The
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
Vt struct sf_buf
will point to an address in the direct map region; on systems without a
direct memory map region, the
Vt struct sf_buf
will manage a temporary kernel address space mapping valid for the lifetime
of the
Vt struct sf_buf.
Call
sf_buf_alloc ();
to allocate a
Vt 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_wire9.
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
Vt struct sf_buf
to become available.
SFB_NOWAIT
Cause
sf_buf_alloc ();
to return
NULL
rather than sleeping if a
Vt 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
Vt 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).
