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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.