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Man Pages
G_BIO(9) FreeBSD Kernel Developer's Manual G_BIO(9)

g_new_bio, g_clone_bio, g_destroy_bio, g_print_bio, g_reset_bio
GEOM bio controlling functions

#include <sys/bio.h>
#include <geom/geom.h>
struct bio *
g_new_bio(void);
struct bio *
g_alloc_bio(void);
struct bio *
g_clone_bio(struct bio *bp);
struct bio *
g_duplicate_bio(struct bio *bp);
void
g_destroy_bio(struct bio *bp);
void
g_print_bio(struct bio *bp);
void
g_reset_bio(struct bio *bp);

A struct bio is used by GEOM to describe I/O requests, its most important fields are described below:
bio_cmd
I/O request command. There are five I/O requests available in GEOM:
A read request.
A write request.
Indicates that a certain range of data is no longer used and that it can be erased or freed as the underlying technology supports. Technologies like flash adaptation layers can arrange to erase the relevant blocks before they will become reassigned and cryptographic devices may want to fill random bits into the range to reduce the amount of data available for attack.
Inspect and manipulate out-of-band attributes on a particular provider or path. Attributes are named by ascii strings and are stored in the bio_attribute field.
Tells underlying providers to flush their write caches.
bio_flags
Available flags:
Request failed (error value is stored in bio_error field).
Request finished.
bio_cflags
Private use by the consumer.
bio_pflags
Private use by the provider.
bio_offset
Offset into provider.
bio_data
Pointer to data buffer.
bio_error
Error value when BIO_ERROR is set.
bio_done
Pointer to function which will be called when the request is finished.
bio_driver1
Private use by the provider.
bio_driver2
Private use by the provider.
bio_caller1
Private use by the consumer.
bio_caller2
Private use by the consumer.
bio_attribute
Attribute string for BIO_GETATTR request.
bio_from
Consumer to use for request (attached to provider stored in bio_to field) (typically read-only for a class).
bio_to
Destination provider (typically read-only for a class).
bio_length
Request length in bytes.
bio_completed
Number of bytes completed, but they may not be completed from the front of the request.
bio_children
Number of bio clones (typically read-only for a class).
bio_inbed
Number of finished bio clones.
bio_parent
Pointer to parent bio.
The g_new_bio() function allocates a new, empty bio structure.
g_alloc_bio() - same as g_new_bio(), but always succeeds (allocates bio with the M_WAITOK malloc flag).
The g_clone_bio() function allocates a new bio structure and copies the following fields from the bio given as an argument to clone: bio_cmd, bio_length, bio_offset, bio_data, bio_attribute. The field bio_parent in the clone points to the passed bio and the field bio_children in the passed bio is incremented.
This function should be used for every request which enters through the provider of a particular geom and needs to be scheduled down. Proper order is:
  1. Clone the received struct bio.
  2. Modify the clone.
  3. Schedule the clone on its own consumer.
g_duplicate_bio() - same as g_clone_bio(), but always succeeds (allocates bio with the M_WAITOK malloc flag).
The g_destroy_bio() function deallocates and destroys the given bio structure.
The g_print_bio() function prints information about the given bio structure (for debugging purposes).
The g_reset_bio() function resets the given bio structure back to its initial state. g_reset_bio() preserves internal data structures, while setting all user visible fields to their initial values. When reusing a bio obtained from g_new_bio(), g_alloc_bio(), g_clone_bio(), or g_duplicate_bio() for multiple transactions, g_reset_bio() must be called between the transactions in lieu of bzero(). While not strictly required for a bio structure created by other means, g_reset_bio() should be used to initialize it and between transactions.

The g_new_bio() and g_clone_bio() functions return a pointer to the allocated bio, or NULL if an error occurred.

Implementation of “NULL-transformation”, meaning that an I/O request is cloned and scheduled down without any modifications. Let us assume that field ex_consumer in structure example_softc contains a consumer attached to the provider we want to operate on.
void 
example_start(struct bio *bp) 
{ 
	struct example_softc *sc; 
	struct bio *cbp; 
 
	printf("Request received: "); 
	g_print_bio(bp); 
	printf("\n"); 
 
	sc = bp->bio_to->geom->softc; 
	if (sc == NULL) { 
		g_io_deliver(bp, ENXIO); 
		return; 
	} 
 
	/* Let's clone our bio request. */ 
	cbp = g_clone_bio(bp); 
	if (cbp == NULL) { 
		g_io_deliver(bp, ENOMEM); 
		return; 
	} 
	cbp->bio_done = g_std_done;	/* Standard 'done' function. */ 
 
	/* Ok, schedule it down. */ 
	/* 
	 * The consumer can be obtained from 
	 * LIST_FIRST(&bp->bio_to->geom->consumer) as well, 
	 * if there is only one in our geom. 
	 */ 
	g_io_request(cbp, sc->ex_consumer); 
}

geom(4), DECLARE_GEOM_CLASS(9), g_access(9), g_attach(9), g_consumer(9), g_data(9), g_event(9), g_geom(9), g_provider(9), g_provider_by_name(9), g_wither_geom(9)

This manual page was written by Pawel Jakub Dawidek <pjd@FreeBSD.org>.
March 7, 2018 FreeBSD 12.0-RELEASE

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