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SF_BUF(9)		 BSD Kernel Developer's	Manual		     SF_BUF(9)

     sf_buf -- manage temporary	kernel address space mapping for memory	pages

     #include <sys/sf_buf.h>

     struct sf_buf *
     sf_buf_alloc(struct vm_page *m, int flags);

     sf_buf_free(struct	sf_buf *sf);

     sf_buf_kva(struct sf_buf *sf);

     struct vm_page *
     sf_buf_page(struct	sf_buf *sf);

     The sf_buf	interface, historically	the sendfile(2)	buffer interface, al-
     lows 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	life-
     time 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 avail-

     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 ac-
		     cesses to the virtual address occur only on the CPU from
		     which sf_buf_alloc() was invoked, perhaps by using

     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

     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).

     sendfile(2), vm_page_wire(9)

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

BSD			       January 28, 2007				   BSD


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