NAME¶
sf_buf
—
manage temporary kernel address space mapping for
memory pages
SYNOPSIS¶
#include
<sys/sf_buf.h>
struct sf_buf *
sf_buf_alloc
(
struct
vm_page *m,
int
flags);
void
sf_buf_free
(
struct
sf_buf *sf);
vm_offset_t
sf_buf_kva
(
struct
sf_buf *sf);
struct vm_page *
sf_buf_page
(
struct
sf_buf *sf);
DESCRIPTION¶
The
sf_buf
interface, historically the
sendfile(2) buffer interface, allows kernel
subsystems to manage temporary kernel address space mappings for physical
memory pages. On systems with a direct memory map region (allowing all
physical pages to be visible in the kernel address space at all times), the
struct sf_buf will point to an address in the
direct map region; on systems without a direct memory map region, the
struct sf_buf will manage a temporary kernel
address space mapping valid for the lifetime of the
struct sf_buf.
Call
sf_buf_alloc
() to allocate a
struct sf_buf for a physical memory page.
sf_buf_alloc
() is not responsible for
arranging for the page to be present in physical memory; the caller should
already have arranged for the page to be wired, i.e., by calling
vm_page_wire(9). Several flags may be passed to
sf_buf_alloc
():
SFB_CATCH
- Cause
sf_buf_alloc
() to abort and
return NULL
if a signal is received
waiting for a struct sf_buf to become
available.
SFB_NOWAIT
- Cause
sf_buf_alloc
() to return
NULL
rather than sleeping if a
struct sf_buf is not immediately
available.
SFB_CPUPRIVATE
- Cause
sf_buf_alloc
() to only arrange
that the temporary mapping be valid on the current CPU, avoiding
unnecessary TLB shootdowns for mappings that will only be accessed on a
single CPU at a time. The caller must ensure that accesses to the virtual
address occur only on the CPU from which
sf_buf_alloc
() was invoked, perhaps by
using sched_pin
().
Call
sf_buf_kva
() to return a kernel mapped
address for the page.
Call
sf_buf_page
() to return a pointer to the
page originally passed into
sf_buf_alloc
().
Call
sf_buf_free
() to release the
struct sf_buf reference. The caller is
responsible for releasing any wiring they have previously acquired on the
physical page;
sf_buf_free
() releases only
the temporary kernel address space mapping, not the page itself.
Uses of this interface include managing mappings of borrowed pages from user
memory, such as in zero-copy socket I/O, or pages of memory from the buffer
cache referenced by mbuf external storage for
sendfile(2).
SEE ALSO¶
sendfile(2),
vm_page_wire(9)
AUTHORS¶
The
struct sf_buf API was designed and
implemented by
Alan L. Cox. This manual
page was written by
Robert N. M.
Watson.