom instance has its own softc.

4.4. Metadata

Format of metadata is more-or-less class-dependent, but MUST start with:

16 byte buffer for null-terminated signature (usually the class name)
uint32 version ID

It is assumed that geom classes know how to handle metadata with version ID's lower than theirs.

Metadata is located in the last sector of the provider (and thus must fit in it).

(All this is implementation-dependent but all existing code works like that, and it is supported by libraries.)

4.5. Labeling/creating a GEOM

The sequence of events is:

user calls geom(8) utility (or one of its hardlinked friends)
the utility figures out which geom class it is supposed to handle and searches for geom_CLASSNAME.so library (usually in /lib/geom).
it dlopen(3)-s the library, extracts the definitions of command-line parameters and helper functions.

In the case of creating/labeling a new geom, this is what happens:

geom(8) looks in the command-line argument for the command (usually label), and calls a helper function.
The helper function checks parameters and gathers metadata, which it proceeds to write to all concerned providers.
This “spoils” existing geoms (if any) and initializes a new round of “tasting” of the providers. The intended geom class recognizes the metadata and brings the geom up.

(The above sequence of events is implementation-dependent but all existing code works like that, and it is supported by libraries.)

4.6. GEOM Command Structure

The helper geom_CLASSNAME.so library exports class_commands structure, which is an array of struct g_command elements. Commands are of uniform format and look like:

  verb [-options] geomname [other]

Common verbs are:

label — to write metadata to devices so they can be recognized at tasting and brought up in geoms
destroy — to destroy metadata, so the geoms get destroyed

Common options are:

-v : be verbose
-f : force

Many actions, such as labeling and destroying metadata can be performed in userland. For this, struct g_command provides field gc_func that can be set to a function (in the same .so) that will be called to process a verb. If gc_func is NULL, the command will be passed to kernel module, to .ctlreq function of the geom class.

4.7. Geoms

Geoms are instances of GEOM classes. They have internal data (a softc structure) and some functions with which they respond to external events.

The event functions are:

.access : calculates permissions (read/write/exclusive)
.dumpconf : returns XML-formatted information about the geom
.orphan : called when some underlying provider gets disconnected
.spoiled : called when some underlying provider gets written to
.start : handles I/O

These functions are called from the g_down kernel thread and there can be no sleeping in this context, (see definition of sleeping elsewhere) which limits what can be done quite a bit, but forces the handling to be fast.

Of these, the most important function for doing actual useful work is the .start() function, which is called when a BIO request arrives for a provider managed by a instance of geom class.

4.8. GEOM Threads

There are three kernel threads created and run by the GEOM framework:

g_down : Handles requests coming from high-level entities (such as a userland request) on the way to physical devices
g_up : Handles responses from device drivers to requests made by higher-level entities
g_event : Handles all other cases: creation of geom instances, access counting, “spoil” events, etc.

When a user process issues “read data X at offset Y of a file” request, this is what happens:

The filesystem converts the request into a struct bio instance and passes it to the GEOM subsystem. It knows what geom instance should handle it because filesystems are hosted directly on a geom instance.
The request ends up as a call to the .start() function made on the g_down thread and reaches the top-level geom instance.
This top-level geom instance (for example the partition slicer) determines that the request should be routed to a lower-level instance (for example the disk driver). It makes a copy of the bio request (bio requests ALWAYS need to be copied between instances, with g_clone_bio()!), modifies the data offset and target provider fields and executes the copy with g_io_request()
The disk driver gets the bio request also as a call to .start() on the g_down thread. It talks to hardware, gets the data back, and calls g_io_deliver() on the bio.
Now, the notification of bio completion “bubbles up” in the g_up thread. First the partition slicer gets .done() called in the g_up thread, it uses information stored in the bio to free the cloned bio structure (with g_destroy_bio()) and calls g_io_deliver() on the original request.
The filesystem gets the data and transfers it to userland.

See g_bio(9) man page for information how the data is passed back and forth in the bio structure (note in particular the bio_parent and bio_children fields and how they are handled).

One important feature is: THERE CAN BE NO SLEEPING IN G_UP AND G_DOWN THREADS. This means that none of the following things can be done in those threads (the list is of course not complete, but only informative):

Calls to msleep() and tsleep(), obviously.
Calls to g_write_data() and g_read_data(), because these sleep between passing the data to consumers and returning.
Waiting for I/O.
Calls to malloc(9) and uma_zalloc() with M_WAITOK flag set
sx and other sleepable locks

This restriction is here to stop GEOM code clogging the I/O request path, since sleeping is usually not time-bound and there can be no guarantees on how long will it take (there are some other, more technical reasons also). It also means that there is not much that can be done in those threads; for example, almost any complex thing requires memory allocation. Fortunately, there is a way out: creating additional kernel threads.

4.9. Kernel Threads for Use in GEOM Code

Kernel threads are created with kthread_create(9) function, and they are sort of similar to userland threads in behavior, only they cannot return to caller to signify termination, but must call kthread_exit(9).

In GEOM code, the usual use of threads is to offload processing of requests from g_down thread (the .start() function). These threads look like “event handlers”: they have a linked list of event associated with them (on which events can be posted by various functions in various threads so it must be protected by a mutex), take the events from the list one by one and process them in a big switch() statement.

The main benefit of using a thread to handle I/O requests is that it can sleep when needed. Now, this sounds good, but should be carefully thought out. Sleeping is well and very convenient but can very effectively destroy performance of the geom transformation. Extremely performance-sensitive classes probably should do all the work in .start() function call, taking great care to handle out-of-memory and similar errors.

The other benefit of having a event-handler thread like that is to serialize all the requests and responses coming from different geom threads into one thread. This is also very convenient but can be slow. In most cases, handling of .done() requests can be left to the g_up thread.

Mutexes in FreeBSD kernel (see mutex(9)) have one distinction from their more common userland cousins — the code cannot sleep while holding a mutex). If the code needs to sleep a lot, sx(9) locks may be more appropriate. On the other hand, if you do almost everything in a single thread, you may get away with no mutexes at all.