man/man9/g_bio.9

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.Dd August 7, 2019
.Dt G_BIO 9
.Os
.Sh NAME
.Nm g_new_bio ,
.Nm g_clone_bio ,
.Nm g_destroy_bio ,
.Nm g_format_bio ,
.Nm g_print_bio ,
.Nm g_reset_bio
.Nd "GEOM bio controlling functions"
.Sh SYNOPSIS
.In sys/bio.h
.In geom/geom.h
.Ft "struct bio *"
.Fn g_new_bio void
.Ft "struct bio *"
.Fn g_alloc_bio void
.Ft "struct bio *"
.Fn g_clone_bio "struct bio *bp"
.Ft "struct bio *"
.Fn g_duplicate_bio "struct bio *bp"
.Ft void
.Fn g_destroy_bio "struct bio *bp"
.Ft void
.Fn g_format_bio "struct sbuf *sb" "const struct bio *bp"
.Ft void
.Fo g_print_bio
.Fa "struct sbuf *sb" "const char *prefix" "const struct bio *bp"
.Fa "const char *fmtsuffix" ...
.Fc
.Ft void
.Fn g_reset_bio "struct bio *bp"
.Sh DESCRIPTION
A
.Vt "struct bio"
is used by GEOM to describe I/O requests, its
most important fields are described below:
.Bl -tag -width ".Va bio_attribute"
.It Va bio_cmd
I/O request command.
There are five I/O requests available in GEOM:
.Bl -tag -width ".Dv BIO_GETATTR"
.It Dv BIO_READ
A read request.
.It Dv BIO_WRITE
A write request.
.It Dv BIO_DELETE
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.
.It Dv BIO_GETATTR
Inspect and manipulate out-of-band
attributes on a particular provider or path.
Attributes are named by ascii strings and are stored in the
.Va bio_attribute
field.
.It Dv BIO_FLUSH
Tells underlying providers to flush their write caches.
.El
.It Va bio_flags
Available flags:
.Bl -tag -width ".Dv BIO_ERROR"
.It Dv BIO_ERROR
Request failed (error value is stored in
.Va bio_error
field).
.It Dv BIO_DONE
Request finished.
.El
.It Va bio_cflags
Private use by the consumer.
.It Va bio_pflags
Private use by the provider.
.It Va bio_offset
Offset into provider.
.It Va bio_data
Pointer to data buffer.
.It Va bio_error
Error value when
.Dv BIO_ERROR
is set.
.It Va bio_done
Pointer to function which will be called when the request is finished.
.It Va bio_driver1
Private use by the provider.
.It Va bio_driver2
Private use by the provider.
.It Va bio_caller1
Private use by the consumer.
.It Va bio_caller2
Private use by the consumer.
.It Va bio_attribute
Attribute string for
.Dv BIO_GETATTR
request.
.It Va bio_from
Consumer to use for request (attached to provider stored in
.Va bio_to
field) (typically read-only for a class).
.It Va bio_to
Destination provider (typically read-only for a class).
.It Va bio_length
Request length in bytes.
.It Va bio_completed
Number of bytes completed, but they may not be completed from
the front of the request.
.It Va bio_children
Number of
.Vt bio
clones (typically read-only for a class).
.It Va bio_inbed
Number of finished
.Vt bio
clones.
.It Va bio_parent
Pointer to parent
.Vt bio .
.El
.Pp
The
.Fn g_new_bio
function allocates a new, empty
.Vt bio
structure.
.Pp
.Fn g_alloc_bio
- same as
.Fn g_new_bio ,
but always succeeds (allocates bio with the
.Dv M_WAITOK
malloc flag).
.Pp
The
.Fn g_clone_bio
function allocates a new
.Vt bio
structure and copies the following fields from the
.Vt bio
given as an argument to clone:
.Va bio_cmd ,
.Va bio_length ,
.Va bio_offset ,
.Va bio_data ,
.Va bio_attribute .
The field
.Va bio_parent
in the clone points to the passed
.Vt bio
and the field
.Va bio_children
in the passed
.Vt bio
is incremented.
.Pp
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:
.Pp
.Bl -enum -compact
.It
Clone the received
.Vt "struct bio" .
.It
Modify the clone.
.It
Schedule the clone on its own consumer.
.El
.Pp
.Fn g_duplicate_bio
- same as
.Fn g_clone_bio ,
but always succeeds (allocates bio with the
.Dv M_WAITOK
malloc flag).
.Pp
The
.Fn g_destroy_bio
function deallocates and destroys the given
.Vt bio
structure.
.Pp
The
.Fn g_format_bio
function prints information about the given
.Vt bio
structure into the provided
.Vt sbuf .
.Pp
The
.Fn g_print_bio
function is a convenience wrapper around
.Fn g_format_bio
that can be used for debugging purposes.
It prints a provided
.Fa prefix
string, followed by the formatted
.Vt bio ,
followed by a
.Fa fmtsuffix
in the style of
.Xr printf 9 .
Any of the prefix or suffix strings may be the empty string.
.Fn g_print_bio
always prints a newline character at the end of the line.
.Pp
The
.Fn g_reset_bio
function resets the given
.Vt bio
structure back to its initial state.
.Fn g_reset_bio
preserves internal data structures, while setting all
user visible fields to their initial values.
When reusing a
.Vt bio
obtained from
.Fn g_new_bio ,
.Fn g_alloc_bio ,
.Fn g_clone_bio ,
or
.Fn g_duplicate_bio
for multiple transactions,
.Fn g_reset_bio
must be called between the transactions in lieu of
.Fn bzero .
While not strictly required for a
.Vt bio
structure created by other means,
.Fn g_reset_bio
should be used to initialize it and between transactions.
.Sh RETURN VALUES
The
.Fn g_new_bio
and
.Fn g_clone_bio
functions return a pointer to the allocated
.Vt bio ,
or
.Dv NULL
if an error occurred.
.Sh EXAMPLES
Implementation of
.Dq Dv NULL Ns -transformation ,
meaning that an I/O request is cloned and scheduled down without any
modifications.
Let us assume that field
.Va ex_consumer
in structure
.Vt example_softc
contains a consumer attached to the provider we want to operate on.
.Bd -literal -offset indent
void
example_start(struct bio *bp)
{
	struct example_softc *sc;
	struct bio *cbp;

	g_print_bio("Request received: ", bp, "");

	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);
}
.Ed
.Sh SEE ALSO
.Xr geom 4 ,
.Xr DECLARE_GEOM_CLASS 9 ,
.Xr g_access 9 ,
.Xr g_attach 9 ,
.Xr g_consumer 9 ,
.Xr g_data 9 ,
.Xr g_event 9 ,
.Xr g_geom 9 ,
.Xr g_provider 9 ,
.Xr g_provider_by_name 9 ,
.Xr g_wither_geom 9
.Sh AUTHORS
.An -nosplit
This manual page was written by
.An Pawel Jakub Dawidek Aq Mt pjd@FreeBSD.org .