sf_buf — manage temporary kernel address space mapping for memory pages

       The structsf_buf API was designed and implemented by Alan L. Cox.   This  manual  page  was  written  by
       Robert N. M. Watson.

Debian                                          January 28, 2007                                       SF_BUF(9)

       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 structsf_buf will point to an address in the direct map region; on systems without a direct memory map  region,
       the  structsf_buf  will  manage  a temporary kernel address space mapping valid for the lifetime of the
       structsf_buf.

       Call sf_buf_alloc() to allocate a structsf_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
                       structsf_buf to become available.

       SFB_NOWAIT      Cause sf_buf_alloc() to return NULL rather than  sleeping  if  a  structsf_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 structsf_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).

       sf_buf — manage temporary kernel address space mapping for memory pages

sendfile(2), vm_page_wire(9)

#include<sys/sf_buf.h>structsf_buf*sf_buf_alloc(structvm_page*m, intflags);

       voidsf_buf_free(structsf_buf*sf);

       vm_offset_tsf_buf_kva(structsf_buf*sf);

       structvm_page*sf_buf_page(structsf_buf*sf);