'\" t
.\"     Title: blk_make_request
.\"    Author: [FIXME: author] [see http://docbook.sf.net/el/author]
.\" Generator: DocBook XSL Stylesheets v1.78.1 <http://docbook.sf.net/>
.\"      Date: May 2018
.\"    Manual: Block Devices
.\"    Source: Kernel Hackers Manual 3.16.56
.\"  Language: English
.\"
.TH "BLK_MAKE_REQUEST" "9" "May 2018" "Kernel Hackers Manual 3\&.16\&" "Block Devices"
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.SH "NAME"
blk_make_request \- given a bio, allocate a corresponding struct request\&.
.SH "SYNOPSIS"
.HP \w'struct\ request\ *\ blk_make_request('u
.BI "struct request * blk_make_request(struct\ request_queue\ *\ " "q" ", struct\ bio\ *\ " "bio" ", gfp_t\ " "gfp_mask" ");"
.SH "ARGUMENTS"
.PP
\fIq\fR
.RS 4
target request queue
.RE
.PP
\fIbio\fR
.RS 4
The bio describing the memory mappings that will be submitted for IO\&. It may be a chained\-bio properly constructed by block/bio layer\&.
.RE
.PP
\fIgfp_mask\fR
.RS 4
gfp flags to be used for memory allocation
.RE
.SH "DESCRIPTION"
.PP
blk_make_request is the parallel of generic_make_request for BLOCK_PC type commands\&. Where the struct request needs to be farther initialized by the caller\&. It is passed a
struct bio, which describes the memory info of the I/O transfer\&.
.PP
The caller of blk_make_request must make sure that bi_io_vec are set to describe the memory buffers\&. That
\fBbio_data_dir\fR
will return the needed direction of the request\&. (And all bio\*(Aqs in the passed bio\-chain are properly set accordingly)
.PP
If called under none\-sleepable conditions, mapped bio buffers must not need bouncing, by calling the appropriate masked or flagged allocator, suitable for the target device\&. Otherwise the call to blk_queue_bounce will BUG\&.
.SH "WARNING"
.PP
When allocating/cloning a bio\-chain, careful consideration should be given to how you allocate bios\&. In particular, you cannot use __GFP_WAIT for anything but the first bio in the chain\&. Otherwise you risk waiting for IO completion of a bio that hasn\*(Aqt been submitted yet, thus resulting in a deadlock\&. Alternatively bios should be allocated using
\fBbio_kmalloc\fR
instead of
\fBbio_alloc\fR, as that avoids the mempool deadlock\&. If possible a big IO should be split into smaller parts when allocation fails\&. Partial allocation should not be an error, or you risk a live\-lock\&.
.SH "COPYRIGHT"
.br