'\" t
.\"     Title: bio_alloc_bioset
.\"    Author: [FIXME: author] [see http://docbook.sf.net/el/author]
.\" Generator: DocBook XSL Stylesheets v1.79.1 <http://docbook.sf.net/>
.\"      Date: April 2019
.\"    Manual: The Linux VFS
.\"    Source: Kernel Hackers Manual 4.9.168
.\"  Language: English
.\"
.TH "BIO_ALLOC_BIOSET" "9" "April 2019" "Kernel Hackers Manual 4\&.9\&." "The Linux VFS"
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.SH "NAME"
bio_alloc_bioset \- allocate a bio for I/O
.SH "SYNOPSIS"
.HP \w'struct\ bio\ *\ bio_alloc_bioset('u
.BI "struct bio * bio_alloc_bioset(gfp_t\ " "gfp_mask" ", int\ " "nr_iovecs" ", struct\ bio_set\ *\ " "bs" ");"
.SH "ARGUMENTS"
.PP
\fIgfp_mask\fR
.RS 4
the GFP_ mask given to the slab allocator
.RE
.PP
\fInr_iovecs\fR
.RS 4
number of iovecs to pre\-allocate
.RE
.PP
\fIbs\fR
.RS 4
the bio_set to allocate from\&.
.RE
.SH "DESCRIPTION"
.PP
If
\fIbs\fR
is NULL, uses
\fBkmalloc\fR
to allocate the bio; else the allocation is backed by the
\fIbs\fR\*(Aqs mempool\&.
.PP
When
\fIbs\fR
is not NULL, if
\fB__GFP_DIRECT_RECLAIM\fR
is set then bio_alloc will always be able to allocate a bio\&. This is due to the mempool guarantees\&. To make this work, callers must never allocate more than 1 bio at a time from this pool\&. Callers that need to allocate more than 1 bio must always submit the previously allocated bio for IO before attempting to allocate a new one\&. Failure to do so can cause deadlocks under memory pressure\&.
.PP
Note that when running under
\fBgeneric_make_request\fR
(i\&.e\&. any block driver), bios are not submitted until after you return \- see the code in
\fBgeneric_make_request\fR
that converts recursion into iteration, to prevent stack overflows\&.
.PP
This would normally mean allocating multiple bios under
\fBgeneric_make_request\fR
would be susceptible to deadlocks, but we have deadlock avoidance code that resubmits any blocked bios from a rescuer thread\&.
.PP
However, we do not guarantee forward progress for allocations from other mempools\&. Doing multiple allocations from the same mempool under
\fBgeneric_make_request\fR
should be avoided \- instead, use bio_set\*(Aqs front_pad for per bio allocations\&.
.SH "RETURN"
.PP
Pointer to new bio on success, NULL on failure\&.
.SH "COPYRIGHT"
.br